In December 2018, an asteroid exploded in the upper atmosphere over the Bering Sea (western Pacific Ocean) with an explosive force initially estimated to be nearly 200 kilotons, or over 10 times that of the Hiroshima bomb.1 This event, which was detected by various sensors and spotted by a Japanese weather satellite, demonstrates that Earth is frequently hit by objects, some of which could cause significant damage if they hit a populated area, as happened almost 6 years earlier over the Russian city of Chelyabinsk. Currently, NASA funds a network of ground-based telescopes and a single, soon-to-expire space-based asset to detect and track large asteroids that could cause major damage if they struck Earth. In 2018, NASA asked the National Academies of Sciences, Engineering, and Medicine to establish the ad hoc Committee on Near Earth Object Observations in the Infrared and Visible Wavelengths to investigate and make recommendations about a space-based telescope’s capabilities, focusing on the following tasks:
- Explore the relative advantages and disadvantages of infrared (IR) and visible observations of near Earth objects (NEOs).
- Review and describe the techniques that could be used to obtain NEO sizes from an infrared spectrum and delineate the associated errors in determining the size.
- Evaluate the strengths and weaknesses of these techniques and recommend the most valid techniques that give reproducible results with quantifiable errors.
THE GEORGE E. BROWN ACT AND NEO DETECTION, TRACKING, AND CHARACTERIZATION
Currently, NASA’s efforts to detect and track NEOs are guided by the 2005 George E. Brown, Jr. Near-Earth Object Survey Act,2 which requires NASA to “detect, track, catalogue, and characterize the physical characteristics
1 This estimate is likely to be revised downward upon further analysis.
2 Technically, this language was included in the 2005 NASA Authorization Act, which states: “This section may be cited as the ‘George E. Brown, Jr. Near-Earth Object Survey Act.’” The committee uses the terms “George E. Brown” and “George E. Brown Act” throughout this report. The goals established by the George E. Brown Act were primarily derived from NASA: “Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters. Report of the Near-Earth Object Science Definition Team,” August 22, 2003, https://www.nasa.gov/sites/default/files/atoms/files/pdco-neoreport030825.pdf.
of near Earth objects equal to or greater than 140 meters in diameter in order to assess the threat of such near Earth objects to Earth. It shall be the goal of the Survey program to achieve 90 percent completion of its near Earth object catalogue (based on statistically predicted populations of near Earth objects) within 15 years after the date of enactment of this Act.”
NASA has not accomplished this goal and cannot accomplish it with currently available assets by December 31, 2020.3 Although Congress has charged NASA with NEO detection and threat characterization, it has failed to provide specific funding to enable NASA to adequately pursue this task.
The George E. Brown Act was based on findings of a 2003 NASA science definition team study of NEOs. A follow-on 2017 NEO science definition team report also used the act as a baseline (e.g., the focus on 140-meter diameter NEOs and 90 percent completion). Any effort to develop a survey of NEOs must have goals to compare to, and most studies and proposals for NEO searches since the act have used its goals as the baseline.
In addition to detecting NEOs and determining their orbits, it is necessary to estimate their mass to quantify their destructive potential. An NEO’s diameter is the most readily available indicator of its mass, a value that can be improved when a density estimate is available. This is the rationale for the 140-meter-diameter requirement included in the act—finding 90 percent of that population or larger would eliminate 90 percent of the hazard to human populations from NEOs (at the time of the publication of the Stokes et al. (2003) report).4 In the 14 years since the passage of the George E. Brown Act, there have been several studies that have reiterated the validity of the 140-meter-diameter requirement and indicated that even smaller size asteroids can pose a significant threat. The asteroid that exploded over Chelyabinsk, for example, is estimated to have been approximately 20 meters in diameter. It damaged more than 7,000 buildings and injured approximately 1,600 people. In comparison, Arizona’s Meteor Crater, which is approximately 50,000 years old, is believed to have been created by a significantly denser (nickel/iron) object approximately 50 meters in diameter (see Figure S.1). Asteroids smaller than 140 meters in diameter are much more numerous than those larger than this size. Although they are far more difficult to detect and track, many of them are still detected in the search for larger asteroids. Although asteroids smaller than the size established in the George E. Brown Act pose a hazard, it is not currently practical to implement systems capable of detecting and tracking a significant proportion of them, and the committee concluded that the requirements established in the George E. Brown Act remain valid.
3 A 2017 report indicated that it would take 9-25 years to complete the survey, depending on search methods (and equipment) that were employed. This places the earliest date for completing the survey in the later 2020s (G.H. Stokes 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, p. iv).
