This chapter discusses the scientific contributions and potential of CubeSats in the context of the science goals set in the decadal surveys from each of five space science subdisciplines: solar and space physics, Earth science and applications from space, planetary sciences, astronomy and astrophysics, and biological and physical sciences in space. As described below, CubeSat mission concepts are highly varied in terms of complexity and scale, filling a role that is distinct in each subdiscipline. For an example list of enabling technologies and potential applications by discipline, see Table 5.1 in Chapter 5. CubeSats are also a platform for demonstration of new technologies that may be used for missions of different sizes.
Before discussing each space science subdiscipline, the overall impact of CubeSats is considered, based on the committee’s review of publications through the end of calendar year 2015. The number and quality of publications are important measures of scientific output. As part of this study, the committee searched for publications using a number of different sources. A detailed account of the methods and findings, including an analysis of Web of Science, Scopus, and NASA/ADS, is provided in Appendix B. Figure 4.1 shows the number of publications as a function of year using the search terms “CubeSat” or “CubeSats” in the NASA/ADS abstract service. About a quarter of the papers (160 of 536) are in refereed journals. Although the primary focus of the committee was on refereed publications, both refereed and non-refereed publications are shown in Figure 4.1. The number of refereed publications is small and often lags missions by several years, thus, the inclusion of non-refereed publications provides another measure of recent activity. The number of both non-refereed and refereed publications increased rapidly between 2008 and 2015.
Not surprisingly, considering the distribution of CubeSat programs, 74 percent (118) of the 160 refereed papers were engineering-focused, while only 26 percent (41) were science-focused, as classified by the committee. The 41 science papers were further classified by subfield (see Figure B.4 in Appendix B). The largest presence by far is in solar and space physics, which has also seen the largest number of science CubeSats launched, primarily supported by the NSF-funded CubeSat program. Science papers from NASA-funded CubeSat science missions are not expected until after they launch, starting in 2016 and later. Refereed publications in astronomy and astrophysics or planetary sciences are mostly focused on the description of new measurement techniques or data strategies enabled by CubeSats. The majority of space physics papers are published in relatively high-impact journals. One paper was published in Nature, and several appeared in Geophysical Research Letters and Journal of Geophysical Research: Space Physics.
In reviewing the scientific literature, the committee found the following:
Finding: The research interest in CubeSats is increasing with time, as demonstrated by the growing number of publications. The majority of CubeSat-related publications are in non-refereed journals, but the number of refereed papers is also increasing with time.
Finding: The majority (74 percent) of refereed papers is engineering-focused, but the number of science-related papers is increasing over time. The majority of science papers is in the field of space physics and is based on NSF-funded CubeSats.
Conclusion: CubeSats have already produced high-value science, as demonstrated by peer-reviewed publications in high-impact journals.
Science Priorities in Solar and Space Physics—Decadal Survey Highlights
The most recent solar and space physics decadal survey, Solar and Space Physics: A Science for a Technological Society,1 was published in 2013 and emphasized that the Sun-Earth system has to be understood as a coupled system. The decadal survey outlined the following four key science goals:
- Determine the origins of the Sun’s activity and predict the variations of the space environment.
- Determine the dynamics and coupling of Earth’s magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs.
- Determine the interaction of the Sun with the solar system and the interstellar medium.
- Discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe.
The decadal survey also focused on the importance of the scientific foundations that improve the ability to forecast the Earth’s space environment.
In addition to the baseline recommendation of completing the ongoing science program, the first recommendation of the decadal survey was to implement the DRIVE initiative to “diversify” observing platforms in solar and space physics, “realize” the scientific potential of already existing assets, “integrate” observing platforms into successful investigations, “venture” forward with technology development, and “educate” the next generation. CubeSats diversify by providing stand-alone, unique measurements and measurements that increase the science-return of larger facilities; venture forward by driving technology development; and educate, as discussed in Chapters 1 and 3. The DRIVE recommendation from the decadal survey identified flight opportunities for very small satellites, including CubeSats, as a key growth area for both NASA and NSF.
The decadal survey also highlighted the importance of NASA’s Explorer program and proposed an expansion of this program, again recognizing the importance of diversified observing platforms in space physics. With regard to new strategic (larger) mission concepts, the review focused on an Interstellar Mapping and Acceleration Probe (IMAP) to follow up on the Interstellar Boundary Explorer (IBEX) and take advantage of the overlap with the historic Voyager missions; the DYNAMIC mission concept to focus on the variability of space weather driven by the lower atmosphere weather on Earth; and MEDICI, which focuses on the magnetosphere-ionosphere-thermosphere system and its coupling under heliospheric forcing. Both DYNAMIC and MEDICI concepts are multi-spacecraft missions of the scale that has been demonstrated previously with missions such as THEMIS, Van Allen Probes, and Magnetospheric Multiscale (MMS) (two to five spacecraft with masses ranging from approximately 100 to 1,000 kg).
The final recommendation for new mission concepts in the decadal survey was Geospace Dynamics Constellation (GDC), which has been a high-priority science recommendation in space physics for many years. GDC would be a constellation of at least six identical satellites in low Earth orbit providing simultaneous global observations of the coupled atmosphere-ionosphere-magnetosphere system. GDC would make measurements critical for understanding how the ionosphere-thermosphere system of Earth responds to driving from above by the solar wind and driving from below by the atmosphere. The notional GDC design presented in the decadal survey does not employ CubeSats. However, a constellation mission on the scale of GDC is expensive to achieve using the current mission paradigm and NASA Heliophysics projected funding profile. CubeSat-derived technology can benefit missions of this scale, as demonstrated by the CYGNSS Earth science mission, a constellation of eight small spacecraft (not CubeSats) measuring ocean surface winds associated with tropical cyclones (see Figure 4.4 below).
The decadal survey also described two future mission concepts that are not achievable within the next decade because significant technology development is required. The Magnetospheric Constellation (MagCon) mission, consisting of several tens of spacecraft measuring particles and fields, would provide a global view of how the magnetosphere stores and releases energy in the magnetotail and accelerates particles that supply the radiation
1 National Research Council (NRC), 2013, Solar and Space Physics: A Science for a Technological Society, The National Academies Press, Washington, D.C.
belts. It would also provide the first set of space weather buoys, making space weather measurements much like those at terrestrial weather stations (Figure 4.2). Magnetospheric Constellation and Tomography (MagCat) would provide global imaging of the magnetosphere for the first time. Although the decadal survey did not specify the size or geometry of spacecraft required to implement these concepts, missions such as MagCat and MagCon, which require tens to hundreds of spacecraft, will never be achievable without a new development approach.
The History and Current Role of CubeSats in Solar and Space Physics
CubeSats have shown their ability to produce high-priority science in space physics. For example, Radio Aurora Explorer (RAX)-2 worked with ground-based radar to measure ionospheric irregularities2 (Chapter 1, Figure 1.8); CSSWE has contributed to the understanding of radiation belt variability (Figure 4.3). A number of other missions are inflight and returning science data or in development. Thus far, most of these successes come from the NSF CubeSat program.
