Why we have Asteroid “Scares”
When an asteroid is discovered it takes a number of observations over a period of time before its orbit around the Sun can be calculated; the longer the time between observations the longer the observed “arc” and the more accurate the orbit that can be calculated.
The initial orbit of a newly discovered near Earth asteroid will be pretty rough, with lots of uncertainties. It will be possible to work out roughly when it will next cross the Earth’s orbit, but not exactly where; all that can be said is that the asteroid will fly through an oval “target” known as the “error ellipse”.
If the Earth lies inside this ellipse, then there is an impact probability. This can be calculated by comparing the area of the Earth’s disk with the area of the ellipse.
As more observations are made over time the uncertainties decrease as more and more accurate orbits are calculated.
The error ellipse
gets progressively smaller, and eventually the Earth ends up outside the ellipse
– there is no possibility of a collision.
It is not always necessary to wait for future observations to extend the arc.
It is sometimes possible to calculate the orbit backwards in time and then search
archive photographs of the relevant patch of sky for signs of the asteroid.
If it is found then the arc can be extended enormously, and a very precise orbit
calculated. This process is called “precovery”.
So, it is quite possible, even normal, that the probability of a collision will rise as the error ellipse gets smaller and smaller, and then suddenly drops to zero when the Earth is no longer inside the ellipse.
It is only if the Earth remains in error ellipse until the ellipse is smaller than the diameter of the Earth that we can say for sure that a collision will occur.
Therefore, as we have seen with a number of recent events, the probability of an impact starts quite low (when the error ellipse is large) then gradually rises as the ellipse shrinks with more observations – the Earth occupies more and more of the area of the ellipse.
Eventually, often with data from precovery work, the ellipse shrinks so much that the Earth is outside, and the impact probability disappears completely.
When an asteroid is first discovered the information is immediately published on the web to help astronomers to make “follow-up” observations to calculate the orbit. Results of these observations are also freely available on the Internet, but can easily be misinterpreted (especially the impact probability figures) by people who don’t understand the process.
In the past scare stories have been generated by orbital data that is very uncertain, and which has been quickly refined (often within a few hours) removing any possibility of impact on the Earth.
However, most science reporters and communicators are now well aware of what goes on, and unnecessary scares should become rare events.