Many NASA missions over previous decades have operated into extended phases and produced significant scientific discoveries. Scientific research is often conducted using extensive data sets collected in both prime and extended mission phases. In the Earth science, heliophysics, and planetary science fields, it is often important to collect data over long periods of time to detect long-term trends; thus a discovery may be made long into extended phase that was only possible after the collection of a lengthy data set. There are also completely new discoveries, either from rare events, new observations of specific features, or new mission destinations or observing modes. Major results have been realized while missions were in extended phase.
This chapter highlights some of the discoveries made in extended mission phase, but certainly is not comprehensive. What this short overview demonstrates, however, is that all of the science disciplines in NASA’s Science Mission Directorate (SMD) have experienced major benefits from the extended phase operations of spaceflight missions. This leads to the first major finding of this report.
Finding: NASA’s extended science missions have made major contributions to scientific discovery over many decades.
The Astrophysics Science Division conducts a broad program of research in astronomy, astrophysics, and fundamental physics. Investigations address issues such as the nature of dark matter and dark energy, discovery of exoplanets and analysis of which planets could harbor life, and the nature of space, time, and matter at the edges of black holes. There were four “Great Observatories” consisting of the Hubble Space Telescope (HST), Compton Gamma-Ray Observatory, the Chandra X-Ray Observatory, and the Spitzer Space Telescope. Except for Compton (de-orbited in 1999), all of these are in extended mission phases (see also Box 2.1 for a discussion of HST). Examples of results from current extended missions are in Table 2.1.
The Chandra X-ray Observatory, which provides 10 times better spatial resolution (0.5 arcsec) than any other X-ray observatory to date or currently in development, was launched into a highly elliptical, geocentric orbit in 1999 and completed its prime mission in 2004. Since that time, it has been extended through the biennial Senior Review process and continues to be in good health. During its extended mission, Chandra has contributed important results over diverse areas of astrophysics, ranging from our solar system to cosmological studies. Chandra has provided
TABLE 2.1 Examples of Science Results Made Possible by Extended Missions in Astrophysics
|Chandra X-Ray Observatory||Discovery of the most recent known supernova explosion in our galaxy with an age of around 140 years, about 200 years younger than previous record-holder (Reynolds et al., 2008).|
|Fermi Gamma-ray Space Telescope||Discovery of a new class: classical novae that produce high-energy gamma rays, indicating acceleration of subatomic particles to cosmic-ray energies (Ackermann et al., 2015).|
|Hubble Space Telescope||The accelerated expansion of the universe due to dark energy is discovered by observations of Type Ia supernovae with HST and ground telescopes, celebrated by the 2011 Nobel Prize (Riess et al., 1998; Perlmutter et al., 1999).|
|Kepler||Great enhancement of the population of small, rocky planets orbiting Sun-like stars and stars with astroseismology periods.|
|NuSTAR (Nuclear Spectroscopic Telescope Array)||Best measurement of the spin rate of a supermassive black hole at the center of a galaxy (Walton et al., 2013).|
|Spitzer Space Telescope||Together with Hubble Space Telescope identified very distant galaxy GNz-11, finding that star formation proceeds much more rapidly than previously known in the early universe (Oesch et al, 2016).|
|Swift||Discovery of bright X-ray emission from a tidal disruption event where a star was torn apart when it orbited too close to a massive black hole (Bloom et al., 2011; Burrows et al., 2011).|
|XMM-Newton||Discovery of the first spinning neutron star in M31 (Esposito et al., 2016).|
strong support for the existence of dark matter (Clowe et al., 2006), and it has recorded the long-term behavior of supermassive black holes, including Sagittarius A* at the center of the Milky Way (Ponti et al., 2015) (Figure 2.1).
The Spitzer Space Telescope was launched into an Earth-trailing heliocentric orbit in 2003. Upon completion of its prime mission in 2009, when its reserve of liquid helium cryogen was exhausted, Spitzer entered into the “warm” Spitzer extended mission phase. Although only two of its four original imaging arrays have remained useful (at wavelengths of 3.6 and 4.5 μm), Spitzer has successfully provided important observations of comets, near-Earth asteroids, brown dwarfs, transient objects, galaxy clusters, and the most distant galaxies (Werner et al., 2015).