There are two general categories where CubeSats have the most potential to contribute to space physics research: targeted science investigations with either unique orbits or new instruments and constellation missions. Both CSSWE and RAX addressed targeted science questions that are part of the broad science goals outlined
2 H. Bahcivan, J.W. Cutler, J.C. Springmann, R. Doe, and M.J. Nicolls, 2014, Magnetic aspect sensitivity of high-latitude E region irregularities measured by the RAX-2 CubeSat, Journal of Geophysical Research: Space Physics 119(2):1233-1249.
in the decadal survey. These missions augmented larger missions (Van Allen Probes in the case of CSSWE) or ground-based instrumentation (PFISR in the case of RAX), increasing the return on investment for large facilities.
Large constellations of CubeSats have not yet flown, even though they appear to be a natural application of this disruptive technology to magnetospheric research. Traditional approaches for gathering multipoint data often have cost estimates well beyond typical budgetary constraints for missions. The Edison Demonstration of Smallsat Networks (EDSN) mission,3 consisting of eight CubeSats operating in a swarm configuration, was a first attempt toward demonstrating larger constellations and inter-spacecraft network operations. The spacecraft were successfully built but were lost when the launch vehicle failed in November 2015. There are several examples of successful missions employing two CubeSats. Both FIREBIRD4 and Aerocube-65 used closely separated spacecraft to study radiation belt losses. Aerocube-6 has successfully demonstrated the use of differential drag for controlling spacecraft separation in low Earth orbit. These and other CubeSat missions under development may be viewed as pathfinders to future space physics missions that will use many spacecraft to carry out critical multipoint measurements necessary for understanding the coupled Sun-Earth system. CubeSats, or the technology they enable, may be the most effective path toward large constellation missions. Future space physics constella-
3 J. Cockrell, R. Alena, D. Mayer, H. Sanchez, T. Luzod, B. Yost, and D.M. Klumpar, 2012, “EDSN: A Large Swarm of Advanced Yet Very Affordable, COTS-based NanoSats that Enable Multipoint Physics and Open Source Apps,” Proceedings of the 26th Annual AIAA/USU Conference on Small Satellites, Technical Session I: The Horizon, SSC12-I-5, http://digitalcommons.usu.edu/smallsat/2012/all2012/89/.
4 A.B. Crew, B.A. Larsen, D.M. Klumpar, E. Mosleh, H.E. Spence, J. Legere, J.B. Blake, L. Springer, M. Widholm, S. Driscoll, S. Longworth, et al., 2012, Focusing on size and energy dependence of electron microbursts from the Van Allen radiation belts, Space Weather 10(11):1-3.
5 J.B. Blake, and T.P. O’Brien, 2016, Observations of small-scale latitudinal structure in energetic electron precipitation, Journal of Geophysical Research: Space Physics, accepted. doi:10.1002/2015JA021815.
tions such as MagCon may require spacecraft of a different form factor or size than traditional CubeSats require, but CubeSat-derived technology can play an important role in their development.
In addition to technology development that will enable constellation missions, CubeSats have already been instrumental in demonstrating miniaturized sensors for use in space physics. For example, the energetic particle detector flown on CSSWE is a miniaturized version of the REPT instrument on Van Allen Probes.
The Future of CubeSats in Solar and Space Physics
In the future, better and more capable miniaturized instruments will continue to enable targeted science. Concepts are being developed that not only provide in situ measurements of energetic particles, plasmas, and fields but also key observations of the Sun. For example, a scaled-down HMI (Helioseismic and Magnetic Imager), which provides key insights into the evolution of the solar magnetic field, could be packaged to fit into a CubeSat, as could the Polarimetric and Helioseismic Imager (SO/PHI) developed for the European Space Agency’s (ESA’s) Solar Orbiter mission. Coronagraphs, ultraviolet (UV), and extreme ultraviolet (EUV) imagers could also be implemented as CubeSats. These imaging applications will require developments in high-rate communication and pointing capability.
CubeSats can have short development cycles that allow for rapid response to targeted opportunities—for example, augmenting larger missions such as CSSWE did for the Van Allen Probes. They can also be used in hazardous orbits not accessible to traditional large observatories—for example, probing the atmospheric boundary region and lower ionosphere (a few hundred kilometers in altitude) where orbital lifetimes are short.
As mentioned previously, constellations can enable transformational understanding of the dynamics and coupling of the Earth’s magnetosphere, ionosphere, and atmosphere. Constellations of tens to hundreds of spacecraft would provide significant advancements over missions like the MMS mission and the Van Allen Probes, allowing for separation of temporal and spatial variation on fast enough timescales to resolve physical mechanisms. A constellation of CubeSats or other small satellites could make fundamental discoveries about how the magnetosphere stores, processes, and releases energy in response to the solar wind, as illustrated by the MagCon concept.6 Another example is a constellation of magnetographs spaced throughout Earth’s orbit that could provide magnetic maps of the entire Sun, enhancing the forecast accuracy for solar outbursts.
In addition to constellations where multiple satellites are in different orbits, a swarm utilizes multiple satellites flying in formation near each other in similar orbits. Formation flying with a positional knowledge of a meter or less is now possible with CubeSats. Swarms could be used to study small-scale structure in the auroral acceleration region, for example. Both constellations and swarms would require propulsion for placing the satellites into their science orbits and for station keeping.
Summary: CubeSats in Solar and Space Physics
CubeSats and platforms taking advantage of CubeSat-derived technology have the potential to make a unique contribution to the field of solar and space physics (heliophysics) and have already proven their value in obtaining breakthrough science. The recommendations in the solar and space physics decadal survey emphasized diversification and integration of platforms of different sizes. It also emphasized that the coupled Sun-Earth system must be investigated using a system-science approach and highlighted the importance of multipoint measurements to accomplish this. Advances in pointing, high-rate communication, sensor technology, and propulsion will enable CubeSats and other small satellites to address a wide array of space physics science goals.
Conclusion: CubeSats have proven their ability to address the high-priority science goals outlined in the solar and space physics decadal survey and are specifically mentioned there. In the area of solar and space physics, CubeSats are particularly useful for achieving targeted science provided by novel measurements or augmentation of larger facilities and for enabling constellation and swarm missions.
6 NASA Science and Technology Definition Team, 2004, Global Dynamics of the Structured Magnetotail: Updated Synopsis of the Report of the NASA Science and Technology Definition Team for the Magnetospheric Constellation Mission, http://www.phy6.org/MagCON.pdf.
Science Priorities in Earth Science and Applications from Space—Decadal Survey Highlights
The first Earth science and applications from space decadal survey7 (hereafter ESAS 2007) was published nearly a decade ago, followed by a midterm review.8 The decadal survey was organized primarily around the following questions central to Earth system science:
- How is the global Earth system changing?
- What are the sources of change in the Earth system and their magnitude and trends?
- How will the Earth system change in the future?
- How can Earth system science improve mitigation of and adaptation to global change?
Many of the recommendations of the Earth science decadal survey present rather well-defined missions and investigations, many which have not yet flown, focusing on a range of science problems, including the Earth radiation budget, surface and ice sheet deformation, land surface composition and vegetation type and structure, atmospheric gas/aerosol composition, and ocean color. The decadal survey also recommended the creation of an innovative, principal-investigator (PI)-led line of missions, the Earth Venture Class.
