From discovery to defense

Almost every asteroid discovery still begins the way the first one, Ceres, did in 1801: with a point of light that moves against the background stars.^[1] What follows is a chain of telescopes, a clearinghouse and automated orbit checks that turns that point into a named asteroid with a known path, and, if a real threat ever appeared, two international groups set up to coordinate a response.

Surveys find a moving point

NASA's Near-Earth Object Observations Program funds wide-field survey telescopes that image large parts of the sky every clear night, among them the University of Arizona's Catalina Sky Survey and the University of Hawaii's Pan-STARRS and ATLAS.^[2] A survey takes three or more images of the same field several minutes apart and looks for anything that shifts position between them.^[1] Each candidate is checked against the predicted positions of every known asteroid and comet; if it matches none, it may be new.^[1]

The Minor Planet Center and the confirmation page

Survey teams send the measured positions of a possible new object to the Minor Planet Center (MPC), the internationally agreed public archive for small-body observations, sanctioned by the International Astronomical Union.^[2] A candidate that could be a near-Earth object is posted on the MPC's NEO Confirmation Page under a temporary label, so that observers elsewhere can look for it.^[1] The page lists each object with a score for how likely it is to be a near-Earth object, its discovery date and its rough position and brightness.^[3]

Follow-up observers, professional and amateur, then find the object again and report new positions.^[2] It usually takes two to three nights of observations to confirm a near-Earth object.^[1] Once confirmed, it leaves the confirmation page and receives a permanent designation that starts with its year of discovery, followed by a code for the half-month of discovery and its order within it, as in 2024 YR4.^[1]

Scout: the first hours

While an object is still unconfirmed, NASA's Scout system at the Center for Near-Earth Object Studies (CNEOS) watches the confirmation page and works out the range of paths it could be on.^[4] For each object, Scout rates the chance of an Earth impact as negligible, small, modest, moderate or elevated, but only once there are at least three observations spanning 20 minutes or more.^[4] Its ratings are meant to flag interesting objects quickly rather than give a precise probability.^[4]

Scout has called real impacts. On 11 March 2022, a 2 m asteroid, 2022 EB5, was first reported two hours before it struck the atmosphere; from 14 observations over 40 minutes, Scout identified it as an impactor, and it entered the atmosphere southwest of Jan Mayen, in the Norwegian Sea, at the predicted time.^[5] At the time, it was only the fifth asteroid ever seen in space before hitting Earth; the first was 2008 TC3, which broke up over Sudan in October 2008.^[5]

Sentry: the next century

Once an object has an official orbit, it passes to Sentry, which receives new observations and orbits from the MPC every day and checks each near-Earth asteroid for possible impacts over at least the next 100 years.^[6] CNEOS also computes the precise orbits of near-Earth objects and, if an impact were predicted, its time and location.^[2] Planetary radar, such as the Goldstone Solar System Radar, can sharpen an orbit further when an asteroid passes within a few million miles.^[2] How Sentry rates a possible impact is explained in Impact risk.

IAWN and SMPAG: if a threat is real

Two international groups take over if the risk becomes serious. Both were formed after a recommendation to the United Nations, and both held their first meetings in 2014.^[7]

• The International Asteroid Warning Network (IAWN), coordinated by NASA, is a worldwide collaboration of asteroid observers, analysts and modellers. If an asteroid is found on a collision course, IAWN assesses the impact time, location and severity, and informs governments through the UN.^[7]
• The Space Mission Planning Advisory Group (SMPAG), chaired by ESA, is a forum of the world's space agencies. It assesses whether spacecraft could study, deflect or destroy an incoming asteroid larger than 50 m with an impact probability above 1%, and advises decision-makers on the options.^[7]

In January 2025, asteroid 2024 YR4 was estimated to be larger than 50 m with an impact chance above 1%, which met the criteria for both.^[8] SMPAG prepared to meet and, if the chance stayed above 1%, to give recommendations to the UN.^[8] Instead, new observations lowered the chance of impact to 0.004% within a month.^[9] The tools for acting on a real threat are covered in Planetary defense.