This picture of asteroid 951 Gaspra is a mosaic of two images taken by the Galileo spacecraft.
Image Credit: NASA/JPL
The questions of how the solar system came into being and how life on Earth might end are two of the "nearest" astronomical issues to mankind. Answering the former requires as an initial step a census of the solar system. But identifying objects in the outer solar system has proven to be a difficult challenge despite some recent success. Answering the latter, at least in the case of a possible asteroid impact, is not strictly a scientific question but might be the most important contribution astronomy makes to life on Earth. Although the odds of a significant impact are slight, the consequences are grave and it would be negligent to ignore capabilities that enable us to learn more about the possibility with relatively modest effort. Indeed, a comprehensive census of potentially hazardous asteroids (PHA) will transform the problem from a statistical one to a deterministic one. Click here for a more detailed discussion NEO Detection, Characterization, and Orbits.
A recent National Research Council report titled *Review of Near-Earth Object Surveys and Hazard Mitigation Strategies *found that LSST is the most cost effective way of surveying the most likely and potentially most damaging Earth threatening objects: here
Ground-based optical surveys are the most efficient tool for comprehensive near-Earth object (NEO) detection, determination of their orbits and subsequent tracking. A survey capable of extending these tasks to PHAs with diameters as small as 100 m requires a large telescope, a large field of view and sophisticated data acquisition, processing and dissemination system. A 10 m-class telescope is required to achieve faint detection limits quickly, and together with a large field of view (~10 square degrees), to enable frequent repeated observations of a significant sky fraction -- producing tens of terabytes of imaging data per night. In order to recognize PHAs, determine their orbits and disseminate the results to the interested communities in timely manner, a powerful and fully automated data system is mandatory. See Ivezic et al. 2006, LSST: Comprehensive NEO Detection, Characterization, and Orbits, (astro-ph/0701506) [PDF].
The LSST is currently by far the most ambitious proposed ground-based optical survey. With initial funding from the NSF, DOE and private sponsors, the design and development efforts are well underway at eighteen institutions, including top universities and leading national laboratories. In addition to dark energy and matter, the Milky Way mapping and transient Universe, the Solar System mapping is one of the four key scientific design drivers, with emphasis on efficient PHA detection and orbital determination.
The LSST would make uniquely powerful contributions to the study of NEOs, which include both asteroids orbiting the Sun (near-Earth asteroids) and comets arriving from the outer solar system. While the frequency of NEO impacts is exceptionally low, the damage they can cause is immense. A 300 meter diameter asteroid impact would be equivalent to 1600 megatons of TNT. In an ocean basin, the resulting tsunami could devastate coastal margins. On the other hand, our ability to predict such events, using available technology, is higher than for any other form of natural disaster.
The LSST system will be sited at Cerro Pachón in Chile, and will provide digital imaging of faint astronomical objects across the entire sky, night after night. In a relentless campaign of pairs of 15 second exposures of its 3,200 megapixel camera, LSST will cover the entire available sky every three nights in two photometric bands to a depth of V=25 per visit, with exquisitely accurate astrometry and photometry. Over the proposed survey lifetime of 10 years, each sky location would be observed over 2000 times. The baseline design satisfies strong constraints on the cadence of observations mandated by PHAs such as closely spaced pairs of observations to link different detections and short exposures to a void trailing losses. Equally important, due to frequent repeat visits LSST will effectively provide its own follow-up to derive orbits for detected moving objects.
What Hazards Do Solar System Objects Present?