The second decadal survey in Earth science and applications from space, sponsored by NASA, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Geological Survey (USGS), was initiated in fall 2015; a final report is anticipated in summer 2017 (ESAS 20179). The survey occurs against a backdrop of highly constrained federal and agency budgets. In addition, for NASA, the survey context includes a backlog of missions recommended by the 2007 survey report, increased responsibility—without commensurate budget increases—for sustained or “continuity” measurements, and growing demands for the information products derived from Earth observations. For NOAA, the survey context includes challenges in ensuring continuity of service from critical operational systems, especially the polar-orbiting spacecraft that feed numerical weather prediction models. USGS looks to ensure continuity of data from its Landsat series of spacecraft.
To meet current and future needs, the relevant agencies are challenged with the need to “accomplish more with less.” Thus, for example, language in the survey’s task statement asks the steering committee to make their recommendations to NASA cognizant of the likely emergence of new technologies; for NOAA and USGS, recommendations are to be made “with the expectation that the capabilities of non-traditional providers of Earth observations continue to increase in scope and quality.”10
The History and Current Role of CubeSats in Earth Science and Applications from Space
CubeSats launched to date have primarily focused on education, technology development, solar and space physics science (see “Solar and Space Physics” above), and commercial Earth imaging. Despite the fact that all of the CubeSats that have flown thus far have done so in low Earth orbit (LEO), the Earth science community has not yet exploited the potential that CubeSats have to offer either in terms of lower-cost, faster science return, or unique observations that are made feasible by the platform (e.g., constellation measurements).
NASA’s Earth Science Technology Office (ESTO) has used CubeSats for Earth science technology development with more than a dozen active CubeSat instrument subsystem development and technology flight maturation
7 NRC, 2007, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond, The National Academies Press, Washington, D.C.
8 NRC, 2012, Earth Science and Applications from Space: A Midterm Assessment of NASA’s Implementation of the Decadal Survey, The National Academies Press, Washington, D.C.
10 The statement of task of the 2017 Decadal Survey for Earth Science and Applications from Space is available at http://sites.nationalacademies.org/DEPS/esas2017/DEPS_169443, accessed April 29, 2016.
projects.11 CubeSat technology development has also been funded internally at individual NASA centers. At the same time, there has been significant commercial development of CubeSats for observations that have some overlap with Earth science objectives—namely, Earth surface imaging (e.g., Planet Labs, Skybox) and more recently atmospheric sounding (e.g., PlanetIQ, Spire). The applicability of commercial Earth observations for Earth science (e.g., purchase of science data as a service from commercial firms) depends on many factors, including duration of data record, data resolution, and quality of calibrations.
Current technology CubeSat projects funded by NASA fall into two categories. The first is the use of the platform to demonstrate technology destined for use on larger missions. The objective of the 3U CubeSat GRIFEX, for example, was to verify the spaceborne performance of a state-of-the-art readout integrated circuit/focal plane array that specifically targets the requirements of the Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission concept in ESAS 2007.12 Increasingly, however, a second category of technology CubeSats is being used to prove technology intended for use in science payloads on future targeted CubeSat science missions themselves or as constellation precursors. The Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) is a 6U mission currently developing a millimeter-wave radiometer for cloud precipitation that could fly as a part of the TEMPEST CubeSat constellation as an Earth Venture mission concept.13 Another example is the Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) 3U CubeSat that will demonstrate a payload that could be incorporated into a CubeSat or hosted payload constellation for measuring Earth’s radiation budget.14
In March 2016, while the present report was being written, NASA, through its Earth Venture program, selected the Time-Resolved Observations of Precipitation Structure and Storm Intensity with a Constellation of Smallsats (TROPICS) investigation, which will be a constellation of 12 3U CubeSats studying the development of tropical cyclones. This CubeSat constellation will be able to make rapid revisits, allowing its microwave radiometers to measure temperature, humidity, precipitation, and cloud properties as frequently as every 21 minutes.15
CubeSat-derived technology is also enabling missions using platforms larger than the 12U CubeSat size limit considered in this report. This is manifested in the Cyclone Global Navigation Satellite System (CYGNSS) Earth Venture mission (Figure 4.4).16 CYGNSS is complementary to traditional spacecraft that measure winds. By going from active to passive technology using GPS, CYGNSS requires significantly less power than does a spacecraft using the traditional technique. This results in smaller, less costly spacecraft that are CubeSat derived, are larger than 12U, and do not have standardized CubeSat dimensions, making a constellation feasible. The combined measurements from multiple spacecraft provide higher time resolution, returning data on timescales relevant for the development of storms.
Near-Term Future Science Opportunities for CubeSats in Earth Science and Applications from Space
Even though not yet proven in flight, it is likely that CubeSats and missions derived from CubeSat technology have the potential to address decadal survey Earth science goals through targeted investigations (e.g., CubeSats
13 S.C. Reising, T.C. Gaier, C.D. Kummerow, V. Chandrasekar, S.T. Brown, S. Padmanabhan, B.H. Lim, S.C. van den Heever, T.S. L’Ecuyer, C.S. Ruf, Z.S. Haddad, et al., 2015, Overview of Temporal Experiment for Storms and Tropical Systems (TEMPEST) CubeSat constellation mission, paper presented at IEEE MTT-S International Microwave Symposium, Phoenix, Arizona, http://ieeexplore.ieee.org/xpls/icp.jsp?arnumber=7167136&tag=1.
14 John Hopkins Applied Physics Laboratory, “Johns Hopkins APL Will Launch RAVAN to Help Solve an Earth Science Mystery,” release date December 10, 2013, http://www.jhuapl.edu/newscenter/pressreleases/2013/131210.asp.
15 NASA, “NASA Selects Instruments to Study Air Pollution, Tropical Cyclones,” release date March 10, 2016, http://www.nasa.gov/pressrelease/nasa-selects-instruments-to-study-air-pollution-tropical-cyclones.
16 University of Michigan, “Cyclone Global Navigation Satellite System Mission,” updated August 3, 2015, http://clasp-research.engin.umich.edu/missions/cygnss/.
are being developed to measure solar irradiance and Earth’s energy budget) and by enabling new kinds of Earth science observations.
Perhaps chief among these are multipoint or constellation-type Earth measurements, which provide much greater temporal coverage than that possible with single, large spacecraft. A single spacecraft in LEO provides high-spatial-resolution imaging, but poor temporal coverage; a single geostationary Earth orbit (GEO) spacecraft provides diurnal temporal coverage, but at the expense of spatial resolution. A LEO constellation comprising several or dozens of individual small spacecraft could provide both global spatial and high temporal resolution. The understanding of many Earth processes benefit from this kind of observation, including severe weather, cloud formation and evolutionary processes, aerosols or air quality related measurements, atmospheric photochemistry, vegetation,
ocean color, and Earth outgoing radiation. Constellations of lower-cost spacecraft also can provide for replenishment over time, allowing technology updates or replacement of failed spacecraft or instruments. To enable such missions, a number of technological advances are required. These include the need for high-rate communication and accurate pointing for high-resolution applications and propulsion needed for station keeping and establishment of constellations.
