Guide
Reading the asteroid close-approach list
Asteroids pass Earth all the time. The list shows which ones will pass closely in the coming weeks, how far away they will be and a rough size. It is a schedule, not an alarm.
Where the list comes from
The list is NASA and JPL's Close-Approach Data service. Distances there are in astronomical units, and times are in a time scale called TDB, which the app converts to UTC. We express distances as multiples of the Moon's average distance from Earth, about 384,400 km, because that is easier to picture. The unit is our choice, not JPL's. A distance of 1 means as far away as the Moon.
About the sizes
A size shown here is an estimate from the object's absolute magnitude, which is a measure of brightness, and an assumed reflectivity of 13 percent. With those assumptions, an absolute magnitude of 22 gives about 147 metres, which matches the figure NASA's CNEOS FAQ gives for objects smaller than about 150 metres. For most of these objects the real reflectivity is not known, so the true size can be quite different. We say "about" for that reason.
What a close approach means
A close approach is a pass, not an impact. The list is a schedule of where objects will be, and the app shows the uncertainty in the time that JPL gives. This page does not claim that any object on the list will hit Earth.
JPL's usage policy asks that its interfaces are not embedded directly in a website. We fetch the list on a schedule and serve our own copy, so the list can be a little behind JPL's.
Sources
- JPL Close-Approach Data API. The list of close approaches
- NASA CNEOS. NASA CNEOS, which the size rule of thumb comes from