Cosmic impact has the potential to eliminate humankind as we know it. Therefore, it is critical for us to systematically assess the magnitude of these threats. The atmospheric, geological, and biological effects of cosmic impact have become apparent only since the early 1980s, when the likely cause of the Cretaceous-Tertiary extinction was first linked to the impact of a 10-km asteroid. Even much smaller impactors still possess enormous energies and may cause local to regional devastation. At Congress's direction, NASA has supported a groundbased program to identify the NEOs larger than 1 km in diameter. This task is about 50 percent complete, with estimates for the date of completion ranging from 2010 to 2020 and beyond. The kilometer-sized impactors would be globally devastating, but much smaller projectiles would wreak unimaginable local havoc and are much more frequent. The high-altitude explosion of an 80-m-diameter body above Tunguska, Siberia, in 1908 flattened trees over a broad area. A differently aimed impact of this scale could flatten a modern city, with deaths in the millions. Bodies larger than about 300 m in size cause ground-level explosions in the gigaton range. Such impacts would devastate whole countries. There is about a 1 percent chance that such an impact will occur in the next century. Assessment of the Potentially Hazardous Asteroids (PHAs) population down to 300 m scales, as part of an organized inventory of the small bodies of the solar system, is recognized as a high priority for NASA's Solar System Exploration program. Extrapolations from existing surveys suggest that the number of PHAs larger than 300 m is on the order of 10,000 to 20,000. These bodies are too faint to have been detected by the current surveys, and almost all remain undetected. For each object, we need to determine the orbital elements with accuracy sufficient to predict the probability of terrestrial impact within the next 100 years. This time scale gives sufficiently early warning for the development of mitigation strategies, as needed, and is compatible with the intrinsic time scale for dynamical chaos among the PHAs. For those objects with a non-negligible impact probability, we also need physical observations to determine the size, which, when combined with a "typical" density yields an estimate of the kinetic energy of the projectile. These goals can be achieved with the Large Synoptic Survey Telescope (LSST).
The discovery of near-Earth Asteroids is time critical and coverage dependent. Both of these are enhanced by high throughput (etendue). With current searches, smaller asteroids are detected only as they get close to the Earth. They have a higher apparent rate of motion as they get close. Any survey with a brighter limiting magnitude will only get a few, since the population is not sampled deeply. The LSST is important because it can catch these asteroids farther out. They are moving slower and are easier to catch using automated software. LSST would sample a larger volume of space for them and so catch a greater number each time we look. Finally, high throughput (etendue) enables more frequent observation of all NEO candidates, permitting orbit solutions of high accuracy. Both throughput and telescope aperture are important for a systematic survey to find nearly all of them. Even 100 meter PHAs could cause significant damage, and there are far more of them. Per unit time, completeness of a NEO survey is related to throughput (etendue); with its 8.4m mirror and ten square degree field of view, LSST has an effective etendue of 320 square meters square degrees. The next largest proposed facility, PanSTARRS-1, has an etendue 24 times smaller. However, software developed for PS-1 will be useful in the LSST survey.
The 100 meter limiting size for significant near-Earth asteroids corresponds to a limiting magnitude of 26. In addition, short exposures are needed since the asteroids trail very quickly at more than 20 second exposures. The discovery of near-Earth Asteroids is time critical and coverage dependent. Both of these are throughput (AΩ) constraints. Today, smaller asteroids are detected only as they get close to the Earth. They have a higher apparent rate of motion as they get close. Any survey with a brighter limiting magnitude will only get a few, since the population is not sampled deeply. The LSST can catch these asteroids farther out. LSST would sample a larger volume of space for them and so catch a greater number each time we look. Both throughput and telescope aperture are important for a systematic survey to find nearly all of them. Even 100 meter NEOs could cause significant damage, and there are far more of them. With its capability to detect moving objects as faint as 25th magnitude, only the LSST will be able to find virtually all significant PHAs 140 meters in size and over 50% of all NEOs 100 meters in size.
The detailed modeling of LSST operations by A. Harris and E. Bowell, using real historical weather and seeing data from Cerro Pachn, shows that LSST could find about 90% of the PHAs with diameters larger than 250 m, and over 50% of those greater than 100 m within ten years. However, this is based on observing in only one filter using the default cadenence. During its survey of the sky, LSST can find 90% of the PHAs over 140 meters in diameter.
It is unlikely that any survey with hardware inferior to that planned for LSST, or less sophisticated and robust data processing system, will be capable of fulfilling the Congressional mandate. It is fortunate that the same hardware, software and cadence requirements are driven by science unrelated to NEOs. Hence, a system like LSST reaches the threshold where a single wide-deep-fast survey achieves seemingly disjoint, but deeply connected, goals.
Advance Warning Capability
LSST will be capable of early detection as well as orbit determination. The warning time before impact depends on the asteroid's size, its orbit, and the cadence and sensitivity of the observing system. For 45m objects, the LSST warning time would be about 1-3 months, depending on their orbits. Note that LSST would also detect such an object during three prior close approaches. As an example of a very different hazardous object - the 3 km large comet C/1996 B2 Hyakutake, which passed within 0.10 AU from Earth in 1996, LSST could provide a warning time of 8 years, with over 500 observations over that period. See attached [PDF].