As an example, CubeSats could offer valuable data for weather and climate forecasting and projections. Global navigation satellite system-radio occultation (GNSS-RO) is a precise, cost-effective technique for measuring Earth’s atmosphere from space, leveraging existing global navigation satellite systems,17 and providing atmospheric measurements similar to those obtained from weather balloons, on a global scale. There are commercial efforts under way utilizing CubeSat technology with the potential of making GNSS-RO measurements analogous to the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission, a constellation of six 70-kg microsatellites that has been making atmospheric soundings of temperature, moisture, and pressure in the troposphere and stratosphere using GNSS-RO for nearly a decade.18 CubeSat-enabled constellations could deliver tens of thousands of occultations daily and make them available in near real time.
An area that is challenging for Earth science satellite observations is both absolute and relative calibration between data sets, especially if the data are used for long-term data analysis for climate-related questions. Some Earth science problems require determination of trends at the percent level per decade. It is not clear whether this can be done inexpensively, but comparative measurements with enough lower-accuracy sensors may be possible.
Mitigation of Data Gaps with CubeSats
An application of CubeSats in Earth science is the potential to make observations that mitigate the gaps in operational data from critical satellite systems, in part because of their lower cost and shorter development cycles.
Gaps in Weather Forecasting Data
Perhaps the most significant gap in satellite data is in observations critical for routine weather forecasting and climate monitoring, the prediction of extreme weather, military operations, and the emergency response to wildfires and other natural disasters. There will likely be a gap in data of 1 to 4 years between the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Preparatory Project (NPP) spacecraft and the next-generation Joint Polar Satellite System (JPSS). There are also potential gaps in Department of Defense (DOD) and European polar satellite programs that provide data for NOAA forecasts. Further, NOAA’s Geostationary Operational Environmental Satellite (GOES) program faces a period of more than 1 year without a backup satellite on orbit.
A number of commercial efforts are developing CubeSat or small satellite constellations seeking to address these gaps using the GNSS-RO sounding technique pioneered by the COSMIC mission. For example, Spire, GeoOptics, and PlanetIQ are developing CubeSat-based GNSS-RO constellations.19
Landsat 8 has been operating since 2013, and Landsat 9 is not planned for launch until 2021. To help mitigate a possible loss of the thermal infrared sensor on L8 before L9 flies, the Class D Thermal Infrared Free Flyer (TIR-FF) was considered as a possibility for launch in 2019. TIR-FF was part of the President’s fiscal year 2016 budget request for USGS, but it was not funded by Congress. TIR-FF would have been a low-cost thermal infrared (TIR) free-flying small satellite—not a CubeSat—possibly larger than 12U, designed to ensure data continuity by flying in formation with L8 and would have carried a microbolometer TIR sensor and cloud camera. If suc-
17 NRC, 1995, The Global Positioning System: A Shared National Asset, National Academy Press, Washington, D.C.
18 C.-J. Fong, D. Whiteley, E. Yang, K. Cook, V. Chu, B. Schreiner, D. Ector, P. Wilczynski, T.-Y. Liu, and N. Yen, 2011, Space and ground segment performance of the FORMOSAT-3/COSMIC mission: Four years in orbit, Atmospheric Measurement Techniques Discussions 4:599-638; University Corporation for Atmospheric Research, “COSMIC Program Office,” updated April 19, 2016, http://www.cosmic.ucar.edu/.
19 E. Hand, 2012, Microsatellites aim to fill weather-data gap, Nature 491(7426):650-651.
cessful at providing data with sufficient quality, TIR-FF would have had the potential to fill this gap and lay the groundwork for filling future gaps in imaging data using a targeted small spacecraft mission.
The solar irradiance reaching Earth is an essential climate variable. The total solar irradiance (TSI) has been measured continuously since the late 1970s. The SORCE mission is near the end of its mission life, and with the loss of the Total Irradiance Monitor (TIM) on the Glory mission and the descope of the Total and Spectral Solar Irradiance Sensor (TSIS) from JPSS-1, the Total Solar Irradiance Calibration Transfer Experiment (TCTE) mission is bridging the gap for TSI. The TCTE mission is expected to end operations after an overlap period with another TSIS, to be installed on the International Space Station (ISS) in 2017. Continuity of the solar irradiance record, however, is precarious. Miniaturized total and spectral irradiance sensors are under development, suitable for flight on CubeSats, and plans exist to use small spacecraft to enable a robust, overlapping solar irradiance record into the future at a lower average operating cost than traditional measurements.
Summary: CubeSats in Earth Science and Applications from Space
Most CubeSats in LEO today are commercial satellites that focus on remote sensing, imagery, and Earth observation. The majority of NASA-funded CubeSats in Earth science to date are technology demonstrations. The Earth science community is just starting to exploit CubeSats and CubeSat-derived small satellites as a platform for doing science—for example, with the TROPICS and CYGNSS missions. CubeSats have the potential to provide high-temporal-resolution measurements through constellations and to mitigate data gaps. To realize this potential, technology developments in sensors and instruments—in particular in their calibration, high-rate communications, and propulsion to set up and maintain constellations—are needed.
Conclusion: CubeSats provide technology demonstration for Earth science missions, but the Earth science community is just starting to exploit CubeSats and CubeSat-derived small satellites for science. CubeSats hold promise for Earth science in several ways:
- CubeSats and CubeSat-associated technologies can enable targeted science and can augment existing capabilities by providing particular Earth science measurements that take advantage of the CubeSat platform.
- CubeSats and CubeSat-associated technologies can provide unique science opportunities as constellations or swarms by providing distributed, multipoint measurements for high-temporal-resolution, global-scale measurements.
- CubeSats or related systems have the potential to mitigate data gaps (e.g., JPSS, Landsat, TSIS) and provide sustained measurements (including monitoring).
- CubeSats are potentially more responsive to observation needs and in the employment of new technologies, owing to their shorter development cycles.
Science Priorities in Planetary Science—Decadal Survey Highlights
The 2011 planetary science decadal survey, Vision and Voyages for Planetary Science in the Decade 2013-2022,20 revolved around the following three major themes:
- Building new worlds—understanding solar system origins,
- Planetary habitats—searching for the requirements for life, and
- Workings of solar system—revealing planetary processes through time.
20 NRC, 2011, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C.
Many of the objectives require close proximity observations with sophisticated instruments for elemental and mineralogy measurements. Some observations pertaining to origin science and habitability may be obtained only from in situ investigations and sampling. Some of these observations and targets are also of interest to human exploration, planetary defense, and reconnaissance for in situ resource utilization.
The high-priority flagship missions recommended by the decadal survey include the first mission in a Mars sample-return campaign, a mission to Europa addressing the planetary habitat goals, and a Uranus orbiter and probe. The decadal survey’s highest recommendations included increasing the research and analysis (R&A) program and the establishment of a technology development program, as well as strengthening the Discovery program, which has produced significant and cost-effective science return operating in a PI mode. Priorities for the next PI-led New Frontiers missions include a comet surface sample return, a Lunar South Pole-Aitken Basin sample return mission, as well as three other missions.
