It’s a funny thing about homing in on an asteroid and calculating its path, future position, and probability of impacting Earth – it will often appear risky during initial observations, get riskier, and then suddenly become entirely safe.
In the case of an asteroid on a definite collision course, the risk would keep growing until it reaches 100%. Fortunately, in most cases, the risk of impact ultimately flattens before rapidly getting down to zero – but why? Does this suggest our results are uncertain? Can we really be sure asteroids removed from ESA’s ‘risk list’ are safe?
The very first observation of an asteroid is ‘just’ a single dot of light in the sky. At this point, it’s not clear what it is or where it’s going. A second observation is needed to reveal an object in motion, and it is generally agreed that at least three are needed to determine an orbit – how quickly our asteroid is going and where it is headed. Further observations refine the orbit a little more, reducing uncertainties until we can be sure of where it won’t go: to Earth.
At first, the future position of an asteroid is uncertain and so the “risk corridor” is a wide tunnel through which the asteroid could fly at any point. When any part of the corridor overlaps with Earth, the asteroid is considered a threat.
As is often the case, the overlap with Earth remains even while the potential corridor gets smaller due to more observations and a more accurate understanding of the asteroid’s path – and so the risk appears to increase.
More often than not, as the hazard zone narrows with more observations, the corridor moves off Earth and the risk suddenly drops. Even if some uncertainty remains about the path of an asteroid, we can know for sure it doesn’t pose a risk.