4

The Advantages of Space-Based Infrared Platforms

A space-based system dedicated to near Earth object (NEO) survey detection, initial orbit determination, and estimation of diameters is necessary to reach 90 percent completeness of the inventory of NEOs larger than 140 meters and to assess the potential impact hazard to Earth. Although ground-based surveys provide larger apertures and greater light collection, space-based infrared surveys

• Can search efficiently the region interior to the orbit of Earth, where some NEOs orbit (see Figure 4.1);
• Provide more time for searching compared to observing from the ground, which is reduced by daylight and by sky brightness due to the Moon and by bad terrestrial weather; and
• Do not contend with the effects of Earth’s atmosphere, which include scattered light and atmospheric absorption, both of which reduce the flux measured from a NEO on the ground, and atmospheric emission, which can blind a telescope to a NEO in the infrared.

Space telescopes operating at low temperature provide unique capabilities for detection of NEOs in the mid-infrared. Since NEOs are at a similar distance from the Sun and Earth, they are at similar temperatures. Thus, ground-based mid-infrared telescopes must pull the signal from a NEO out of the far larger (by a factor of up to a million) foreground emission by the telescope and atmosphere. The spectacular sensitivity of cooled infrared telescopes in space has been demonstrated by a series of very productive space astronomy missions such as Infrared Astronomical Satellite (IRAS), Infrared Space Observatory (ISO), Spitzer Space Telescope, Akari (previously known as ASTRO-F or IRIS-InfraRed Imaging Surveyor), and Wide-Field Infrared Survey Explorer (WISE).

Furthermore, the information obtained in the visual and infrared wavelength regimes is different. Infrared observations are better suited to estimating asteroid diameters with the least uncertainty. Due to the enormous numbers of NEOs that a space-based infrared survey will discover, use of a relatively simple first-cut model for size determination will be essential. Even relatively simple analyses of mid-infrared measurements can return reasonable estimates for NEOs, whereas visible light and near-infrared measurements are severely compromised for size determination. More accurate estimation of sizes using the Near Earth Asteroid Thermal Model (NEATM) is discussed in the Chapter 5.

Given a typical reflected light level of only about 15 percent, a substantial majority of the energy emerges in the mid-infrared, making the objects easier to detect there. The combination of all these factors results in an infrared space telescope potentially achieving the George E. Brown, Jr. Near-Earth Object Survey Act goal faster than other feasible approaches (see Figure 4.2).

With regard to the fact that a space-based, infrared survey has advantages over a visible-alone survey, visible surveys have a bias against discovering small low-albedo NEOs regardless of the limiting magnitude, suggesting there may be a population of undiscovered small low-albedo NEOs. Infrared surveys have a bias against discovering small high-albedo NEOs, although this bias is smaller than the bias of visible surveys.

Visible and near-infrared measurements are preferred for obtaining measurements of orbits over extended time and for determining the surface properties of an asteroid through multicolor photometry and spectroscopy. More advanced modeling of the NEO size and other properties can be based on the combination of visible and mid-infrared measurements (see Figure 4.3). The committee notes that while space-based surveys, unlike their ground-based counterparts, are not limited to observing the night-time sky and are able to discover relatively faint objects, they have the disadvantage that in general ground-based telescopes will be unable to provide short-term follow-up observations to secure orbits. Therefore, an effective space-based survey will have to be designed with a scanning cadence that maximizes the probability that detected objects are observed multiple times at intervals that allow accurate orbits to be established.

A space-based telescope operating in the thermal infrared is the approach best capable of giving sufficiently accurate diameters to meet the intent of the George E. Brown Act (see Figure 4.4). This telescope would need to be ~50 centimeters or larger in aperture.¹ For example, a number of much smaller infrared telescopes would not have the sensitivity or the resolution to reach the necessary diameter level and to determine accurate orbits.

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¹ G.H. Stokes, B.W. Barbee, W.F. Bottke, Jr., M.W. Buie, S.R. Chesley, P.W. Chodas, J.B. Evans, et al., 2017, Report of the Near-Earth Object Science Definition Team: Update to Determine the Feasibility of Enhancing the Search and Characterization of NEOs, NASA Science Mission Directorate, https://cneos.jpl.nasa.gov/doc/2017_neo_sdt_final_e-version.pdf, p. 138.

SPACE-BASED VISUAL WAVELENGTH TELESCOPES

A team of scientists and engineers at NASA Goddard Space Flight Center (GSFC) studied multiple NEO survey system designs at both visual and infrared wavelengths. While the discovery rate of NEOs increases with time using existing ground-based assets, the 90 percent completeness by 2020 specified in the George E. Brown Act will not be met, extrapolating current discovery rates of ground-based telescopes. This point was repeatedly made by numerous reports and those reporting to this committee.