The History and Current Role of Deep Space CubeSats for Planetary Science
There are currently no deep space CubeSats in flight launched by NASA or NSF, hence no track record of success, no heritage hardware, and no lessons learned. However, there are eight planetary CubeSat missions and three non-flight planetary technology systems under development by NASA. Classification based on the committee’s databases, referenced in Chapter 1, are as follows: two technology and instrument demonstration missions (INSPIRE, MarCO) at and beyond lunar orbit; three science (LunaH-Map, Q-PACE, Lunar Flashlight) and two technology missions (Lunar IceCube, Skyfire) at or near the Moon; and one exploration science mission at a Near Earth Asteroid (NEA) Scout—all to be launched in the 2016-2018 time frame. The three non-flight planetary technology demonstrations are DAVID, HALO, and MMO.
Significant progress in electronics miniaturization in the past decade has led to increased interest for small platforms, including CubeSats. JAXA’s Hayabusa mission carried the Minerva hopper (~1U in size, although with a different geometrical shape) for in situ imaging of an NEA. Despite a failed deployment, Minerva demonstrated its functionality by transmitting images of the mothership.21 In 2005, the CubeSat form factor was introduced into planetary exploration concepts with the Planetary Hitchhiker,22 which would use an Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter (ESPA) as the main vehicle and telecom relay, dropping CubeSats at multiple asteroids (this concept was not funded to move forward). Since then, a large number of CubeSat concepts have been proposed and presented at conferences (e.g., Interplanetary SmallSat Conference, iCubeSat, LunarCubes), based on the premise that enabling technologies and launch opportunities will soon become available. Interest for developing efficient, long-range propulsion systems that could fit within the CubeSat form factor began during the same period with the demonstration of solar sail deployment from the 3U NanoSail-D2 CubeSat in 2008.
Interest in deep space exploration concepts using independent CubeSats expanded in 2011 after Staehle et al. published results from a NASA Innovative Advanced Concepts (NIAC) technology gap assessment study.23 This was followed by the NASA/JPL INSPIRE24 technology demonstrator (3U), which is still awaiting launch. The prospect of obtaining space-qualified CubeSat transponder, propulsion, and attitude control then led to the selection in 2013 of three CubeSat missions to be launched with the Space Launch System’s Exploration Mission 1 (EM-1) in 2018, which was focused on a range of science topics, including asteroid multiscale imaging, detection of lunar water, and the effect of deep space radiation on plants.
21 T. Yoshimitsu, T. Kubota, and I. Nakatani, 2006, “MINERVA rover which became a small artificial solar satellite,” Proceedings of the 20th Annual AIAA/USU Conference on Small Satellites, Session IV: The Past & Coming Years, SSC06-IV-4, http://digitalcommons.usu.edu/smallsat/2006/All2006/27/.
22 I. Garrick-Bethell, R.P. Lin, H. Sanchez, B.A. Jaroux, M. Bester, P. Brown, D. Cosgrove, M.K. Dougherty, J.S. Halekas, D. Hemingway, P.C. Lozano, et al., 2013, Lunar magnetic field measurements with a CubeSat, Proceedings of SPIE 8739.
23 R.L. Staehle, D. Blaney, H. Hemmati, D. Jones, A. Klesh, P. Liewer, J. Lazio, M. Wen-Yu Lo, P. Mouroulis, N. Murphy, P.J. Pingree, et al., 2013, Interplanetary CubeSats: Opening the solar system to a broad community at lower cost, Journal of Small Satellites 2(1):161-186.
24 A. Klesh, J. Baker, J. Castillo-Rogez, L. Halatek, N. Murphy, C. Raymond, B. Sherwood, J. Bellardo, J. Cutler, and G. Lightsey, 2013, INSPIRE: Interplanetary Nano-Spacecraft Pathfinder in Relevant Environment, Proceedings of the 27th AIAA/USU Conference on Small Satellites, Technical Session XI: Around the Corner, SSC13-XI-8, http://digitalcommons.usu.edu/smallsat/2013/all2013/127/.
In 2015, NASA’s Planetary Science Division introduced the SIMPLEx (Small, Innovative Missions for Planetary Exploration) program whose selections in September 2015 included LunaH-Map (Figure 4.5) and Q-PACE (see Figure 4.7 in the “Astronomy and Astrophysics” section), a low-gravity laboratory in Earth orbit. Other missions in development include the Mars CubeSat One (MarCO) (Figure 1.9), a technology demonstration.
CubeSats have also been proposed as secondary payloads on bigger spacecraft to increase the science return of the whole mission by acquiring complementary observations. In October 2014, ESA selected five CubeSat concepts for further development to accompany the AIDA mission (Asteroid Impact and Deflection Assessment) currently in Phase A.25
Near-Term Future Science Opportunities for CubeSats in Planetary Science
CubeSats and platforms taking advantage of CubeSat technology have the potential to make unique contributions to planetary science by creating unique vantage points or multipoint measurements (e.g., in situ package(s) complementary to an orbiter); exploring high-risk or uncharted regions; and serving as low-gravity laboratories. As an example of exploring uncharted regions, NASA’s Planetary Science Division selected for study 10 university-led concepts that would complement the Europa Clipper mission. CubeSats in LEO contribute to planetary exploration by providing natural low-gravity laboratories (see the section “Other U.S. Government Programs”), as well as observation platforms for astronomical observations of planetary bodies. Possible applications include the long-term monitoring of planetary atmospheres (Jupiter, Mars) and the tracking of meteors as they break up upon entering Earth’s atmosphere.
25 European Space Agency, “CubeSat Companions for ESA’s Asteroid Mission,” release date November 2, 2015, http://www.esa.int/Our_Activities/Space_Engineering_Technology/Asteroid_Impact_Mission/CubeSat_companions_for_ESA_s_asteroid_mission.
CubeSats can also be used as platforms for technology demonstration to enable future large missions. Two of the Discovery-13 proposals selected for Phase A study (September 2014) included CubeSats. One would obtain field measurements at an asteroid in coordination with its mothership while the other would obtain noble gas measurements in Venus’s atmosphere, a high-priority objective of the 2011 decadal survey. Those technology demonstration options, if funded, would include the development of a deep space deployer capable of sustaining long cruise time and equipped with its own telecommunication and computer to limit the impact of the CubeSat on the mothership. CubeSat-based constellation networks have also been suggested as part of the telecom infrastructure to support the human exploration of the martian system,26 which the MarCO mission plans to demonstrate.
Unique Challenges for Planetary Science CubeSats
Deep space CubeSat missions can have lower risk tolerance, and thus higher cost, than traditional CubeSats (although they may still be cheaper than the traditional alternative for planetary science) due to the constrained launch date and single launch opportunity typical of planetary missions. Therefore, the fly-learn-refly paradigm also does not apply. Additionally, planetary protection requirements may apply, depending on the destination. While these may be more easily implementable on CubeSats because of their small size, there is a perceived risk of contamination of a mothership. Other perceived risks, such as post-deployment impact with a mothership or pressurized containers, may pose a barrier to the use of CubeSats on future deep space missions.
