Finding an asteroid is only the first step. The harder question is what its orbit will look like in ten, fifty or a hundred years, and whether any of those futures includes Earth. NASA answers it with a small set of tools run by the Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory. This article explains what those tools do and how to read the risk ratings that sometimes make headlines.
Surveys and observatories around the world photograph the sky night after night. When software spots a point of light that moves against the stars, the position is reported to the Minor Planet Center, the international clearing house for asteroid observations. A new object does not get an official designation right away. First it needs follow-up observations from other telescopes, often within hours, to confirm that it is real and to pin down its path.
Every measurement has a small error, so an orbit is never a single line. It is a "cloud" of possible paths consistent with the data. Right after discovery the cloud is large. With each new observation it shrinks. Nearly everything in planetary defense follows from this one idea. For a deeper look at what counts as a near-Earth object, see our NEO explainer.
New candidates wait on the Minor Planet Center's NEO Confirmation Page until enough data arrive. NASA's Scout system watches that list continuously. It looks only a short way ahead, days to weeks, and asks whether any orbit in the cloud could bring the object close to Earth soon.
Scout's results are deliberately rough. With so few observations, its scores are meant to flag objects that deserve quick follow-up, not to give a rigorous probability. Most flagged objects fade from concern within a day or two as better data arrive. Scout is also designed to pick out the rare small object that is about to strike, a job that matters because tiny asteroids can be found only hours before they arrive.
Once an object is confirmed and has an orbit, monitoring passes to Sentry. According to NASA, Sentry examines the full range of possible orbits for each object and searches for Earth approaches over roughly the next 100 years. For every approach that could end in a collision, it estimates the probability. These "virtual impactors" are listed in the public Sentry Risk Table, so anyone can check them.
A newer version, Sentry-II, was developed to handle subtle effects such as the Yarkovsky effect, the tiny push from sunlight that slowly changes an asteroid's path, and to cope better with awkward cases. Independent European systems run by ESA and partner institutions do similar calculations, and the teams compare results.
The key habit to learn is this: an object on the risk table is not a prediction of impact. It is a list of cases that cannot yet be excluded. Most objects leave the table as their orbits improve. This is exactly what happened with Apophis, as described in Is Apophis Dangerous?
The Torino Scale was introduced at a 1999 conference in Turin, Italy, to communicate risk simply. It runs from 0 to 10 and combines two things: how likely an impact is, and how much damage it would do.
No object has ever gone above level 4. Apophis reached it briefly in late 2004. In early 2025, asteroid 2024 YR4 rose to level 3 when its estimated chance of hitting Earth in 2032 reached about 3%, and then fell back to zero as more observations came in. Its Earth impact was ruled out.
The Palermo Technical Impact Hazard Scale answers a different question: how does this risk compare with the background risk? The background is the average chance that an object of similar size or larger hits Earth over the same time span. The scale is logarithmic, so each step is a factor of ten.
Almost every object on the risk table has a strongly negative Palermo value. Astronomers use it to rank which objects deserve scarce telescope time, while the Torino Scale is for explaining risk to everyone else.
These tools are only as good as the discoveries feeding them. Large objects are mostly found, but many smaller ones are not, and they can still damage a city. That gap is why NASA is building the NEO Surveyor space telescope, which is meant to find more of them. If a real threat were ever found early, the next step would be deflection, covered in our planetary defense guide. For terms like asteroid and meteoroid, see this short glossary.
Apophis is now one of the best-tracked objects in the sky. The 2029 flyby will let scientists test and improve the very models described here. See the Apophis 2029 guide for details.
Facts in this article are drawn from public scientific sources. For primary information, see: