Contrary to the impression that classroom discussions might give, finding out whether or not an asteroid will hit the Earth is anything but trivial. I decided to take a look at some factors are involved in finding whether point A,the Asteroid, meets point E, the Earth.

Gravitational Perturbations: When determining the long-term trajectory of an interplanetary object, it’s very important to take gravitational influences into account, and not just the big players. On this Harvard site listing upcoming close approaches to the Earth, the writers note that “orbital solutions consider perturbations by eight major planets (Mercury to Neptune), three minor planets (Ceres, Pallas and Vesta) and treat the earth and the moon as separate perturbing bodies.” In other words, part of the computations for finding an object’s interplanetary trajectory involves accounting for gravitational forces from objects well under 1/10,000^th the mass of Earth (calculation based on mass of Earth and Pallas). And that’s for the “rough” calculations. Further calculations take into account the spin on the asteroid and smaller objects in the solar system. The equation of motion for the asteroid certainly has no small numbers of parameters.

Error: Unfortunately, empirical data and predictions are neither perfect nor complete. As you may have noticed in the past decade or so, media reports of our planet’s impending doom are a favorite standby on slow news days. Just for examples:

CNN in 1998
BBC in 2002
USA TODAY in 2004

While everybody enjoys a little sensational story, a passing look at the predictions shows that the scientific “conclusions” are anything but certain. For example, the BBC 2002 report notes the following:

“Dr Benny Peiser, of Liverpool John Moores University in the UK, told BBC News Online that ‘this asteroid has now become the most threatening object in the short history of asteroid detection’.”

Yet, further on, the report also notes that the possible error is “several tens of millions of kilometres”. The CNN 1998 report says that the asteroid in question “should pass just under 30,000 miles from Earth”, but also says that the estimate has a “margin of error of more than 180,000 miles”, so that the actual distance may be 7x as far away as predicted! The 2004 USA Today story also concluded some time later with the threat of the asteroid dropping to zero.

See no evil: So, making predictions on whether two point objects intersect on a scale of 30+ years is hard in something as large as the Solar System. Our best estimates include margins of error of hundreds of thousands of miles, and our models may be incomplete. Still, the greatest danger is not looking. According to the NASA Ames Research Center, when considering the amount of warning we will have of an incoming asteroid, “[in] almost all cases, we will either have a long lead time or none at all.” Even though initial trajectory calculations may be inaccurate, additional data and years of observation allow predictions that come closer to reality. They believe that investment in mapping out the rocks in our sky is certainly worth it, as the probability that we’ll be caught off guard approaches zero as we complete the mapping.

For more on asteroid tracking and such, check out NASA’s Near Earth Object Program site, which is regularly updated with events in this field.