The traditional CubeSat form factor is too restrictive for some planetary applications due to instrument or aperture size, thermal control issues, and radiation environment. For example, it can be difficult to maintain low temperatures for focal plane arrays and to maintain the thermal stability of optical systems in the presence of tightly stacked electronics and frequent and long radio-communication passes needed for long-distance communication. Free-flying planetary CubeSats can suffer from stringent limitations such as telecommunication back to Earth, implying the need for a larger antenna or supporting communication infrastructure. Existing and upcoming propulsion systems can provide the change in velocity required to reach a variety of targets, but the very low thrust implies flight times beyond the expected lifetime of CubeSat parts. Extra spacecraft volume may be required for larger propulsion systems.
Summary: CubeSats in Planetary Science
Even though there are no active planetary CubeSats or published science results from CubeSats in planetary science, there is demonstrated interest by the planetary science community, and multiple CubeSats are currently under development. However, the traditional CubeSat form factor is often not viable for planetary science due to telecommunications, propulsion, thermal, and other constraints.
Conclusion: CubeSats in planetary science have potential in three areas: creating unique vantage points or multipoint measurements, exploring high-risk or uncharted regions, and serving as low-gravity laboratories. However, they can have unique challenges: the traditional form factor may not be appropriate, and there may be lower risk tolerance due to the nature of single mission opportunities and potential risk to a mothership.
Science Priorities in Astronomy and Astrophysics—Decadal Survey Highlights
The science goals for 2012-2021 put forward in the astronomy and astrophysics decadal survey report New Worlds, New Horizons in Astronomy and Astrophysics,27 called “Astro2010,” are the following:
26 A. Babuscia, K-M. Cheung, and C. Lee, Jet Propulsion Laboratory, “Augmenting and Evolving the Mars Relay Network Using a Constellation of Identical CubeSats,” presentation to the Mars CubeSat/NanoSat Workshop, November 20, 2014.
27 NRC, 2010, New Worlds, New Horizons in Astronomy and Astrophysics, National Academy Press, Washington, D.C.
- Cosmic dawn: searching for the first stars, galaxies, and black holes,
- New worlds: seeking nearby habitable planets, and
- Physics of the universe: advancing understanding of the fundamental physics of the universe.
The program envisioned to accomplish these goals primarily involved large observatories in space, such as the Wide-Field InfraRed Survey Telescope (WFIRST), the gravitational wave observatory LISA, and the X-ray observatory IXO. Large observatories such as these recommended missions and the James Webb Space Telescope (JWST)—the top priority from the 2001 decadal survey28—are required for many applications due to the natural faintness of observation targets that can only be detected with sufficient collecting power. Also included in Astro2010 was a major augmentation to the program of Explorer missions—small (i.e., hundreds of kilograms), cost-capped, PI-led spacecraft that are competitively selected. Smaller efforts in space include technology development for finding and characterizing nearby terrestrial exoplanets.
The History and Current Role of CubeSats in Astronomy and Astrophysics
Due to the large distances of the objects studied in astronomy and astrophysics, telescopic observations are needed. Space-based observations provide the ability to observe in wavelengths absorbed by the atmosphere or ionosphere (e.g., ultraviolet, X ray, <50 MHz radio, and parts of the infrared), observe without interruptions due to daylight and clouds, have very stable sensitivity and image quality, and observe without the distorting effect of atmospheric turbulence. Like any spacecraft, CubeSats can potentially provide the access to space required for these kinds of measurements, potentially at relatively low cost.
Astro2010 recognized that significant contributions have been made over the past decade by Explorer missions such as WMAP, Swift, WISE, GALEX, and NuSTAR, which utilized small (hundreds of kilograms) spacecraft. However, there is little mention of much smaller systems, such as CubeSats or nanosatellites, in the decadal survey. Furthermore, the astrophysics community’s interest in the use of CubeSats has continued to lag behind that of other NASA science divisions because there are unique challenges in astrophysics. Many applications require large apertures for high angular resolution and sensitivity, precise attitude control, and large data rates. Nevertheless, the interest in the use of CubeSats is growing, and there are a number of mission concepts that address Astro2010 science goals.
The aperture of an instrument hosted by a CubeSat typically is constrained by the CubeSat’s small size. However, CubeSats can provide dedicated observations of specific targets, thus somewhat compensating for the limited aperture size. Only a limited number of astronomy CubeSats have been selected to date, and none have resulted in published scientific results.
The Cosmic X-ray Background Nanosatellite-1 (CXBN-1) is a 2U CubeSat that was launched by the NASA CubeSat Launch Initiative’s ELaNa program in 2012 to make precise measurements of cosmic (diffuse) X-ray (20-50 keV) background. The mission did not meet its science objectives due to a telemetry problem. However, a follow-on mission (CXBN-2) was selected for a 2016 launch.
Other missions make use the of the “stop and stare” ability of CubeSats, where each CubeSat in an array can be dedicated to observing a single object for long periods of time. The BRITE constellation (Figure 4.6) used this technique to study stellar variability. ExoPlanetSat29 is a mission under development employing a suite of CubeSats to make ultra-stable brightness observations of Sun-like stars to search for transiting exoplanets.
28 NRC, 2001, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C.
29 M.W. Smith, S. Seager, C.M. Pong, J.S. Villaseñor, G.R. Ricker, D.W. Miller, M.E. Knapp, G.T. Farmer, and R. Jensen-Clem, 2010, ExoplanetSat: Detecting transiting exoplanets using a low-cost CubeSat platform, Proceedings of SPIE: the International Society for Optical Engineering 7731:773127, http://hdl.handle.net/1721.1/61644.
Near-Term Future Science Opportunities for CubeSats in Astronomy and Astrophysics
Addressing many of the science goals set out in the decadal survey requires arcsecond or better pointing to enable locking onto a astrophysical object or to allow integration of data from multiple exposures. The best CubeSat pointing capability currently claimed is 11 arcseconds,30 although it has not yet been demonstrated in flight.31 The 10 cm optics that fit in a 1U form factor give a 1 arcsecond FWHM diffraction limit at visible wavelengths, thus requiring arcsecond or better pointing for observations in the visible part of the spectrum. Several technology demonstration missions are under way to improve CubeSat pointing capabilities. The Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA) is a JPL Phaeton early-career project to achieve arcsecond-level line of sight pointing error and highly stable focal plane temperature control. If successful, missions such as this will enable the high-precision photometry required for transiting exoplanet detection, for example.
Deployables offer a potential workaround for the size constraints of a CubeSat. A structure such as a solar panel or antenna is folded up and housed in the CubeSat for launch and then unfolded into a larger structure after orbit insertion. A deployable petal telescope is currently under development to enable larger apertures.
Swarms or constellations will enable interferometric applications, another workaround for the size-constraint of a CubeSat. An interferometer at very low frequencies (<30 MHz) is currently being developed as a Chinese-European collaboration. The DSL mission, consisting of 10-50 CubeSats in lunar orbit, could provide images at frequencies not accessible from the ground and at significantly higher spatial resolution than ever achieved before at these frequencies. This mission would conduct a full sky survey to study transients, map diffuse galactic emission, possibly probe signals from the early universe, and address a number of other science goals. Precision guidance, navigation, and control—or at least knowledge of the relevant quantities—are critical for enabling interferometers at shorter wavelengths. In principle, interferometers could provide spacecraft separations larger than Earth, allowing for higher spatial resolution than is currently available from Earth-based Very Long Baseline Interferometry (VLBI).
