Planetary Defence

How NASA tracks
dangerous asteroids

Planetary defence is the scientific and engineering effort to find potentially hazardous asteroids before they find us - and to have options ready if one ever needs to be deflected. Here is how the system works.

How asteroids are discovered →

The detection network

Finding asteroids requires scanning large areas of sky repeatedly and identifying objects by their motion. Several programmes do this on a nightly basis.

Catalina Sky Survey, operating from the Catalina Mountains in Arizona, is one of the most productive. Pan-STARRS, based on Haleakala in Hawaii, uses a 1.8-metre telescope with an unusually wide field of view. The ATLAS network operates from multiple sites and is designed specifically for rapid detection of objects that could arrive with little warning.

Each discovery is reported to the Minor Planet Center, the international clearinghouse for solar system object data, operated at the Smithsonian Astrophysical Observatory. The Minor Planet Center assigns a provisional designation and coordinates follow-up observations to confirm and refine the orbit.

NASA's Center for Near Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory then assesses whether the object qualifies as a near-Earth object (NEO) and runs impact probability calculations. Any non-zero probability appears in the Sentry risk table, which is updated automatically as new data arrive.

DART: the first deflection test

The Double Asteroid Redirection Test (DART) launched in November 2021 and struck Dimorphos - a 160-metre moonlet of the binary asteroid Didymos - on 26 September 2022 at a closing speed of approximately 22,000 km/h.

The impact shifted Dimorphos's orbital period around Didymos by 33 minutes - reducing it from 11 hours 55 minutes to 11 hours 22 minutes. The minimum mission success target had been 73 seconds. The actual result was more than 25 times that.

ESA's Hera mission is due to arrive at the Didymos system in 2026 to characterise Dimorphos's altered orbit in detail, providing data to refine models of kinetic impactor effectiveness for future use.

Deflection methods

The right approach depends on asteroid size, composition, and - most critically - how much warning time is available.

Kinetic impactor Demonstrated (DART, 2022) Lead time needed: Decades

A spacecraft rams the asteroid at high speed, changing its velocity by a fraction of a millimetre per second. Small changes applied early produce large trajectory shifts by the time of closest approach.

Gravity tractor Theoretical - feasible with current technology Lead time needed: Decades

A spacecraft hovers near the asteroid and uses mutual gravitational attraction to slowly alter its course. Requires no physical contact. Works for smaller objects given enough time.

Nuclear standoff Theoretical - rapid deflection option Lead time needed: Years to decades

A nuclear device detonated near (not on) the asteroid vaporises surface material, creating a propulsive effect. Effective for larger objects or shorter warning times than other methods.

Civil defence Last resort for short warning times Lead time needed: Days to months

If deflection is not possible, evacuation of the predicted impact zone. Effective only for localised impacts from smaller objects. Cannot address global-scale events.

Current status

NASA's planetary defence goal - set by the 2005 George E. Brown Jr. Near-Earth Object Survey Act - is to catalogue all near-Earth objects 140 metres or larger. NASA estimates it has found more than 95% of NEOs in the 1-kilometre-plus range and is working through the 140-metre category.

The Nancy Grace Roman Space Telescope, planned for launch this decade, will substantially advance the catalogue of smaller objects. NEO Surveyor, a dedicated infrared space telescope for planetary defence, is in development at JPL and will detect objects that are difficult to spot from the ground - particularly those approaching from the direction of the Sun.

None of the currently known near-Earth objects have a meaningful probability of impacting Earth in the next century.

Will an asteroid hit Earth?

Current risk summary and historical context.

How NASA discovers asteroids

The telescopes and software behind every new discovery.

Danger ratings explained

The Torino Scale and Palermo Scale described.

The Apophis 2029 flyby

The most-watched asteroid event of the coming decade.

Common questions

What is planetary defence?

Planetary defence refers to the scientific and engineering efforts to detect, track, and if necessary deflect near-Earth objects (NEOs) that could pose a collision threat. It encompasses survey telescopes that find new objects, orbital mechanics centres that calculate trajectories and impact probabilities, and mission concepts for changing an asteroid's course. NASA's Planetary Defense Coordination Office (PDCO) coordinates US activity; the European Space Agency and other agencies contribute globally.

How does NASA track potentially dangerous asteroids?

NASA funds a network of ground-based survey telescopes that scan the sky repeatedly for moving objects. The largest contributors are Catalina Sky Survey in Arizona, Pan-STARRS in Hawaii, and the ATLAS network. Each discovery is reported to the Minor Planet Center in Cambridge, Massachusetts, which assigns a provisional designation. NASA's Center for Near Earth Object Studies (CNEOS) then computes the orbit and assesses impact probability.

What is the DART mission?

The Double Asteroid Redirection Test (DART) was NASA's first full-scale planetary defence test. In September 2022, the DART spacecraft deliberately crashed into Dimorphos - a 160-metre moonlet of the binary asteroid Didymos. The impact changed Dimorphos's orbital period around Didymos by 33 minutes, significantly more than the minimum target of 73 seconds. The mission proved that a kinetic impactor can measurably alter an asteroid's trajectory.

Could we deflect an asteroid if one were heading for Earth?

With sufficient lead time - ideally decades - yes. The DART mission demonstrated that a kinetic impactor can change an asteroid's velocity, which over time translates into a large positional change. Other techniques include a gravity tractor (a spacecraft that flies alongside the asteroid and uses gravitational attraction to nudge it slowly), and for extreme cases, a nuclear standoff detonation. The key variable is warning time: the earlier a threat is confirmed, the more options are available.

How much warning would we have before an asteroid impact?

For large, well-studied objects, NASA can project close approaches decades ahead. Apophis, for example, was identified 24 years before its 2029 flyby. Smaller objects can be discovered much later - sometimes days before they pass. The goal of survey programmes is to find all potentially threatening objects as early as possible. For any object in the current risk catalogue, the lead time is measured in years to decades, not days.