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.
The LSST would make uniquely powerful contributions to the study of near-Earth objects (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 100 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.
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
For up-to-date news about the most threatening objects currently known, see JPL's NEO page.
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 100 m in size cause ground-level explosions in the giga-ton range. Such impacts would devastate whole countries. There is about a 1 percent chance that such a impact will occur in the next century. Assessment of the Potentially Hazardous Asteroid (PHA) population down to 100-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 100 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 throughput (A&Omega) 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 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. 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.
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. So large aperture is important. This instrument would be unique. Its utility is diminished if it cannot cover the entire visible sky several times per month as there would be no competing telescopes to cover its holes or even follow its discoveries. At the present time, systematic surveys like Spacewatch have difficulty finding all 21st magnitude near-Earth Asteroids.
LSST will be able to detect objects as faint as 24.5 in magnitude in a 30s visit, enabling it to detect 140m NEOs as far away as the Main Belt asteroids. Depending on survey strategies, LSST could detect between 60-90% of all PHAs larger than 140m in diameter.
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.