Finally, CubeSats are being used to demonstrate enabling technologies that may feed into the development of the large missions of the future. The AAReST (Autonomous Assembly of a Reconfigurable Space Telescope) mission is a proof of concept mission to demonstrate the autonomous assembly of a large primary mirror using small independent spacecraft, each with a single mirror.32
Summary: CubeSats in Astronomy and Astrophysics
The small aperture size and pointing accuracy currently available with CubeSats has so far limited the use of the platform for astronomy and astrophysics. However, CubeSats can currently achieve niche mission objectives. They can be used as a dedicated spacecraft to stare at single bright targets for long periods of time, making them ideal for studying sources that vary on a variety of timescales. CubeSat constellations create opportunities for interferometry and other multi-aperture applications. Arcsecond pointing capability is needed to achieve many science objectives such as exoplanet detection.
Conclusion: Although many astronomy and astrophysics science goals require larger mission platforms than CubeSats, some science opportunities can be enhanced by the use of CubeSats. These include but are not limited to the following:
- Observations of variable sources including variable stars and transiting planets—a CubeSat can stare for long time periods at targets of interest, for example;
- Interferometry—CubeSats can form swarms and arrays that create new opportunities for multi-aperture observations; and
30 The current Blue Canyon XACT appears to deliver 11 arcsecond 1-sigma 1-axis, which gives about 25 arcsecond FWHM.
31 The first on-orbit test of the BCT XACT Attitude Determination and Control System Technology will be on the MinXSS mission that launched to the ISS in December, 2015.
32 K. Fesenmaier, 2013, Using space wisely, Engineering and Science 76(4):14-19, http://www.its.caltech.edu/~sslab/2013_Winter_Using_Space_Wisely.pdf.
- Technology de-risking—CubeSats can be platforms for new technology development and testing of sensors and system methodologies that will enable larger missions.
Relevant Science Priorities in Biological and Physical Sciences in Space from the Decadal Survey
This section addresses other areas where CubeSats have proved or have the potential to advance scientific knowledge, encompassing biology research in natural gravity and radiation environments and microgravity research for material physics. A comprehensive list of science objectives are presented in the 2011 biological and physical sciences in space decadal survey report Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era.33 Some example objectives potentially relevant to CubeSats include the following:
- Plant and microbial growth and response to the space environment,
- Study of complex fluids and soft matter in the microgravity laboratory, and
- Advanced materials design and development for exploration.
Biology research is of particular interest to NASA because it helps characterize the impact of space environment on a human crew while characterizing the potential limits of terrestrial biology to move beyond Earth. Information of interest includes monitoring the metabolic activity and genome alteration of plants and possibly also microbes and small animals. This information has intrinsic value to terrestrial biology in extraterrestrial environments and can also be extrapolated to predict the impact of long-term exposure to deep space on the human body. It is also of interest to astrobiology in order to assess the potential for life to survive beyond Earth.
Spaceborne in situ laboratories also provide novel ways to approach fundamental and material physics questions. Indeed, it is very difficult to simulate microgravity on Earth over extended periods of time and expensive to develop and run that type of experiment on the ISS. Moreover, there are advantages in that CubeSats can more readily achieve longer periods of microgravity than on an operational platform such as the ISS. The need for microgravity research is also explicitly called out in the planetary science decadal survey.34
The History and Status of CubeSats in Biological and Physical Sciences in Space
Biology research in space motivated the early development of fully automated CubeSats, starting with GeneSat in 2006, the first science CubeSat. GeneSat provided a life support environment and nutrients to bacteria and tracked the production of proteins as a result of genetic activity. In 2007, NASA released a call for small payloads for astrobiology research that received a large number of concepts and resulted in the selection of the O/OREOS mission (Organism/Organic Exposure to Orbital Stresses) (Figure 1.8). More recently, NASA’s Advanced Exploration Systems selected the BioSentinel 6U mission, led by NASA’s Ames Research Center, to pursue radiation studies in an Earth trailing heliocentric orbit.
Two recent examples illustrate the value and potential offered by small and automated laboratories in space. O/OREOS monitored the effects of space exposure on organic molecules and biological organisms, advancing the state of the art by the first real-time analysis of the dynamic response of organics and biomarkers to direct solar irradiation for long exposures under simulated and controlled environments.35 Another goal of the O/OREOS mis-
33 NRC, 2011, Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era, The National Academies Press, Washington, D.C.
34 NRC, 2011, Vision and Voyages.
35 A. Mattioda, A. Cook, P. Ehrenfreund, R. Quinn, A.J. Ricco, D. Squires, N. Bramall, K. Bryson, J. Chittenden, G. Minelli, E. Agasid, et al., 2012, The O/OREOS mission: First science data from the space environment viability of organics (SEVO) payload, Astrobiology 12(9):841-853; A.M. Cook, A.L. Mattioda, A.J. Ricco, R.C. Quinna, A. Elsasser, P. Ehrenfreund, A. Ricca, N.C. Jones, and S.V. Hoffmann, 2014, The Organism/Organic Exposure to Orbital Stresses (O/OREOS) satellite: Radiation exposure in low Earth orbit and supporting laboratory studies of iron tetraphenylporphyrin chloride, Astrobiology 14(2):87-101.
sion was to demonstrate the capability for CubeSats to autonomously perform in situ biological experiments in a relevant environment at low cost. Q-PACE (Figure 4.7), currently under development and selected for launch under the CSLI program in fall 2016, uses the microgravity environment to study the mechanics of early planetesimal development and accretion, important for understanding solar system formation. A similar mission, called AOSAT36 (Asteroid Origins Satellite), is also under development, internally funded by the Arizona State University, selected in 2015 for launch as part of the ELaNa program.
For many applications, a micro-laboratory lends itself to the advantages provided by the CubeSat standard. Performance requirements (e.g., mass, power, pointing) are generally compatible with CubeSat resources. Data downlink is manageable with a UHF or S-band patch antenna, although the small data rates limit the return of high-quality science images (e.g., uncompressed). The observing sensors themselves have benefited from miniaturization advances over the past decade and the introduction of new technologies (e.g., 3-D printing, micro-eletromechanical systems), although the absence of off-the-shelf components in most cases require new technological and engineering solutions that may be resource intensive.
36 J. Thangavelautham, A. Thoesen, F. Gadau, G. Hutchins, E. Asphaug, and I. Alizadeh, 2014, Low-cost science laboratory in microgravity using a CubeSat centrifuge framework, Proceedings of 65th International Astronautical Congress, http://space.asu.edu/IAC-2014-cubesat_centrifuge_laboratoryc.pdf.
