What is
CNEOS?

The Center for Near Earth Object Studies operates at NASA's Jet Propulsion Laboratory in Pasadena, California. It was established in 1998, following a Congressional directive to NASA to catalogue near-Earth objects (NEOs) larger than 1 kilometre. That mandate has since expanded to objects down to 140 metres.

CNEOS receives position measurements from ground-based observatories worldwide, computes orbital solutions, and projects each asteroid's trajectory forward and backward over long timescales. From those projections, it identifies every predicted close approach and calculates the probability that any future pass might result in an Earth impact.

The results feed into two automated monitoring systems - Sentry and Scout - and into the publicly accessible close-approach database. All of that data is also available programmatically through the NASA NeoWs API, which this tracker uses.

Sentry runs continuously. Every time an asteroid's orbital solution is updated - which happens whenever new observations come in - Sentry recalculates the probability of Earth impact over the next 100 years. It samples thousands of possible orbital paths consistent with the observations and counts what fraction lead to impact.

Most objects exit the Sentry risk table quickly. Additional observations narrow the orbital uncertainty until all sampled paths miss Earth. A few objects remain on the list for longer, but with probabilities well below one in a million. The highest impact probability of any current Sentry entry is roughly 1 in 600 for a small object - far below the threshold for planetary defence action.

Apophis was removed from the Sentry risk table in 2021, after radar observations during its 2021 pass definitively ruled out impact through at least 2068.

When a telescope detects a moving object, the observation goes to the International Astronomical Union's Minor Planet Center within hours. Scout picks up that report and runs a preliminary orbital calculation immediately - often within minutes of the MPC posting.

Scout's output tells observers whether the object is worth urgent follow-up. If the preliminary orbit shows a non-trivial impact probability, Scout issues a notice through NASA's planetary defence channels. Observers with time on large telescopes can then prioritise that object above routine targets.

The 2008 TC3 case - the first asteroid predicted to hit Earth before it arrived - predates Scout, but it is the kind of scenario Scout was designed to handle. With Scout running continuously today, a similar object would be flagged within an hour of discovery.