Planetary Defence

NASA Sentry -
automated impact monitoring

Sentry runs around the clock at the Jet Propulsion Laboratory, scanning every known near-Earth asteroid's orbit for potential Earth impacts over the next century. When a new asteroid is discovered anywhere in the world, Sentry analyses it automatically.

CNEOS explained →

What Sentry is

Sentry is an automated collision monitoring system operated by NASA's Center for Near Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory in Pasadena, California. It runs continuously, scanning the orbital solution for every known near-Earth asteroid (NEO - a body with an orbit that brings it within 1.3 AU of the Sun, where one AU is approximately 150 million kilometres) and computing the probability that any of them will collide with Earth over the next 100 years.

The results are published in real time on the CNEOS website as the Sentry risk table. Any object with a non-zero impact probability for any date in that window appears on the list. When the probability reaches zero - as it does for most objects after additional observations - the object is removed automatically.

Sentry replaced an earlier manual process in the early 2000s. The volume of new asteroid discoveries - now running into tens of thousands per year - makes automated analysis essential. A human analyst reviewing each new orbit by hand could not keep pace.

How Sentry works

When an asteroid is discovered, observers measure its position on multiple nights. Those measurements are never perfect - each one carries small observational errors from atmospheric distortion, instrument precision limits, and timing uncertainties. A single night's observations cannot pin down an orbit precisely.

Sentry represents the orbital uncertainty as a statistical distribution: a cloud of possible orbits, each one consistent with the available observations and their associated errors. It then propagates every orbit in that cloud forward through time, computing where the asteroid could plausibly be on every future date.

If a fraction of those possible orbits intersect Earth at some future date, that fraction is the impact probability. An object with 100,000 possible orbits, 10 of which hit Earth on a given date, has a 0.01% probability for that date. Subsequent observations narrow the cloud. As the range of possible orbits shrinks, the number that intersect Earth typically falls to zero.

The method is conceptually similar to a Monte Carlo simulation - not exact propagation of a single orbit but probabilistic sampling of the full uncertainty space.

The Yarkovsky effect

Gravitational forces alone do not fully describe asteroid motion. Small asteroids are also subject to the Yarkovsky effect - a subtle, cumulative force caused by the asymmetric emission of thermal radiation.

As sunlight heats a rotating asteroid's surface, the warmest point is not directly facing the Sun but slightly offset, because the surface takes time to heat up and cool down as the asteroid rotates. The slightly off-centre emission of infrared radiation creates a tiny thrust - comparable to the force of a few paperclips on a boulder. Applied continuously over decades, this force can shift an orbit by tens of thousands of kilometres. For long-range predictions, that shift matters.

Sentry-II, an upgraded version of the system deployed in 2023, explicitly models the Yarkovsky effect for all monitored objects. Earlier versions had to treat it separately or ignore it for objects where it was difficult to measure. The upgrade substantially improves the reliability of assessments for encounters that are decades away - exactly the timescale where early warning and intervention would be possible.

Reading the risk table

The CNEOS Sentry table includes the following columns for each monitored object.

Column	What it means
Object name	Asteroid designation or name
Year range	Date range of potential impacts assessed
Impact probability	Cumulative probability across all dates in range
Estimated diameter	Inferred from brightness; carries significant uncertainty
Palermo Scale	Logarithmic measure vs background impact rate; values above -2 warrant monitoring
Torino Scale	Public-facing 0–10 integer risk rating; all current entries sit at 0

The Palermo Scale value is the most useful single number for comparing entries. Objects with a Palermo Scale below -2 are considered below the threshold of concern. None of the current entries exceed 0.

How objects enter and leave the table

An object joins the Sentry risk table when the initial orbital solution, given its uncertainties, includes at least one Earth-intersecting possibility. This is not unusual for newly discovered asteroids - the first night of observations provides only a rough arc, and the orbital uncertainty is large.

Follow-up observations, gathered over days to weeks, shrink that uncertainty. As the possible orbit range narrows, the Earth-intersecting possibilities usually disappear entirely. The object is removed from the table automatically. This is not a correction or a cover-up; it is the normal process by which orbital knowledge improves from rough to precise.