Near-Term Future Science Opportunities and Challenges for CubeSats in Biological and Physical Sciences in Space
The missions discussed in the previous subsection have been developed for LEO, which simplifies their design. Building on the success of the GeneSat, PharmaSat, and O/OREOS missions, NASA’s Ames Research Center is now developing the Bio-Sentinel mission, which is scheduled to launch in 2018 with the first launch of the Space Launch System on EM-1. This 6U CubeSat includes a 4U science payload that will track the degradation and repair of yeast DNA as a result of radiation at the lunar orbit. This mission takes advantage of the space offered by a 6U CubeSat to increase the number, quality, and control of the samples and monitor radiation with multiple sensors.37 Experience gained from the development of CubeSat-based laboratories was also leveraged to develop automated experiments that run on ISS NanoRacks without the need for crew monitoring.38
The implementation of CubeSat-based autonomous in situ laboratories comes with a number of challenges. The small resources available to the science experiment limit the extent of testing and monitoring. The small CubeSat form factor may introduce boundary effects in material physics experiments, which decreases the effective volume usable for science observations. Low downlink rates may prevent the use of high-resolution imaging that would be complementary to analytical measurement techniques, unless resources are traded between payload and telecommunications. Automated experimental protocols are complex to implement and in many cases, require the introduction of new technologies or engineering solutions that cannot be attained with commercial off-the-shelf components.39
Another major limitation that applies to both biological and, to lesser extent, material physics study is the long lead time required for flying on certain rockets (e.g., EM-1), which would require that the specimens be left under limited temperature control for periods of months and may also lead to contamination of the samples. This hinders biology investigations using complex organisms (e.g., small animals, most mammalian cultures, multicellular microorganisms). The long lead time may also impact the stability of reagents or drugs used as part of the experiment. This aspect may be addressed with new approaches to thermal engineering and packaging, which in turn adds to development complexity.
The scale of the CubeSat-based laboratory (~1U) is a constraint to the type of processes that can be simulated and may pose boundary issues. Still, they offer a novel avenue for studying processes that would remain poorly understood otherwise, due to the lack of access to low-gravity environments.
Summary: CubeSats in Biological and Physical Sciences in Space
CubeSats offer a platform for investigating processes in environments that cannot be reproduced on Earth or under constrained conditions. In particular, CubeSats can provide access to microgravity and relevant radiation environments for extended periods of time.
Conclusion: CubeSats have already performed science in microgravity and biological sciences and continue to offer opportunities for future investigations. However, the use of CubeSats as microgravity laboratories for live specimens is limited by size constraints and the difficulty of maintaining life support during satellite integration and launch delays.
CubeSats play a different role in each science discipline, and therefore, there are a wide range of CubeSat mission concepts in terms of complexity and scale. The science potential of CubeSats has already been demonstrated
37 W. Nicholson, University of Florida, and T. Ricco, NASA Ames Research Center, “Biological Science in Space: Role of CubeSats (aka Nanosatellites),” presentation to the committee, October 28, 2015.
38 NASA, “NanoRacks-Ames Fruit-Fly Experiment (NanoRacks-AFEX),” release date September 24, 2015, http://www.nasa.gov/mission_pages/station/research/experiments/1360.html.
39 W. Nicholson, University of Florida, and T. Ricco, NASA Ames Research Center, “Biological Science in Space: Role of CubeSats (aka Nanosatellites),” presentation to the committee, October 28, 2015.
in the field of solar and space physics where CubeSats have delivered high-impact results and have augmented larger facilities. In Earth science, CubeSats and CubeSat-enabled technologies have so far been underutilized for science. However, the CYGNSS Venture-class mission heavily relies on CubeSat technology. A number of missions are under development in planetary science but have not yet flown. These missions can be higher cost and have lower risk tolerance than traditional CubeSats do (although they may still be cheaper than the traditional alternative for planetary science), and the fly-learn-refly paradigm generally does not apply. Most astronomy- and astrophysics-themed CubeSat concepts are still notional, with many requiring significant advances in pointing and other technologies to be scientifically useful. In biological and physical sciences in space, CubeSats complement research on the ISS and are the only viable alternative to the ISS as well, with the potential to provide access to deep space environments.
Technology development continues to play an important role in promoting the use of CubeSats. In particular, sensor development, optical, or other methods for high-rate communication, arcsecond pointing, and propulsion will enable future capabilities and science applications.
The set of science goals where the use of CubeSats would be enabling is evolving too quickly to comprehensively list, and, per the statement of task in Appendix A, this committee has not been tasked with prioritizing CubeSat missions. However, the following provides a sampling of high-priority science goals that could potentially be pursued using the CubeSats:
- Solar and space physics, Earth science and applications from space—Exploration of Earth’s atmospheric boundary region. CubeSats are uniquely suited because of their expendability to explore the scientific processes that shape the upper atmospheric boundary using short-lifetime, low-altitude orbits.
- Solar and space physics—Measurement of plasma processes in the magnetosphere-ionosphere system. A 10-100 satellite constellation of CubeSats carrying magnetometers and plasma instrumentation can provide detailed information about the spatial and temporal evolution of magnetospheric plasmas.
- Earth science and applications from space—Multipoint, high temporal resolution of Earth processes. Satellite constellations in low Earth orbit could provide both global and diurnal observations of Earth processes that vary throughout the day, such as severe storms, and are currently under-sampled by Sun-synchronous observatories.
- Earth science and applications from space—Mitigation of data gaps and continuous monitoring. Anticipated and potential gaps (caused by launch or instrument failures and budget constraints) in weather satellite data, land surface imaging, and solar irradiance measurement may have the potential to be mitigated by observations from small spacecraft enabled by CubeSat technology.
- Planetary science—Measuring the distribution of lunar water. CubeSat concepts could map the distribution of water on the Moon with a variety of complementary techniques, such as neutron spectroscopy and infrared spectroscopy.
- Planetary science—In situ investigation of the physical and chemical properties of planetary surfaces or atmospheres. Deployable (daughter-ship) CubeSats could expand the scope of the motherships with complementary science or site exploration.
- Planetary science—Measurements of planetary magnetospheres. Constellation of CubeSats could provide simultaneous fields and particle measurements at multiple sites in planetary magnetospheres. Such measurements in the vicinity of large icy satellites could help determine the magnetic field induced in deep oceans.
- Astronomy and astrophysics—Search for extrasolar planets. A CubeSat could “stop and stare” for a long time at one bright Sun-like star to search for transiting exoplanets.
- Astronomy and astrophysics, solar and space physics—Low-frequency radio science. Interferometers made of CubeSats could explore the local space environment and also galactic and extragalactic sources with spatial resolution in ways not accessible from Earth.
- Biological and physical sciences in space—Investigate the survival and adaptation of organisms to space. CubeSats offer a platform to understand the effects of the environment encountered in deep space, such as microgravity and high levels of radiation.
The committee made the following conclusions related to science:
Conclusion: CubeSats have already produced high-value science as demonstrated by peer-reviewed publications that address decadal survey science goals. CubeSats are useful as instruments of targeted investigations to augment the capabilities of large missions and ground-based facilities, they are enabling new kinds of measurements, and they have the potential to mitigate gaps in measurements where continuity is critical.
Conclusion: Although all science disciplines can benefit from innovative CubeSat missions, CubeSats cannot address all science objectives and are not a low-cost substitute for all platforms. Some activities, such as those needing large apertures, high-power instruments, or very-high-precision pointing, most likely will always require larger platforms because of fundamental and practical constraints of small spacecraft.
Conclusion: Constellations of 10 to 100 satellites can provide transformational science, particularly in solar and space physics and Earth science where high-cadence or multipoint measurements are essential for studying highly coupled systems. Constellations or swarms may also provide important science capabilities in astronomy. CubeSats provide a realistic and possible path toward such constellation missions.