Space-based visual wavelength systems presented to the committee were explored for low cost and high reliability, examining different telescope apertures as well as launch ridesharing and piggybacking on existing space-borne platforms such as the International Space Station (ISS). All space-based systems, especially ones designed specifically for detection of NEOs, have space-based platform advantages including the following:

• Near full-time operations;
• Access to large sky coverage;
• Scanning flexibility optimized for NEO discoveries; and
• Ability to observe above Earth’s atmosphere, enhancing signal and reducing noise.

Visual wavelength space-based systems take longer to reach a high completion rate than space-based infrared telescopes and have inherent biases that cannot be overcome in either ground-based or space-borne platforms. This bias includes the inability to differentiate between large dark objects and small bright ones, a limitation of all visual wavelength telescopes whether ground or space based. This limitation is the result of large uncertainties in diameter estimates because of the compounding H-magnitude uncertainty, and the uncertainty inherent in assuming an albedo to estimate diameter.

GSFC’s telescope performance tool allows survey simulations with the following variables:

• Detectors and their performance;
• Telescope pointing as a function of sky background related to zodiacal light and Earth and moon shine;
• NEO orbital populations of both size frequency and orbital distributions;
• Telescope pointing and field of view; and
• Search results as a function of discovery criteria.

Among the visual wavelength systems looked at by GSFC’s engineers and scientists are the following:

• 1.5 m telescope platform in geostationary, equatorial orbit;
  • High sensitivity
  • Mature technologies
  • Low risk
  • Fast development
• 0.5 m and 0.8 m large field of view, low-cost platform attached to the ISS;
• 1.0 m survey telescope co-manifested with various launch vehicles and locations in space; and
• 4-0.4 m telescopes, two visual wavelength, two infrared, co-manifest with various launch vehicles to different locations in space (LEO, GEO, L2).

Michael Shao of the Jet Propulsion Laboratory presented a concept of a constellation of five micro satellites (20- to 30-cm-aperture telescopes) for NEO search and survey featuring synthetic tracking to optimize detections. Large-sky coverage would be achieved by the multiple, small-aperture telescopes and automated analysis of moving targets’ signal. This approach is very interesting and has promise for NEO detection and survey.

The committee concluded that none of the visual wavelength-alone platforms matched the predicted performance of infrared, space-based platforms. Yet the combination of ground-based visual wavelength and infrared space-based platforms reached the desired completion rate in the shortest time period.

The committee, after hearing from scientists and engineers who considered visual wavelength space-based platforms, found no contradiction to numerous previous reports that concluded that an infrared space-based platform dedicated to the survey of NEOs will reach a completeness level of 90 percent for 140-meter-diameter objects sooner than any visual wavelength platform on Earth or in space. The physics of the interaction of sunlight with asteroid surfaces responds such that measurements of emitted radiation in the infrared spectral region can find NEOs more efficiently than any visual wavelength space-based system operating alone. Additionally, the estimated diameters derived from infrared measurements have lower uncertainty when combined with visual wavelength measurements. Thus the two spectral wavelengths regions complement each other in the detection, cataloguing, and diameter estimates of NEOs.²

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² At the committee’s first meeting in mid-December 2019, Professor Richard P. Binzel of MIT, a planetary scientist who has made significant contributions to the study of asteroids and Kuiper Belt objects across five decades, commented on the committee’s statement of task, point by point. Previous reports written by experts from both government and private sectors on the subject of NEO surveys include search simulations, analyses of technical approaches and their costs, as well as preparedness and mitigation strategies that were solicited by NASA. Some of these reports were carried out under the auspices of the Space Studies Board of the National Academies of Sciences, Engineering, and Medicine. The list of reports numbers more than 18 in the past 11 years and can be found at https://www.nasa.gov/planetarydefense/supporting_documents and at links from this site. Those reports assessing the technical aspects of NEO search, cataloguing, and characterization conclude that the most comprehensive approach toward meeting the task at hand is to employ space-based infrared surveys that both discover and estimate diameters upon discovery.

COST OF SPACE TELESCOPES

The cost of space-based telescopes can be estimated roughly as parameterized by Stahl and Henrichs (2016).³ Their formulation shows a strong dependence on aperture and only weak dependences on operating temperature and wavelength. This behavior is confirmed by detailed costing of a range of space-based telescopes in the Near-Earth Object Science Definition Team (SDT) report (2017).⁴ They estimated that a 0.5-meter-aperture visible telescope in either low Earth orbit or geosynchronous orbit would cost about $480 million, while one at the L1 or L2 Lagrangian point would be about $120 million more. The numbers for a 1-meter telescope were about $700 million, or $850 million at L1 or L2.

