page_title: Asteroid impact prediction
section_title: Impact probability calculation pattern
hierarchical_section_title: Asteroid impact prediction / Impact calculation / Impact probability calculation pattern
caption_attribution_description: This figure shows why asteroid impact probability goes up, then down.
context_page_description: Asteroid impact prediction is the prediction of the dates and times of asteroids impacting 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.
  In addition, although not strictly part of the prediction process, once an impact has been predicted, an appropriate response needs to be made.
  Most asteroids are discovered by a camera on a telescope with a wide field of view. Image differencing software compares a recent photograph 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 up can then be carried out by any telescope powerful enough to see the newly detected object.
context_section_description: The ellipses in the diagram on the right show the predicted position of an example asteroid at closest Earth approach. At first, with only a few asteroid observations, the error ellipse is very large and includes the Earth. Further observations shrink the error ellipse, but it still includes the Earth. This raises the predicted impact probability, since the Earth now covers a larger fraction of the error region.  Finally, yet more observations (often radar observations, or discovery of a previous sighting of the same asteroid on archival images) shrink the ellipse revealing that the Earth is outside the error region, and the impact probability is near zero.
  For asteroids that are actually on track to hit Earth the predicted probability of impact continues to increase as more observations are made. This initially very similar pattern makes it difficult to quickly differentiate between asteroids which will be millions of kilometres from Earth and those which will actually hit it. This in turn makes it difficult to decide when to raise an alarm as gaining more certainty takes time, which reduces the time available to react to a predicted impact. However raising the alarm too soon has the danger of causing a false alarm and creating a Boy Who Cried Wolf effect if the asteroid in fact misses Earth.
  In December 2004 when Apophis was estimated to have a 2.7% chance of impacting Earth on 13 April 2029, the uncertainty region for this asteroid had shrunk to 83000 km.
caption: Why predicted asteroid impact probability often goes up, then down.
exif: {}