One of the most important questions in astrophysics involves the details of star formation and galaxy growth in the early universe. On the basis of colors determined from Hubble and Spitzer (warm/extended mission) images in different wavebands, a galaxy named GNz-11 had an estimated distance and age suggesting it was one of the most distant and youngest observed to date. These Spitzer and Hubble images indicated that GNz-11 is about 25 times smaller than our Milky Way galaxy and about 100 times less massive. Nonetheless, GNz-11 forms stars at a rate about 20 times higher than the present rate of star formation in the Milky Way. Motivated by these prior Hubble and Spitzer data, spectroscopic observations made in 2015 with the Hubble Wide Field Camera 3 (during the Hubble extended mission) determined a precise redshift of 11.1 for this galaxy, meaning that it is being observed as it appeared just 400 million years after the Big Bang and about 200 million years earlier than the previous record holder (Oesch et al., 2016). This more precise distance determination tells us that star formation proceeds much more rapidly than previously known in the very early universe and promises many more such results from the upcoming James Webb Space Telescope (JWST) and Wide-Field Infrared Survey Telescope (WFIRST) missions.
Recent engineering modifications have enabled Spitzer to become an additional tool in the identification, confirmation, and classification of exoplanets. Moreover, Spitzer’s warm mission has become an essential tool for studying atmospheric properties of hot Jupiters and determining whether super-Earth-size planets have an atmosphere (see Figure 2.2). Thus, one of the lessons from Spitzer’s experience is that extended missions can be surprisingly useful and resilient, even to the people who developed them. There was widespread perception within the astrophysics community that the warm Spitzer phase would not be very productive, and yet it has resulted in numerous important scientific discoveries. There are many reasons for this, including the fact that new technologies on the ground, and new concepts, questions, and ideas generated by its mission team, can be applied to a spacecraft many years after the end of its prime phase.
The Swift Gamma-Ray Burst Mission studies the most powerful explosions the universe has seen since the Big Bang. In its extended phase, Swift discovered the first jetted emission from a tidal disruption event (TDE). TDEs are a unique probe of dormant supermassive black holes in galaxies that are too distant for resolved kinematic studies. They occur when a star passes too close to a supermassive black hole and is ripped apart by the tidal forces. In an unexpected development, the TDE world was revolutionized in 2011 by Swift’s discovery of the high-energy transient SwJ1644+57. While initially thought to be an exotic gamma-ray burst, SwJ1644+57 turned
out to be the birth of a relativistic jet triggered by the tidal disruption process. It was located at the center of an inactive galaxy nucleus, where a supermassive black hole is likely to exist. The initial bright flaring emission lasted for 1 day, followed by 1 year of fading afterglow. The formation of a relativistic outflow also powered a bright radio emission, visible for months after the onset of SwJ1644+57. Based on this Swift discovery, the new class of relativistic TDEs are predicted to be one of the most numerous class of extragalactic transients to be discovered by forthcoming wide-field radio surveys.
The Nuclear Spectroscopic Telescope Array (NuSTAR) provided the first orbiting telescopes to focus light in the high energy X-ray (6-79 keV) region of the electromagnetic spectrum to study highly energetic phenomena. In its extended mission, NuSTAR, working together with Chandra, for the first time witnessed a Type Ib supernova—the explosion of a massive star without a hydrogen envelope—metamorphose into a supernova with a shock wave interacting strongly with material previously ejected by the progenitor star (Margutti et al., 2016). The data for SN2014C (Figure 2.3) imply that the shell of material was ejected by the progenitor star 10 to 1,000 years before the explosion. This phenomenology challenges the current theories of massive stellar evolution and argues for a revision of the understanding of mass loss in evolved massive stars. In turn, such revisions would affect estimates of the stellar initial mass function in galaxies and of star formation through cosmic time, which rely on the predictions of stellar evolution models.
Earth is a complex, dynamic system and to fully understand it requires understanding Earth’s atmosphere, lithosphere, hydrosphere, cryosphere, and biosphere as a single interconnected system. Earth is changing on all
spatial and temporal scales. The purpose of NASA’s Earth science program is to develop a sufficient understanding of Earth’s system and its response to natural or human-induced changes to make accurate predictions of climate impacts under various scenarios. NASA Earth science missions are a mix of large directed (flagship) missions such as Terra and Aqua, plus smaller, competitively selected missions and instruments. Examples of major results from a sub-sample of extended missions are given in Table 2.2.
The Gravity Recovery and Climate Experiment (GRACE) is an Earth system science Pathfinder mission launched in 2002 and initially planned for 5 years. The Pathfinder Program provides periodic, competitively selected opportunities to accommodate new and emergent scientific priorities. GRACE goals included monthly measurements of Earth’s gravity field with unprecedented accuracy, to help define Earth’s geoid and help measure the dynamic ocean surface topography resulting from the general ocean circulation. The measurements contribute to understanding the temporal variations in global and regional sea level and are essential for separating the contributions of sea level rise due to thermal expansion from those of increasing seawater mass. This separation allows determination of the change in heat stored by the oceans. The monthly measurements also contribute to assessing ground water storage in aquifers, ocean mass change from melting of glaciers, measuring the change in mass distribution of polar ice and the episodic mass change associated with large earthquakes.
GRACE entered extended mission phase in 2008 and has been extended several times since then. Due to its unique measurements and well-designed spacecraft and instruments, this international partnership mission continues to play a vital role in assessing Earth’s water resources. The long time series from the extended mission phase has enabled water resources to be monitored worldwide (e.g., Feng et al., 2013; Moiwo et al., 2013, Joodaki et al., 2014; Chen et al., 2011), and assessed relative to precipitation changes in El Niño years and La Niña years.1
The demonstrated value of GRACE measurements for global water resource monitoring led to the decision to implement the GRACE Follow-On (GRACE-FO) mission, which is scheduled for launch in late 2017. To maintain the climate record, there is a strong desire within the Earth science community to continue extended operations of GRACE until GRACE-FO is launched and the overlapping data sets can be compared. If NASA does this,
TABLE 2.2 Examples of Science Results Made Possible by Extended Missions in Earth Science
|Aqua||MODIS fractional snow cover, sea ice extent, and ice surface temperature products showed that the melting of the Greenland ice sheet in 2012 was the most extensive surface melting observed in the satellite era to that date (Hall et al., 2013).|
|Aura||Microwave Limb Sounder and Ozone Monitoring Instrument data revealed unprecedented ozone loss during the 2010-2011 Arctic winter (Manney et al., 2011).|
|CALIPSO||CALIPSO observations showed gradually increasing stratospheric aerosol loading from 2006-2011 due to a series of relatively moderate volcanic eruptions (Vernier et al., 2011) and resulting in a global cooling of about −0.07°C (Solomon et al., 2011), sufficient to offset a significant portion of the surface warming expected from increasing greenhouse gas concentrations over the past decade.|
|CloudSat||CloudSat data from 2008-2010 showed that trapping of heat by clouds is enhancing Greenland ice sheet meltwater runoff (Van Tricht et al., 2016).|
|EO-1||As a technology demonstration mission, EO-1 demonstrated over 12 years the practicality and stability of using ground-based calibration sites in support of sensor cross-comparisons and carbon flux measurements (Campbell et al., 2013).|
|GRACE||GRACE documented dramatic ice mass loss in Patagonia (Ivins et al., 2011), the Russian High Arctic (Moholdt et al., 2012), coastal Alaska (Sasgen et al., 2012), the Canadian Arctic (Gardner et al., 2011), and in the high mountains of central Asia (Jacob et al., 2012). GRACE data revealed groundwater depletion in the Colorado River basin from 2002-2014 during the recent drought in the western United States (Castle et al., 2014), as well as groundwater depletion in China (Feng et al., 2013; Moiwo et al., 2013), the Middle East (Joodaki et al., 2014), Turkey (Gokmen et al., 2013), the Aral Sea watershed (Zmijewski and Becker, 2014), Mexico (Castellazzi et al., 2014), and India (Chen et al., 2011; Chinnasamy et al., 2013).|
|Jason-1/Jason-2 (OSTM)||The Jason-1/Jason-2 (OSTM) observation record now stretches over 20 years, providing the most accurate and complete understanding of sea level change. The extended mission phases of Jason-1 and Jason-2 improved estimates of deep ocean topography, resolving many presently unknown seamounts and geologic features on the ocean bottom.|
|QuikSCAT||From 1999-2009, QuikSCAT provided ocean vector winds used by operational weather centers and the U.S. Navy. Since 2009, QuikSCAT provided a stable calibration of other spaceborne ocean wind vector measurements to enable a long-term, high-quality ocean wind vector database.|
|SORCE||SORCE observations have extended the record of solar irradiance to determine that warming over the past century is attributable mainly to increasing anthropogenic gases, with solar irradiance variability estimated to cause about 10 percent of the 0.74°C per century increase in global surface temperature (Lean and Rind, 2008). Furthermore, SORCE total solar irradiance data from the Total Irradiance Monitor instrument revealed a smaller solar irradiance than previously thought (Kopp and Lean, 2011).|
|Terra||MOPITT data between 2000-2003 and 2004-2008 show a clear decrease in carbon monoxide concentration worldwide (Worden et al., 2013) and over megacities (Pommier et al., 2013). MISR data show that human-caused fires limit rainfall in Africa, exacerbating dry conditions in the region (Tosca et al., 2015).|
NOTE: CALIPSO, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation; EO-1, Earth Observing-One Mission; GRACE, Gravity Recovery and Climate Experiment; MISR, Multi-angle Imaging Spectroradiometer; MODIS, Moderate Resolution Imaging Spectroradiometer; MOPITT, Measurement of Pollution in the Troposphere; OSTM, Ocean Surface Topography Mission; QuikSCAT, Quick Scatterometer; SORCE, Solar Radiation and Climate Experiment.