The 2017 SDT report estimates the cost of constructing a dedicated 8-meter, ground-based visible-light telescope as about one-seventh the cost of a 0.5-meter telescope in space.⁵ The collecting area of the ground-based telescope would be 250 times that of the space-based version. This comparison dramatizes the ability of the ground to provide important capabilities by telescope size. Of course, ground-based telescopes have operating limitations that space-based telescopes do not, such as the inability to operate during the day, before sunrise or immediately after sunset, and in poor weather.

Operation at L1 or L2 is strongly favored for an infrared telescope; the 2017 SDT study found that an infrared telescope in the L1 or L2 orbit requires a cost increase of just a few percent over the same-size visible telescope in the same orbit and provides better NEO detection and characterization.⁶ The SDT study provided a cost estimate that agreed well with the independent, detailed cost modeling for NEOCam (see the following section).

The committee draws two conclusions. First, it is reasonable to compare performance in the two spectral regions for telescopes of the same size; the costs for both rise so steeply with increasing aperture that only a modest increase in size of a visible light telescope would make it match the cost of an infrared one. Second, since an infrared telescope in space significantly outperforms an in-space visible wavelength telescope, from a cost/benefit analysis it is unnecessary to consider space-borne visible telescopes in depth. The advantage for the visible region comes from operating on the ground where very large telescopes are feasible.

TEST OF FEASIBILITY OF A SPACE-BASED INFRARED TELESCOPE

In addition to a detailed cost estimate, the Near-Earth Object Camera (NEOCam) provides a test of the detailed mission design and technical risk of a dedicated NEO-detecting telescope. NEOCam is a proposed mid-infrared space-based telescope that would scan the solar system to detect and track NEOs. It would use a passively cooled mid-infrared telescope with a 0.5-meter-diameter mirror and would operate at the Sun-Earth L1 point. NEOCam has received NASA funding for risk reduction and for mission concept development. All the relevant technologies are at a technology readiness level (TRL) of 5 or higher. According to the NEOCam project, if launched, the spacecraft would be able to detect approximately 80 to 87 percent of the >140-meter-diameter NEOs within 5 years.⁷ In an extended 10-year mission, NEOCam would meet the George E. Brown Act goal.

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³ H.P. Stahl and T. Henrichs, 2016, Multivariable parametric cost model for space and ground telescopes, Proceedings of the Society of Photo-Optical Instrumentation Engineers 9911, Modeling, Systems Engineering, and Project Management for Astronomy VI, 99110L.

⁴ Stokes et al., 2017, p. 157.

⁵ Ibid., p. 150.

⁶ Ibid., p. 158

⁷ A. Mainzer, 2006, NEOCam: The Near-Earth Object Camera, Bulletin of the American Astronomical Society, 38, p. 568, http://adsabs.harvard.edu/abs/2006DPS....38.4509M.

The committee notes that NEOCam is primarily a planetary defense mission that has been forced to compete within NASA’s Discovery program against other proposals that have primarily scientific objectives. This has placed NEOCam and similar planetary defense-related missions at a competitive disadvantage.

CONCLUSION

Visible, ground-based surveys are compromised by the day-night cycle and weather, as compared to space-based surveys. A major advantage of an infrared space-based system is its ability to provide a more reliable diameter shortly after detection, as soon as orbital parameters are available. Visible light and near-infrared measurements are severely compromised for size determination, whereas even relatively simple analyses of mid-infrared measurements can return accurate sizes for NEOs. As a result, a space-based infrared survey is better able to detect and characterize the NEO population to meet the requirements of the George E. Brown Act goal. A detailed study of a mid-infrared mission has concluded that the proposed system can reach the George E. Brown Act goal more quickly than currently considered alternatives.

The cost of a ground-based capability designed to meet the George E. Brown Act goal in a timely fashion is substantial. In addition, there is a range of possible costs and capabilities from the ground. The NEO SDT report⁸ quantifies the benefits relative to a space-borne infrared telescope, and they appear to scale roughly with cost.

This could be addressed by having a separate planetary defense mission line with an announcement of opportunity that is focused on planetary defense needs, something that is currently under discussion within NASA. Observations in the thermal infrared can provide far more accurate asteroid sizes than observations in the visible region, provided the illumination and observing geometries and rotation of the asteroid are taken into account.

Chapter 5 provides a quantitative discussion of the methods and relative accuracies of asteroid size determinations.

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⁸ Stokes et al., 2017.