When I was growing up in the 70′s and 80′s, the opportunities available to the amateur for making useful scientific observations were somewhat limited. One could observe variable stars, record stellar occultations by the moon, planets, or asteroids, or hunt for comets and supernovae. While such activities will continue to appeal to backyard observers, large automated survey telescopes that can repeatedly image the entire night sky to extremely faint magnitudes will certainly change the role of the amateur and the potential for novel discoveries by those of us with more modest equipment. However, with future surveys promising to produce petabytes of data annually and the creation of massive catalogs containing hundreds of millions of objects, new and important discoveries await the desktop-bound amateur with a penchant for data analysis.
For example, it was widely reported last year that a group of undergraduate astronomy students out of the University of Washington went looking for supernova using images obtained by the Sloan Digital Sky Survey as part of a class project. What they found instead were 1,300 newly discovered asteroids. From the original press release:
“We started searching for supernovae using data from the second phase of the Sloan Digital Sky Survey and all these asteroids were in the way,” said Andrew Becker, a UW research assistant professor in astronomy. “We decided that rather than get frustrated by the asteroids we should do some science and note details about our observations. I kept asking the students what they had found and they kept saying, ‘More asteroids. No supernovae, but lots of asteroids.’”
This is a story that underscores the exciting opportunities for novel discoveries and scientific contributions available to the amateur as we enter the age of high-throughput digital astronomy. The story got me to thinking about what we know about asteroids, and the dangers they pose to life on Earth. Asteroids are small rocky objects revolving around the sun, mostly between the orbits of Mars and Jupiter in the so-called asteroid belt between 2.1 and 3.3 A.U from the sun. The majority of these have shallow orbital eccentricities. Over time, some of these asteroids have been purturbed gravitationally by Mars or more particularly Jupiter into more highly eccentric orbits.
Asteroid 243 Ida, imaged by the Galileo spacecraft
Of particular concern are the Near-Earth Asteroids (NEA’s) which cross the orbit of the Earth and thus have the potential for major devastation.
Some notable impacts from both recent and ancient history:
- Asteroid 2008 TC3 (October 7, 2008). The first case of an asteroid’s impact time and location being predicted in advance. The entry of the asteroid, estimated to be no more than 5 meters in diameter, generated a spectacular fireball over Northern Sudan. The event was even capture by satellite imaging. Energy release: 1 kiloton TNT.
- Hodges Meteorite. (1954) The only documented case of a human getting hit from a rock from outerspace. The 4 Kg meteorite crashed through the roof of 31-year old Ann Hodges of Sylacauga, Alabama, bouncing off her radio before striking her on the left hip. She was badly bruised. The radio was destroyed. Energy: 16 lbs TNT (personal rough estimate based on size), with the roof and radio absorbing the brunt of the impact!
- The Tunguska Event (1908) flattened 2000 sq kilometers of trees in a remote region of Siberia. It is believed to have been caused by an asteroid about 60 meters in diameter. Energy: 10 Megatons TNT.
- Meteor crater Arizona (50,000 years ago.) A 45-50 meter asteroid left a 1.2 km diameter crater and probably leveling everything within about 16 km (10 miles). Energy: 2.5 megatons TNT.
- Cretaceous-Tertiary Extinction Event (65.5 million years ago). A 10 km asteroid striking the Yucatan leading to the immediate extinction of the dinosaurs and 70% of the life on Earth. Energy: 100 million Megatons TNT.
NASA’s Jet Propulsion Laboratory (JPL) has an on-going program to study Near Earth Objects. There you can obtain orbital elements for some 5800+ catalogued NEAs. Of these, a little over 1000 are designated as potentially hazardous, having an MOID (minimum orbit intersection distance) of less than 0.05 AU (about 5 million miles).
The site also lists upcoming close approaches. For example, the asteroid 2008 XC1, a Tunguska-sized asteroid, will pass within about 1 million miles of the Earth, or about 4 times the distance to the moon in the next couple of days (Friday December 12, 2008).
You can also plot the orbit of any of the 437 thousand asteroids whose orbital elements have been catalogued. The figure below depicts Earth’s upcoming close encounter with 2008 XC1.
Although only about 1000 potentially hazardous asteroids (PHAs) have been identified, it has been estimated that there may be as many as 10-20,000 such objects awaiting discovery by larger more powerful survey telescopes such as the LSST due to come online in the coming decade.
So what can we do about mitigating the risk of death-by-asteroid? Obviously cataloging asteroids in order to assess the risk is a critical first step. To avert an imminent collision, a number of ideas have been proposed. Simply nuking the asteroid might actually be problematic. One particularly interesting idea involves the use of a “tugboat” spacecraft which would rendezvous with the asteroid and by applying a small but steady force, would physically nudge the asteroid just enough to avert disaster. In an article by Schweickart, Lu, Hut, and Chapman describing the concept, they note that with an asteroid with an orbital period of two years, a 1 cm per second velocity change would increase the period by 45 seconds, creating a delay of 225 seconds over 10 years, enough to insure a near miss, but a miss nevertheless (Scientific American, 2003). Of course, such a scheme underscores the importance of early detection with decades of advance notice.
So ultimately it turns out the the new generation of deep-sky surveys not only leads to unprecendented insights into the nature of the universe, but may even one day save the world!
Posted by John Rachlin 