then GRACE will have operated for over 15 years, only 5 of those in prime phase and the rest in extended phase. The GRACE experience demonstrates another typical value of Earth-science extended missions: providing cross-calibration of sensors. By enabling GRACE to continue operating until GRACE Follow-On is operational, scientists can remove any bias in the data caused by transferring from the current sensor to the next sensor, even though the two sensors theoretically have the same specification. Such cross-calibration has been important for other Earth science missions, such as missions for measuring solar irradiance (e.g., Acrimsat), sea-surface topography (e.g., Jason, OSTM), and ocean vector winds (e.g., QuikSCAT), and can be important for planetary missions.
Figure 2.4 shows the ground water storage percentage over the continental United States in September 2015 compared to the average historical results from 1948-2012, showing the severe drought in California and the Pacific Northwest.
Terra is a flagship EOS (Earth Observation System) mission launched in December 1999, whose prime mission ran through September 30, 2005. It has been extended through the Earth Science Senior Reviews in 2005, 2007, 2009, 2011, 2013, and 2015, and all five instruments are still operating nearly as well as at launch, with the exception of the 1999 failure of the shortwave-infrared instrument on ASTER (Advanced Spacebourne Thermal Emission and Reflection Radiometer, a contribution from the Japanese Ministry of Economy, Trade, and Industry). There were more than 1,600 peer-reviewed science publications using Terra data in 2014 alone (NASA, 2015). Of the many science products produced over an increasingly long time period is the record of carbon monoxide (CO) concentration produced by the Canadian Space Agency-provided instrument Measurements of Pollution in the Troposphere (MOPITT), which has shown a steady decrease of CO concentration globally since Terra’s 1999 launch. Due to its relatively long lifetime of several weeks in the troposphere, CO is used as a tracer of pollution transport in satellite or model studies and is an important precursor of ozone (O3). Of particular note is the use of the shortwave and thermal infrared channels of MOPITT to increase the capability to assess CO concentration in the lower atmosphere, an algorithm enhancement developed well into the extended phase of Terra. Most megacities studied by MOPITT show a clear reduction in CO emission between 2000 and 2003 and 2004 and 2008, reaching −43 percent over Tehran, Iran, and −47 percent over Baghdad, Iraq (Pommier et al., 2013). Figure 2.5 shows a cross section of CO concentration upwind and downwind of Baghdad in 2000 to 2003 (blue line, prime mission)
and 2004 to 2008 (red line, extended mission). In addition to this focused study on various megacities around the world, MOPITT’s long time series has enabled studies of the overall decrease of CO concentration worldwide, which shows an approximately 1 percent per year decrease in total column CO over the Northern Hemisphere from 2000 to 2011 (Worden et al., 2013), with a somewhat smaller but still decreasing trend in the Southern Hemisphere.
One of the lessons that Terra illustrates is that, although the spacecraft itself represents aging hardware, new technologies and techniques developed on the ground during an extended phase can be applied to the data. Thus, even a spacecraft that has been operating for many years and no longer represents the state of the art can be used in new and sophisticated ways.
Heliophysics is the study of the Sun, the heliosphere, and the interactions of the Sun and the solar wind with planetary environments. The heliosphere is a vast region of space carved out of the local interstellar medium by the solar wind, the magnetized plasma that flows outward at high speeds from its source in the solar corona. Heliophysics addresses fundamental properties of space plasmas. Using in situ spacecraft measurements of charged particles from low to high energies, the magnetic field, electromagnetic radiation, and energetic neutral atoms produced by charge exchange with energetic ions in regions remote from the observation point, studies in this area elucidate processes that apply to astrophysical systems throughout the universe. Research addresses the properties and the variability of the Sun and the solar wind, the interaction of the solar wind with planetary environments, and the outer heliosphere and its interaction with the interstellar medium, the latter a new frontier in the field. The interaction of the solar wind with planetary environments produces magnetospheres or analogous structures, and study of Earth’s magnetosphere has profoundly contributed to our understanding of the complexities of magnetized plasmas.
