AUSTRALIAN SPACEGUARD SURVEY

Additional Information on Asteroids and Comets

Predicting a NEO's orbit

How difficult is it to predict the orbit of a newly discovered asteroid to determine if it is a threat to the Earth?

The recent excitement with asteroid 1997XF11 gives an indication of the difficulty of the task. Consider an asteroid which is 2 kilometres in diameter and has an orbital period of just over 1 year. The length of its orbit will be about 1,000 million km and in 30 years (the timescale of concern with 1997XF11) it will have travelled about 30,000 million km. Imagine this huge distance is equivalent to the circumference of the Earth (about 40,000 km) - our scaling factor is therefore about 800,000:1. On this scale the Earth is about the size of a house (15m) and the asteroid is the size of a small ant (a few millimetres), crawling along a straight line at 150 metres per hour (4km/day, 100km/month, 1,300km/year). We watch the ant crawling along for a while and need to determine whether it will hit our house by the time it has crawled right around the world (40,000km) some 30 years later. It is very important that the ant's bearing (trajectory) is measured precisely!

Now, just to make it more challenging, we are only allowed to watch the ant with a telescope from a distance of 150 km (assuming 120 million km between the Earth and the asteroid when it is first spotted).

For an idea of the damage that one "ant" can cause if it hits the "house" see this  table. Of course, we need to find and track more than 1,000 ants (estimated number of asteroids larger than 1 kilometre in our region of the solar system) and sometimes these pesky ants deviate from their path (due to the gravitational effects of the planets and some unpredictable effects such as outgassing). Finally, there are many more smaller "ants" that we also need to find and track.

Note: this analysis is not intended to trivialise the importance of the Spaceguard Program. On the contrary, it is intended to give some idea of the exceptional work that a handful of scientists are undertaking (or should be undertaking).

See also Tracking Asteroids - Astrometry

Books

• "Rogue Asteroids and Doomsday Comets" by Duncan Steel, John Wiley and Sons, 1995.
• "Hazards Due to Comets and Asteroids" by Tom Gehrels, ed.
• New "Carl Sagan's Universe" edited by Yervant Terzian and Elizabeth Bilson, Cambridge University Press, 1997. A collection of excellent papers by scientists at the Sagan Symposium - held to celebrate Carl's 60th birthday. Many of the papers refer to the influence of asteroid/comet impacts on the development (and future) of life on Earth. In particular, see the papers by Chris Chyba and David Morrison.
• Scientific American
• March 1996: "Collisions with Comets and Asteroids" by Tom Gehrels "Small rocky or icy bodies, left over from the formation of the planets, normally follow distant, stable orbits, but rare mischance can send one hurtling into the inner solar system. A leader of the Spacewatch team that tracks near-earth comets and asteroids describes their awesome beauty, the odds of a collision with our world and what could be done to prevent a cataclysm."
• October 1990: "What caused the mass extinction" by Walter Alvarez and Frank Asaro. At the time this article was written the Chicxulub Crater had not been discovered. The authors talk about the world-wide signs of a huge comet/asteroid impact at the KT* boundary and the search for a 150km impact crater (Chicxulub is the right age and is about 170km in diameter). The same issue contains an article "A volcanic eruption" in which Vincent Courtillot proposes that a massive volcanic eruption caused the extinctions. The interesting thing is that, 65 millions years ago the volcanic eruptions in the Indian sub-continent appear to be directly opposite Chicxulub and there could be a relationship.
• April 1990: "Impact cratering on Earth" by Richard Grieve.
• January 1982: "The mass extinctions of the late Mesozoic" by Dale Russell. Destined to be a historic piece. The detective story which led Walter Alvarez and his team to propose that an asteroid or comet wiped out the dinosaurs.
• "Rain of iron and ice : the very real threat of comet and asteroid bombardment" by John Lewis, Reading, Mass.(240 p.)
• "Impact!" The threat of Comets and Asteroids by Gerrit Verschuur.
• "Comet and Asteroid Impact Hazards on a Populated Earth" by John S. Lewis, Academic Press. It was released late in 1999. See review. If you have a copy of the software check out, and contribute to, the Users Page.

Last update 30 July 1998

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