Topic summary
Asteroid impact prediction
Asteroid impact prediction is the prediction of the dates and times of asteroidsimpacting Earth, along with the locations and severities of the impacts.
The process of impact prediction follows three major steps:
- Discovery of an asteroid and initial assessment of its orbit which is generally based on a short observation arc of less than 2 weeks.
- Follow-up observations to improve the orbit determination
- Calculating if, when and where the orbit may intersect with Earth at some point in the future.
The usual purpose of predicting an impact is to direct an appropriate response.
Most asteroids are discovered by a camera on a telescope with a wide field of view. Image differencing software compares a recent image with earlier ones of the same part of the sky, detecting objects that have moved, brightened, or appeared. Those systems usually obtain a few observations per night, which can be linked up into a very preliminary orbit determination. This predicts approximate positions over the next few nights, and follow-ups can then be carried out by any telescope powerful enough to see the newly detected object. Orbit intersection calculations are then carried out by two independent systems, one (Sentry) run by NASA and the other (NEODyS) by ESA.
Current systems detect an arriving object only when several factors align, mainly its direction of approach relative to the Sun, the weather, and the Moon's phase. Smaller objects are less likely to be detected before impact. A few near misses by medium-size asteroids have been predicted years in advance, with a tiny chance of striking Earth, and a handful of small impactors have successfully been detected hours in advance. All of the latter struck wilderness or ocean, injuring no one. The majority of impacts are by small, undiscovered objects. They rarely hit a populated area, but can cause widespread damage when they do. Detection of smaller objects is improving as existing systems are upgraded and new ones come online, but all current systems have a around the Sun that can be overcome only by a dedicated space-based system or by discovering objects during an earlier approach to Earth, years before a potential impact.