The solar wind is confined within the heliosphere, a plasma bubble within the local interstellar medium, and the study of the outer heliosphere is a new frontier in the field. Heliophysics applies lessons of basic physics to the analysis and prediction of space weather, which is increasingly important to our technological civilization. Key objectives of heliophysics include unraveling of fundamental phenomena such as particle acceleration in turbulent plasmas and magnetic reconnection in space plasmas, goals that require multi-spacecraft measurements on scales
pertinent to exposing the details of this ubiquitous and critically important process. The science conducted by extended missions has been essential to advancing knowledge in all of the principal areas comprising heliophysics. Examples of major scientific results from a subsample are provided in Table 2.3 and the text that follows.
One outstanding example of discovery science emerging from data acquired during the extended phase of a mission is the first in situ exploration of the outer heliosphere. The evidence comes from the two Voyager spacecraft, initially approved for flybys of Jupiter and Saturn. Voyager 1 and 2 are perhaps the most remarkable spacecraft ever launched. (Voyager 1 flew by Jupiter in 1979 and Saturn in 1980. Voyager 2 flew by Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989.) Once past Neptune, the ongoing extended Voyager mission has provided unprecedented information about the outer boundaries of the region of interstellar space in which we live. The scientific benefits of the extended mission include the first observation of the termination shock (Stone et al., 2005), a front across which the solar wind slows markedly, and the first crossing of the outer boundary of the heliosphere and the first direct encounter with interstellar space (Stone et al., 2013; Krimigis et al., 2009) (see Figure 2.6). The dramatic results obtained at the outer boundary of the solar system are particularly remarkable in view of the small cost of extended operation. Even today, the in situ measurements of plasma and magnetic
TABLE 2.3 Examples of Science Results Made Possible by Extended Missions in Heliophysics
|ACE||Continuous observation of solar wind conditions for studies of energy, mass, and momentum flow through the geospace system (Gopalswamy et al., 2005). Long-term (over multiple solar cycles) observation of the solar wind is an essential part of the Heliophysics System Observatory (King and Papitashvili, 2005).|
|AIM||Long-distance relationships (“teleconnections”) were discovered between noctilucent clouds in one polar region and meteorological activity in the other (Holt et al., 2015).|
|ISEE-3||Launched in 1978, ISEE became ICE in 1982, and well into extended phase, it was retargeted to Comet Giacobini-Zinner, becoming the first spacecraft to traverse the plasma tail of a comet, where it measured particles, fields, and waves (Scarf et al., 1986).|
|STEREO||In its extended mission, STEREO obtained the first 360 degree images of the Sun.a|
|THEMIS/ARTEMIS||Conversion of magnetic energy in the magnetotail to particle energy in the inner magnetosphere was observed (Angelopoulos et al., 2013), particularly in conjunction with the Van Allen Probes (THEMIS). Retargeting two of the five spacecraft to circumlunar orbits (ARTEMIS) allowed for the first fully quantitative analysis of the structure and dynamical processes characteristic of the lunar wake (Wiehle et al., 2011).|
|TIMED||Dramatic cooling in the upper atmosphere was observed that correlated with the deep solar minimum in 2009 (Solomon et al., 2010).|
|Voyager 1 and 2, IBEX, Cassini||In situ measurements by Voyagers 1 and 2 of magnetized plasmas and energetic particles in the outermost regions of the heliosphere, combined with remote sensing energetic neutral atoms observations by IBEX and Cassini have led to development of new models of the heliosphere required to explain plasma properties of these strange plasma regions.|
|Wind||Direct observation of the electron diffusion region in collisionless reconnection (Øieroset et al., 2001).|
|HSO||HSO is not a single mission. It brings together the sum of spacecraft in both prime and extended phase. In particular, through the use of extended phase missions (including those not in this table), HSO has been able to document changes in the geospace environment over several solar cycles, especially the anomalously deep 2009 solar minimum (Russell et al., 2010), allowing for heliospheric wide observational studies (Gibson et al., 2009) and comparisons to models (Wiltberger et al., 2012) of entire Carrington rotations of the Sun.|
a NASA Science, “First Ever STEREO Images of the Entire Sun,” release date February 6, 2011, http://science.nasa.gov/science-news/scienceat-nasa/2011/06feb_fullsun/.
NOTE: ACE, Advanced Composition Explorer; AIM, Aeronomy of Ice in the Mesosphere; ARTEMIS, Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun; HSO, Heliophysics System Observatory; ICE, International Cometary Explorer; ISEE, International Earth-Sun Explorer; STEREO, Solar Terrestrial Relations Observatory; THEMIS, Time History of Events and Macroscale Interactions during Substorms; TIMED, Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics.
field properties made by the two Voyager spacecraft and the remote sensing of the plasma and field properties by the Interstellar Boundary Explorer (IBEX) spacecraft in Earth orbit continue to provide information about the farthest reaches of the heliosphere; the new data challenge our scientific preconceptions and are generating new understanding.
