This month we continue with Near Earth Asteroids (NEAs) and describe some projects to catalog and study them, after last month's discussion of the tale of 1997 XF11. The hazard due to NEAs (some of which are certainly extinct short-period comets traveling incognito) has been slow to dawn upon us, partly because large impacts are so rare, and also because even those that occur are likely to escape notice in the absence of global monitoring. Indeed, it was only in about 1806 that the very possibility of rocks falling out of the sky became scientifically respectable! The great Tunguska event of 1908 was not even universally recognized as due to impact until fairly recently, part of the confusion probably being due to the absence of any large associated crater. More recently, the collision of Shoemaker-Levy 9 with Jupiter in July 1994 has served to remind us that enormous impacts are still possible.
How many of these objects are there that could actually hit the Earth?. According to the IAU Minor Planet Center (MPC) at SAO, as of April 1998, there are about 113 "Potentially Hazardous Asteroids" (PHAs) known, based on distance of closest approach. These are a subset of some 261, the "Atens" and "Apollos", whose orbits cross ours. Unfortunately these numbers are biased in favor of larger sizes, and seriously incomplete even at the (catastrophic) 1 km level. It is estimated that roughly 2000 asteroids larger than 1 km (where the observations are believed to be incomplete by roughly a factor of 10) cross Earth's orbit. Based on extrapolations (across a vastly under-represented gulf of a factor of hundreds in linear size and millions in mass) from larger sizes, and also from studies of the distribution of lunar craters, until we reach impacts and fireballs which have actually been observed in historic times, it is believed that the corresponding numbers for 100 m and 10 m NEAs are roughly 500,000 and 100,000,000, respectively.
In any impact, the kinetic energy (energy of motion) is the most important single factor determining the effects, as this energy is rapidly released as heat and for collisions with the Earth is always large compared to an equal mass of high-explosive. The kinetic energy E is ½mv² for a body of mass m, moving at velocity v. This turns out to very nearly equal the energy of an equal mass of TNT for v = 3 km/s, and is proportional to the square of the velocity. Thus, a 1000 ton object arriving at the minimum possible velocity, 11.2 km/s (the escape velocity of the Earth) would very nearly equal the Hiroshima bomb (~13 kT) in energy. If it had the density of water, such a body need have a radius of only about 6 m. With the end of the Cold War, interesting data is being declassified that was collected by US Department of Defense and National Reconnaissance Office spacecraft, in connection with Nuclear Test Ban Treaty verification, early warning, and other intelligence-gathering activities. These data indicate that moderate sized impacts, with objects of < 30 m size, are not uncommon on a global scale. Figures of the order of one Hiroshima-size event per year, or possibly more, have been reported.
Two major projects are under way in the US to survey the sky for NEAs, the Near Earth Asteroid Tracking project (NEAT) at JPL, and the Spacewatch Project at the Lunar and Planetary Laboratory (LPL) of the University of Arizona at Tucson. Both projects have significant pure scientific objectives beyond the cataloging of potential impact threats. Of them, Spacewatch is the earlier, having begun operations in 1990. Under the leadership of Tom Gehrels and Robert McMillan of the LPL, it pioneered the use of CCDs to scan semi-automatically for comets and asteroids. Since beginning operation Spacewatch has achieved many "firsts", such as the first discovery of an NEA by computer, discovery of the smallest asteroid known (~6 m), the closest approach of any asteroid to Earth (105,000 km, 1994 XM1), and numerous other distinctions. It was also J. Scotti of the Spacewatch group who discovered 1997 XF11 last December. Spacewatch uses the old 0.9 m telescope on Kitt Peak, with a 2048 X 2048 CCD, a bandpass of 0.5-1.0 µ, an effective exposure time of about 2.5 min, and a field of view about 30 arc min (1" per pixel) on a side, resulting in a limiting magnitude of about 22 under good conditions. Observations are taken on 20 nights per lunation. A series of three exposures of the same region of the sky is taken in about 90 min, allowing the automatic detection of moving objects. Simultaneous real-time visual observation of the images is used to catch fainter fast-moving objects and re-observe them immediately, so that they are not lost. About a third of the known NEAs have been discovered by this project, once again showing what older, moderate-sized instruments can do when equipped with modern detectors and analysis software. A new 1.8 m Spacewatch telescope is currently under construction at the same site.
NEAT, which began operation in Dec. 1995, is a joint project between NASA and the US Air force, using a USAF fully-automatic, remote-controlled 1 m, f/2.2 Ground-based Electro-Optical Deep Space Surveillance (GEODSS) telescope on Maui in Hawaii, with a 4096 X 4096 CCD camera and electronics developed at JPL. At present, with a modern telescope, an excellent site (at 3,000 m), and a state-of-the-art detector system, NEAT seems mainly limited by access to the telescope, currently just 6 nights per lunation. NEAT's stated objective is to make possible the discovery of all the potential 1 km threats within about 10 years. Since the estimated mean time between impacts of this order of magnitude is around 100,000 years or more, the chance of our being struck without warning should thereby be reduced to the order of 1 in 10,000. The leader of the effort, Eleanor Helin, has long been one of the world's premier asteroid discoverers, previously using photographic plates with the Palomar 0.45 m Schmidt camera. It was two prediscovery plates of 1997 XF11, acquired by her group in 1990, that improved the orbit determination enough to show that it will unquestionably miss the Earth in 2028.
Last month I expressed the hope that false alarms like 1997 XF11 would help us to prepare ourselves for the real thing when it inevitably occurs. Deflection of even a 1 km asteroid is certainly within our capabilities at this point, given sufficient warning. Nudged along by the 1997 XF11 experience, NASA just this week announced formation of a new program office to coordinate US efforts to survey the sky for PHAs, along with its intention to approximately double the funding devoted to the effort.
A number of WWW sites are available concerning NEAs.
For those interested in making current astrometric observations
(which would be especially useful in the southern hemisphere), or
obtaining information on the orbits of cataloged objects, the
Minor Planet Center is essential, at:
http://cfa-www.harvard.edu/cfa/ps/mpc.html.
Dave Morrison of NASA Ames maintains a useful clearinghouse of impact-related
information at:
http://neo.arc.nasa.gov.
The Spacewatch Project is described at:
http://www.lpl.arizona.edu/spacewatch,
while the JPL/USAF NEAT project is at:
http://huey.jpl.nasa.gov/~spravdo/neat.html.
Finally, the Spaceguard Foundation is an international organization
"aimed at the protection of the Earth environment against bombardment
of objects in the solar system":
http://cfa-www.harvard.edu/~marsden/SGF/
They have a particularly good collection of links at:
http://cfa-www.harvard.edu/~marsden/SGF/links.html.