4 The probability of impact by long-period comets (LPCs) is much lower than the probability of impact by NEOs.
Recommendation: Objects smaller than 140 meters in diameter can pose a local damage threat. When they are detected, their orbits and physical properties should be determined, and the objects should be monitored insofar as possible.
The committee concluded that the accuracies of NEO diameters derived from thermal-infrared measurements and simple modeling usually far exceed those based on measurements of visible brightness alone. For this reason, thermal-infrared detection and tracking of asteroids, which can be accomplished only by a space-based platform (due to the properties of Earth’s atmosphere, which block infrared wavelengths), is highly valuable. A thermal-infrared search program that can detect NEOs, determine their orbits, and measure NEO sizes to 25 percent typical uncertainty or better is preferable to separate search and characterization programs. To gain the same information about an NEO’s size with ground observations would require both a search program and a separate characterization program.
Characterization—that is, determining the physical properties of NEOs—is critical for a full understanding of the impact hazard. Characterization observations include radar as well as photometry and spectroscopy in the visible and near infrared. Although planetary defense missions are not science driven, significant scientific input is essential to optimally design a planetary defense task.
SPACE-BASED NEO DETECTION AND TRACKING
After hearing from representatives of different organizations, including persons who had sought to develop alternative proposals for both ground- and space-based NEO detection systems, the committee concluded that a space-based thermal-infrared telescope designed for discovering NEOs is the most effective option for meeting the George E. Brown Act completeness and size requirements in a timely fashion (i.e., approximately 10 years) (see Figure S.2). The most important justification for a shorter time span is that mitigation by deflection requires early detection.
A thermal-infrared discovery survey will provide an immediate measure of asteroid diameters—and hence a mass estimate—even without a measurement of the asteroids’ optical brightness. An optical discovery survey is not able to provide this diameter measurement/mass estimate with the same accuracy within a similar timeframe, as it depends on thermal-infrared follow-up observations. Furthermore, the availability of an observation asset capable of obtaining this thermal-infrared follow-up is not guaranteed (ground-based observations are strongly limited in wavelength range and sensitivity, while future space-based infrared observatories like the James Webb Space Telescope are not able to perform quick-turnaround observations of nearby NEOs). Hence, only a space-based thermal-infrared survey is capable of meeting the requirement of obtaining a diameter/mass estimation. A major advantage of an infrared space-based system is its ability to provide the 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. Visible, ground-based surveys are also compromised by the day-night cycle and weather, as compared to space-based surveys. 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.5 (See Appendix C for a summary table of advantages and disadvantages of ground- and space-based options for infrared and visible observations of NEOs.)
The committee found that in-space infrared telescopes
- Are more effective at detecting NEOs than visible wavelength in-space telescopes,
- Provide diameter information that visible wavelength telescopes cannot provide, and
- Do not cost significantly more than in-space visible wavelength telescopes (a primary driver of space telescope cost is aperture).
5 G.H. Stokes, 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, p. 187.
Although ground-based visible telescopes can be significantly less expensive than space telescopes, currently existing and planned visible ground-based telescopes (such as the Large Synoptic Survey Telescope [LSST]) cannot accomplish the goals of the George E. Brown Act. The committee heard from experts on LSST that in 10 years LSST would be 50-60 percent complete for NEOs with an absolute magnitude (H) of less than 22. When combined with other search efforts, this would be approximately 77 percent.6
Recommendation: If the completeness and size requirements given in the George E. Brown, Jr. Near-Earth Object Survey Act are to be accomplished in a timely fashion (i.e., approximately 10 years), NASA should fund a dedicated space-based infrared survey telescope. Early detection is important to enable deflection of a dangerous asteroid. The design parameters, such as wavelength bands, field of view, and cadence, should be optimized to maximize near Earth object detection efficiency for the relevant size range and the acquisition of reliable diameters.
For more than a decade, NASA has provided technology development funding for a space-based, passively cooled, thermal-infrared telescope designated NEOCam, but has not pursued this project to full-scale development. The committee heard from representatives from NEOCam. The committee also heard from a representative from
6 “LSST’s Projected NEO Discovery Performance,” Steve Chesley & Peter Vereš, Briefing to NAS Committee on Near Earth Object Observations in the Infrared and Visible Wavelengths, Irvine, California, February 25, 2019.