From the large scale and the outer reaches of the solar system to the smallest scale in our own backyard, important scientific discoveries have been made and are continuing to be made using data from extended missions. A key example is the developing understanding of the process of magnetic reconnection. This dynamical phenomenon, ubiquitous in space plasmas, transfers energy from magnetic fields to plasmas and powers solar flares and magnetic storms. However, many details of the reconnection process are still poorly understood. There had been an ongoing argument whether resistive or collisionless processes were at the heart of reconnection in Earth’s magnetosphere. The question was hard to answer because space is big, and the electron diffusion region where the critical processes take place is very small. But in 2001, NASA’s Wind spacecraft, well into its extended mission, was in the right place at the right time to capture crucial evidence that collisionless reconnection was occurring (Øieroset et al., 2001). Data from the ongoing THEMIS (Time History of Events and Macroscale Interactions during Substorms) extended mission have been illuminating in considerable detail the fundamental mechanisms through which energy released in magnetic reconnection is converted into plasma energy that powers the aurora and helps create the Van Allen radiation belts (Angelopoulos et al., 2013).
Tracking energy flows through the magnetospheric system is central to our understanding of space weather. We live in the neighborhood of a variable star, and understanding its variations is fundamental to understanding our space climate. In the past decade, something has been happening with the Sun. In 2009, Earth experienced the deepest prolonged solar minimum of the space age with almost no sunspot activity (e.g., Russell et al., 2010). Fortunately, the Wind and ACE (Advanced Composition Explorer) extended missions were operating and were able to monitor the state of the Sun and the solar wind. The deep solar minimum was felt throughout the system; for example, data from the TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) extended mission revealed a link between the anomalously low solar extreme ultraviolet irradiance and the thermospheric density (Solomon et al., 2010). This type of correlated response highlights the need for the constellation of spacecraft that comprise the Heliophysics System Observatory (HSO) to provide a long-term monitoring of Earth’s space environment (see Figure 2.7).
Additional questions addressable through heliophysics observatories include the following: How will the Sun evolve over the next solar cycle or two? Will it enter into a new extended minimum in solar magnetic activity
like that of the Dalton minimum of the 19th century or even the Maunder minimum of the 17th century?2 What will be the effect on space weather, or even on terrestrial climate? Only a continuous monitoring of all of the components of the system can help to answer these questions. Fortunately, the armada of spacecraft that comprise the HSO are already operating and most are still functioning well. Given that it would never be possible to launch all of the elements of the HSO simultaneously as new missions, it is essential that existing spacecraft be operated as long as they are functioning effectively because they are needed to provide the required long-term records that can reveal temporal changes of key elements of the heliosphere.
The strategic goal of NASA’s Planetary Science Division (PSD) is to advance scientific knowledge of the origin and history of the solar system, the potential for life elsewhere, and the hazards and resources present as humans explore space. Planetary science differs from the other science disciplines in a key way: it commonly takes significant time and energy for a spacecraft to reach its operating location and begin collecting data. For planetary science missions, a number of major science results have been possible only because of extended missions (see Table 2.4 for examples from some current extended missions). This section focuses on three examples to demonstrate the value of extended missions: recent extended mission discoveries about Mars, about ocean worlds, and near-Earth objects. In the first two cases, these discoveries have been critically important to shaping future exploration to achieve the highest priorities of NASA PSD. In the latter case, a relatively recent discovery revealed that Earth may have previously unknown companions in its orbit.
During the 2014 Planetary Science Senior Review, both the Lunar Reconnaissance Orbiter and the Opportunity rover were rated highly for their continued scientific contributions. However, they were both zeroed out for funding in the President’s fiscal year (FY) 2015 and FY2016 budgets. The scientific discoveries made by both missions during their extended phase are addressed in Appendix B of this report.
NASA’s Mars Exploration Program has benefited from missions lasting well beyond their primary missions, including the Mars Global Surveyor (MGS), Mars Odyssey, Mars Reconnaissance Orbiter (MRO), and the Mars Exploration Rovers (MER) Spirit and Opportunity. Each of these missions has spent far more time in extended phases than in the prime missions. For example, Spirit did not arrive at the Columbia Hills until well into its extended mission, where it achieved its most important results, describing a habitable ancient hydrothermal environment (Squyres et al., 2008; Ruff et al., 2011). This region is now one of the top candidate landing sites for the Mars 2020 rover, designated to cache samples for future return to Earth.
MRO was launched in 2005, achieving orbit around Mars in 2006. After completing its 4-year prime mission, MRO then entered into the extended mission phase in 2010, in which it continues to be operated. During the extended mission, the MRO science team first observed recurring slope lineae (RSL) on the surface of Mars (see Figure 2.8)—dark streaks that grow and fade with the seasons (McEwen et al., 2011). In the extended missions, these features were systematically monitored to understand their temperature behavior, consistent with briny water, and geographic distribution (McEwen et al., 2014). Spectral data collected during the extended missions enabled the detection of hydrated salts at some of these locations, confirming a role for briny water (Ojha et al., 2015). The RSL and other discoveries are of key importance for understanding potential present-day habitability, “Special Regions” for planetary protection plans, and resources for future humans on Mars, leading to the major science focus of the next recommended orbiter (MEPAG, 2015).
The outer planet moons with confirmed subsurface oceans are the Saturnian moons Titan and Enceladus and the icy Galilean satellites of Jupiter. Europa is the most interesting case because water is in contact with tidally heated silicates. An ocean in Europa was only suspected following three close encounters during the Galileo prime mission (Pappalardo et al., 1999). It was not until eight successful encounters in the extended missions that new
2 The Dalton minimum was a period of low sunspot count, representing low solar activity, named after the English chemist, physicist, and meteorologist John Dalton, lasting from about 1790 to 1830. The Maunder minimum is the name used for the period starting in about 1645 and continuing to about 1715 when sunspots became exceedingly rare, named after the solar astronomers Annie Russell Maunder (1868-1947) and E. Walter Maunder (1851-1928).
TABLE 2.4 Examples of Major Science Results Made Possible by Extended Missions in Planetary Sciences
|Cassini||Global subsurface oceans were discovered in Titan (Lorenz et al., 2008; Iess et al., 2012) and in Enceladus (Thomas et al., 2016).|
|LRO||Hundreds of new impact events (Speyerer et al., 2016) as well as recent or active tectonics (Watters et al., 2015) were detected, and polar ice was quantified (Hayne et al., 2015; Patterson et al., 2016).|
|MERs Spirit and Opportunity||A habitable hydrothermal environment was discovered by the Spirit rover (Squyres et al., 2008; Ruff et al., 2011). The Opportunity rover, along with MRO, mapped hydrated magnesium and calcium sulfate minerals that formed from rising ground waters (Arvidson et al., 2015).|
|Mars Odyssey||Extensive chloride-bearing deposits were discovered, likely ancient playas (Osterloo et al., 2008).|
|MRO||Recurring slope lineae were discovered (McEwen et al., 2011) and their association with hydrated salts was studied (Ojha et al., 2015).|
|Mars Science Laboratory||The Curiosity rover arrived at the base of Mt. Sharp and discovered evidence for a long-lived lake (Grotzinger et al., 2015). Evidence of refractory organic material on Mars was discovered (Eigenbrode et al., 2015).|
|NEOWISE||Earth’s Trojan asteroid was discovered (Connors et al., 2014).|
|Voyager 2||The first exploration of ice giant systems was completed of Uranus (Stone, 1987) and Neptune and Triton (Stone and Miner, 1989).|
NOTE: LRO, Lunar Reconnaissance Orbiter; MER, Mars Exploration Rovers, MRO, Mars Reconnaissance Orbiter, NEOWISE, Near-Earth Object Wide-field Infrared Survey Explorer.
geophysical (Kivelson et al., 2000) and other results (Pappalardo et al., 2009) were considered definitive evidence for an ocean. This changed the focus of future Europa exploration from confirmation of an ocean to habitability of that ocean. The multiple flyby mission to study Europa’s habitability is now in Phase A development, and a Europa lander is also being studied.3
Cassini-Huygens is a flagship mission originally launched in 1997 that, after 7 years in transit, reached Saturn in 2004 to begin its 4-year prime mission of exploring the local system and landing the Huygens probe on the surface of Saturn’s largest moon, Titan. Upon completing the prime mission, the orbiter was extended in 2008 for the 2-year Cassini Equinox Mission, including a series of close approaches to the icy moon, Enceladus. Having previously discovered active cryo-volcanism near the southern pole of this moon, Cassini was able to engage its suite of remote sensing and fields and particle experiments to determine the trace constituents within the plumes, as well as the conditions near the surface fractures where the jets emanate. These observations provided strong additional evidence for the existence of a liquid water reservoir beneath the surface of Enceladus (e.g., Waite et al., 2009; Figure 2.9) and for hydrothermal activity in the deep subsurface (Hsu et al., 2015). Cassini was extended again in 2010 for the Cassini Solstice Mission in order to study seasonal-temporal changes within the Saturn system, with an additional 12 encounters with Enceladus and 56 of Titan. In the Cassini prime mission, a subsurface ocean (perhaps not global) was only suspected in Enceladus, and confirmation came from the extended mission with many more encounters (Iess et al., 2014; Thomas et al., 2016) (see Figure 2.10). For Titan, surface hydrocarbon lakes or seas were known, but confirmation of a deep global water ocean was a key extended mission result (Iess et al., 2012). Based on these extended mission results, Congress has recommended, and NASA is acting on, creating a new Ocean Worlds program with a series of future missions.