2024 YN4 is a recent example. Briefly appearing on the risk table with a meaningful impact probability, it attracted widespread attention. Follow-up observations resolved the orbit and removed it from the table. The same cycle plays out dozens of times per year for newly discovered objects.

Sentry and Scout

Sentry handles long-range predictions for objects whose orbits are reasonably well determined. Its companion system, Scout, handles rapid assessment for newly reported objects that have not yet been confirmed.

Scout analyses preliminary orbit data within minutes of a new object being posted to the Minor Planet Center's Near-Earth Object Confirmation Page. It flags anything whose preliminary orbit might intersect Earth and alerts observers to prioritise follow-up on that object. Most Scout alerts resolve within hours. A small fraction escalate to Sentry for longer-range analysis.

Together, the two systems provide continuous, automated coverage from the moment of first detection through to long-term orbital characterisation.

No object on the current Sentry risk table carries an impact probability above 0.5% for any date in the next 100 years. The vast majority of entries have probabilities far below 0.01%.

CNEOS explained

NASA's Centre for Near Earth Object Studies and what it does.

Will an asteroid hit Earth?

Current impact risk and what the monitoring data actually shows.

Impact probability explained

How scientists calculate and communicate collision risk.

The Torino Scale explained

How the 0-10 asteroid risk rating works and what each level means.

Common questions

What is NASA's Sentry system?

Sentry is an automated collision monitoring system operated by NASA's Center for Near Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory in Pasadena, California. It runs continuously, scanning the orbital solution for every known near-Earth asteroid and calculating whether any of them could strike Earth over the next 100 years. Any object with a non-zero impact probability appears on the publicly available Sentry risk table.

How does Sentry calculate impact probability?

Asteroid observations are never perfect - each measurement carries small uncertainties in position and velocity. Sentry represents this uncertainty as a statistical distribution: a cloud of possible orbits, each consistent with the available observations. It propagates all of those possible orbits forward in time. If some fraction of the possible orbits intersect Earth on a specific future date, that fraction becomes the impact probability for that date. As more observations arrive and refine the orbit, the cloud of possibilities narrows - usually to the point where none of the remaining possibilities intersect Earth.

What is the Yarkovsky effect?

The Yarkovsky effect is a subtle, continuous force caused by the uneven thermal emission of a rotating asteroid. As sunlight heats the asteroid's surface, the warmest part is not the point directly facing the Sun but slightly off to one side, due to the lag as the asteroid rotates. The slightly asymmetric emission of infrared radiation creates a tiny thrust. Over years and decades, this cumulative push can shift an asteroid's orbit enough to matter for long-range impact predictions. Sentry-II, deployed in 2023, explicitly models the Yarkovsky effect for all monitored objects, improving the accuracy of assessments for encounters decades in the future.

What does it mean if an asteroid is on the Sentry risk table?

It means the object has a non-zero calculated impact probability for at least one date within the next 100 years. It does not mean an impact is likely or expected. The vast majority of entries on the risk table have impact probabilities well below 0.01%, and most are there only because the orbit has not yet been observed long enough to rule out every possible Earth-crossing trajectory. Follow-up observations typically remove objects from the table within weeks or months of their initial appearance.

How often is the Sentry table updated?

Continuously. Sentry is an automated system that recalculates orbital solutions and impact probabilities whenever new observational data refine an asteroid's orbital solution. New observations are reported to the Minor Planet Center and flow automatically into the CNEOS systems. The publicly available risk table at cneos.jpl.nasa.gov/sentry reflects these updates in real time.

Is there a way to see the Sentry risk table?

Yes. The Sentry risk table is published publicly at cneos.jpl.nasa.gov/sentry and updated in real time. It lists all objects with a non-zero impact probability, along with the date range of potential impacts, cumulative impact probability, estimated size, and Palermo Scale value. The European Space Agency also publishes a comparable resource called the ESA Risk List.