NASA Goddard Space Flight Center who had proposed alternative space-based telescope projects and a representative from the Jet Propulsion Laboratory who is proposing a small satellite (SmallSat) telescope constellation. Proposed alternatives include visible wavelength ground- and space-based telescopes and SmallSat constellations. The committee concluded that, at the moment, none of these alternatives is competitive with a thermal-infrared space telescope in terms of detection capabilities or cost.
To date, opportunities for a space-based NEO survey telescope have been primarily available via the Discovery program. However, Vision and Voyages for Planetary Science in the Decade 2013-2022 (the 2011 planetary science decadal survey), a report that prioritizes the planetary science program and exerts great influence on the selection of Discovery mission proposals, explicitly does not address “issues relating to the hazards posed by near Earth objects and approaches to hazard mitigation.”7 As a result, there is a bias against selection of planetary defense-focused missions in this program or any other program without an explicit planetary defense component.
Recommendation: Missions meeting high-priority planetary defense objectives should not be required to compete against missions meeting high-priority science objectives.
CURRENT NASA NEO SURVEY EFFORTS
NASA currently funds several ground-based telescopes for NEO detection, including the Catalina Sky Survey, Pan-STARRS, among others. It also funds the space-based NEOWISE spacecraft, which will likely not operate much longer (possibly less than 1 year). No existing ground- or space-based platform can satisfy the size and completeness requirements of the George E. Brown Act goals in the foreseeable future. A new, dedicated survey mission is required to achieve the George E. Brown Act goals.
The LSST, which is expected to enter into operation in 2023, has—in addition to a number of astrophysics missions—the mission to detect solar system objects and NEOs at a higher rate than current ground-based telescopes. However, LSST will not achieve the George E. Brown Act goals even after a decade. Contrary to what was expected when LSST was still in a concept phase, even a dedicated LSST optimized for NEO detection would not achieve the George E. Brown Act goals for several decades. The committee heard from representatives of LSST about its capabilities for NEO detection and concluded that, even though it cannot meet the completeness goal at the appropriate time, it would be useful for NASA to fund work to discover NEOs in the LSST archive as a complement to other methods.
Observation by ground-based systems equipped with specific instrumentation is necessary for subsequent characterization of NEOs after discovery.
Recommendation: If NASA develops a space-based infrared near Earth object (NEO) survey telescope, it should also continue to fund both short- and long-term ground-based observations to refine the orbits and physical properties of NEOs to assess the risk they might pose to Earth, and to achieve the George E. Brown, Jr. Near-Earth Object Survey Act goals.
ARCHIVAL RESEARCH AND CATALOGUING NEOS
Archival research can and has played an important role in detecting and characterizing NEOs. Archiving all data and images to support future improved thermal modeling, searching for serendipitous “precovery” observations (i.e., NEOs that were imaged but not noted at the time, but are located when data is later reviewed), and other types of studies not considered during the survey mission are critical to detecting and characterizing NEOs. The current system for archiving NEO data is not optimized for accessing data and analyzing data in an automated fashion. As new systems become operational, such as LSST and a space-based infrared telescope, this will become a more pressing issue.
7 NRC (National Research Council), 2011, Vision and Voyages for Planetary Science in the Decade 2013-2022, Washington, DC: The National Academies Press, p. S-2.
Recommendation: All observational data, both ground- and space-based, obtained under NASA funding supporting the George E. Brown, Jr. Near-Earth Object Survey Act, should be archived in a publicly available database as soon as practicable after it is obtained. NASA should continue to support the utilization of such data and provide resources to extract near Earth object detections from legacy databases and those archived in future surveys and their associated follow-up programs.
There is currently no consistent NASA policy on archiving NEO survey data, especially images. Access to archived data is important for future threat evaluation and research by the general science and planetary defense community.
ORGANIZATION OF THIS REPORT
This report is divided into seven chapters. Chapter 1 provides an introduction and background, including an explanation of the recent policy history for planetary defense. Chapter 2 discusses the challenges of conducting planetary defense in terms of estimating key parameters for NEOs. Chapter 3 discusses current and near-term observation systems, which are primarily ground-based telescopes funded by NASA. Chapter 4 explains the advantages of space-based platforms and addresses infrared versus visual space-based telescopes in terms of capability and costs. Chapter 5 discusses techniques to obtain NEO sizes, a key factor in determining their mass and therefore their destructive potential if they impact Earth (and one of the components of the George E. Brown Act requirements for NEO survey and detection). Chapter 6 addresses the importance of archiving the large amounts of data generated by NEO survey systems. Last, Chapter 7 discusses some other relevant objects that are not part of the George E. Brown Act survey criteria, but that are nevertheless important for understanding the overall impact threat.