Earth is now known to share its orbit with a Trojan asteroid that librates around its L4 Lagrange point, joining Venus, Mars, Jupiter, Neptune, and Uranus among the list of planets known to host such co-orbital objects. The first and only known Earth Trojan, 2010 TK7, was discovered by the Wide-field Infrared Survey Explorer (WISE) (Wright et al., 2010) satellite and its enhancement for solar system science, known as NEOWISE (Mainzer et al., 2011). WISE, launched in December 2009, surveyed the full sky in four infrared wavelength bands (3.4, 4.6, 12, and 22 μm) until the frozen hydrogen cooling the telescope was depleted in September 2010. The survey continued as NEOWISE for an additional 4 months used the two shortest wavelength detectors. The spacecraft was placed into hibernation in February 2011 after completing its search of the inner solar system. NEOWISE was brought out of hibernation (now supported by PSD) to learn more about the population of near-Earth objects and comets that could pose an impact hazard to Earth. NEOWISE observations resumed in December 2013. Shortly after the survey start, NEOWISE discovered its first potentially hazardous near-Earth asteroid, 2013 YP139. Earth Trojan 2010 TK7 was discovered on October 1, 2010, approximately a day after the cryogen was fully depleted and the survey was originally scheduled to stop. Numerical integrations have shown that 2010 TK7is likely to remain a Trojan asteroid for thousands of years (Connors et al., 2011, 2014; Figure 2.11). Subsequent fits to the data yielded diameter and albedo estimates for the object, indicating that it is several hundred meters across (Mainzer et al., 2012). It is possible that 2010 TK7 represents the first of a population of Earth Trojans, some of which may be primordial. The decision to operate the WISE spacecraft beyond its original lifetime has provided a first glimpse into this unique and rare population of small bodies.
What the planetary science extension examples demonstrate is that sometimes new scientific discoveries are only possible after a spacecraft moves into a new orbit or to a new location that could not be achieved during the prime mission, such as Cassini making multiple orbits around Saturn enabling it to make more and better planned observations of Enceladus, or a Mars rover reaching a new location far from its landing site. In addition, as in the earlier example of Earth science missions, sometimes data collected later in a mission (such as repeated observations of the time-varying RSL on Mars) enables fuller interpretation of earlier data. Finally, as NEOWISE demonstrates, surprising discoveries, like Earth’s Trojan asteroid, can be made at any time, including long after a prime mission has ended.
Extended missions in all four divisions of NASA’s Science Mission Directorate have made major scientific contributions at low cost relative to the initial investments for the prime missions.
Finding: Extended science missions are valuable assets in NASA’s portfolio because they provide excellent science at low incremental cost.
In numerous cases, the long-baseline data is critical to recognizing changes over time, especially in understanding the dynamic Earth system, the large and dynamic heliosphere, and for active planetary bodies such as Mars. Long-baseline data are also essential to discovery of rare events, such as supernova explosions and X-ray flares and relativistic jets from supermassive black holes.
Finding: Continuity, long-baseline data sets, and statistically significant observations of infrequent events require continuity of measurement over years or decades and are best provided through missions in extended phase.
In multiple cases, extended missions are able to accomplish surprising new results, either from a new orbit or observation profile or from new data analysis techniques. Examples include the Voyager spacecraft exploring the outer heliosphere, new Cassini orbits advancing understanding of the ocean and erupting jets of Enceladus, and development of a new algorithm to track carbon monoxide using Terra.
Finding: Extended missions may accomplish surprising new results via new destinations, observation types, or data analysis methods.
NASA extended mission science results have been sufficiently compelling to change the future exploration priorities of NASA and the decadal surveys. Examples include GRACE leading to GRACE-Follow On, Mars discoveries leading to new landing sites and future orbiter science priorities, and discovery of subsurface oceans leading to new missions such as the Europa multiple flyby mission and a new Ocean Worlds program.
Finding: NASA’s extended missions are an important part of both achieving science objectives of the decadal surveys (see Appendix D) and determining priorities or approaches for future exploration.
Recommendation: NASA should strongly support a robust portfolio of extended-phase science missions. This support should include advance planning and sufficient funding to optimize the scientific return from continued operation of the missions.
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