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Use NASA/JPL’s Center for Near-Earth Object Studies (CNEOS) as your primary source. Its live Sentry Impact Risk page lists confirmed near-Earth objects (NEOs) with calculated impact possibilities, while Scout evaluates unconfirmed candidates over the next few days or weeks. Open an object’s detail page to see the observation arc, orbit uncertainty, close-approach dates, nominal and minimum possible distances, impact probability, Torino category, Palermo value and update time.
A listing is not an impact prediction or a statement that a collision is likely. It means that at least one orbit still allowed by the observations intersects Earth sometime in the search interval. New observations usually narrow the possible orbits and remove the object from the risk list.
What counts as a near-Earth object?
NASA’s Asteroid Watch definition covers asteroids and comets whose orbits bring them within 120 million miles (195 million kilometers) of the Sun. Most known NEOs are asteroids, ranging from a few meters to nearly 40 kilometers across. Their orbits are calculated by fitting an elliptical path to the object’s observed positions in the sky.
That definition is about orbital geometry, not danger. An object can be a NEO without ever coming close to Earth, and a close approach does not automatically become an impact. The quality and length of the observation record determine how tightly its future path is known.
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NASA’s live asteroid-monitoring tools
CNEOS separates confirmed-object risk analysis from the early stage at which an object has only a few observations. Use the tool that matches the object’s status and the question you are asking.
| Tool or data set | Objects covered | Time scale | Best use | Important limitation |
|---|---|---|---|---|
| Sentry Impact Risk Page | Confirmed near-Earth objects | Possible impacts during the next 100 years | Check the current public risk list and open technical object records | Values can change as additional observations refine the orbit |
| Scout | Unconfirmed candidates on the Minor Planet Center’s NEO Confirmation Page | Days to weeks | See whether a newly reported candidate deserves follow-up observations | Short observation arcs and uncertain astrometry make the ratings preliminary, not rigorous impact probabilities |
| Close-approach tables | Objects with calculated Earth encounters | Past and future approaches listed by the service | Find approach dates, distances and object designations | A close approach is not an impact assessment by itself |
| Small-Body Database (SBDB) | Asteroids and comets with orbit and physical records | Depends on the object and observations | Check designations, orbital elements and observation information | Orbit data are estimates that can be updated |
| JPL Horizons | Objects with available ephemerides | User-selected dates and observing locations | Generate positions, rise/set information and other observing ephemerides | An ephemeris is for locating an object, not a replacement for Sentry’s impact analysis |
| CNEOS and SSD APIs | Several CNEOS services, including Sentry, Scout, close approaches, SBDB and Horizons interfaces | Live data | Automate repeat checks or build a monitoring workflow | Use fair, necessary requests and submit one request at a time |
How to check NASA’s current impact-risk list
- Start at CNEOS. Go to NASA/JPL’s Center for Near-Earth Object Studies rather than relying on a reposted “asteroid danger” headline. CNEOS computes the high-precision orbits and publishes the close-approach and impact-risk products.
- Open the Sentry Impact Risk Page. This is the live summary for confirmed NEOs. Treat the page as a changing data set, not a dated chart copied into an article.
- Find the object by designation. Use the asteroid’s official provisional or numbered designation. News stories may use an informal spelling or omit punctuation, so verify the designation on the CNEOS record.
- Open the object’s technical page. The summary row is only an index. The detail record contains the impact solutions, close-approach information, uncertainty context and the time at which the calculations were updated.
- Record the fields that answer your question. Write down the possible impact date or dates, impact probability, nominal close-approach distance, minimum possible distance, Torino category, Palermo value, observation arc or uncertainty information and the page’s update time.
- Check again after new observations. CNEOS may revise a probability, move a date, add a possible impact solution or remove the object entirely. Keep the retrieval time with anything you quote or publish.
The Sentry page also links to technical records and provides access to machine-readable services. A current list has no permanent membership: discoveries are added, and entries are often removed when the orbit becomes sufficiently constrained.
How to read an individual Sentry record
Impact probability and its time horizon
The probability applies to a particular possible encounter in a stated future interval. Do not detach the percentage from its date or from the calculation’s update time. If a record contains several possible encounters, read each date and probability as a separate solution rather than adding percentages without understanding the service’s presentation.
A small nonzero probability is compatible with an object that is overwhelmingly likely to miss Earth. The number exists because the uncertainty region still contains some trajectories that intersect Earth; it is not a claim that the impact path is the nominal or most likely orbit.
Nominal and minimum possible distance
The nominal distance is based on the best-fit orbit. The minimum possible distance reflects the edge of the currently allowed uncertainty region for that encounter. A large difference between the two is a warning to read the observation arc and uncertainty details before drawing conclusions.
Distances are meaningful only with their units and reference point. Keep the unit shown by CNEOS and note whether the value describes an Earth encounter, a possible impact solution or another close-approach metric.
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Observation arc and uncertainty
An object observed for a short time can fit many future paths. A longer arc with observations from different dates and viewing geometries usually narrows the set of viable orbits. CNEOS explains the underlying issue this way: “Because orbit computations are based on imperfect data, each asteroid will have a range possible orbits that match the observed sky positions.”
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When comparing two objects, an apparently higher probability for the newly discovered object may simply reflect a broader uncertainty region. Compare the length and quality of the observation record before treating the two percentages as equivalent warnings.
Update time
Every quoted value needs the page’s last-update time. Sentry recalculates when new astrometry is incorporated, so a value copied yesterday may not be the value displayed today. For a public post or an automated report, store the UTC retrieval time, the object designation and the exact encounter date alongside the result.
Why an asteroid can appear and then disappear
CNEOS states that when a newly discovered near-Earth asteroid is posted on the Sentry Impact Risk Page, “by far the most likely outcome is that the object will eventually be removed as new observations become available, the object’s orbit is improved, and its future motion is more tightly constrained.”
The normal sequence is:
- Survey observations provide a short arc of sky positions.
- Orbit determination produces a family of paths that fit those positions within their measurement errors.
- A subset of those paths may intersect Earth in a future calculation, so Sentry lists a possible impact solution.
- Follow-up observations eliminate incompatible paths and shrink the uncertainty region.
- The impact solution’s probability can fall, the date can shift, or the object can leave the list.
Removal does not mean the object vanished. It means the current orbit solution no longer meets Sentry’s impact-list criteria, or that the object’s risk has been recalculated below the displayed range. Conversely, a new observation can briefly create or increase a listed probability before later data reduce it.
Torino and Palermo: two different risk scales
Do not treat the Torino and Palermo values as interchangeable grades. Torino is designed to communicate the seriousness of an impact scenario to the public. Palermo is a technical comparison between the calculated scenario and the background impact risk from the general near-Earth-object population.
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| Scale | What it tells you | How to use it |
|---|---|---|
| Torino | A public-facing category that combines impact probability and potential consequences | Use it as a communication aid, alongside the encounter date and probability; a Torino 0 entry generally indicates a background-level or lower concern |
| Palermo | A logarithmic technical comparison with the expected background impact risk | Values below zero indicate risk below the background level in the usual interpretation; values above zero are very rare and warrant close technical attention |
CNEOS notes that Palermo values below zero and, in most cases, Torino zero indicate risk below the background level. A nonzero probability therefore does not automatically imply an exceptional hazard. Read the scales with the object’s uncertainty and update time, not as standalone scores.
Scout versus Sentry
The names are easy to confuse, but the systems answer different questions and operate at different stages of the discovery process.
| Question | Scout | Sentry |
|---|---|---|
| Is the object confirmed? | No. Scout watches unconfirmed candidates listed on the Minor Planet Center’s NEO Confirmation Page. | Yes. Sentry analyzes confirmed near-Earth objects with established designations and orbit solutions. |
| How far ahead does it look? | Usually days to weeks, while observers decide whether to obtain follow-up measurements. | Possible Earth impacts over the next 100 years. |
| What is the output? | Preliminary ratings intended to flag interesting candidates for observation. | Impact solutions, probabilities and associated risk indicators on a technical object page. |
| How reliable is the probability? | Scout warns that its data concern unconfirmed objects and may be unreliable; poor astrometry and a short arc can dominate the result. | More rigorous orbit and risk calculations, still subject to revision when new observations arrive. |
| What happens after confirmation? | The candidate leaves the unconfirmed workflow. | Long-term monitoring moves to Sentry and related CNEOS services. |
Use Scout to decide whether a candidate merits immediate observing attention. Use Sentry to discuss the current impact-risk status of a confirmed object. A Scout rating should not be quoted as if it were a Sentry probability.
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When two asteroids are listed, compare the same fields for the same type of encounter. A simple headline comparison of percentages can hide major differences in timing and uncertainty.
- Impact probability: Include the encounter date and the calculation update time.
- Time horizon: A possibility decades away has a different observation opportunity than one in the next few years.
- Nominal and minimum possible distance: Keep the two distance measures distinct and retain their units.
- Torino category: Use the public communication category as context, not as a substitute for the underlying data.
- Palermo value: Check whether the technical comparison is below or above the background level.
- Observation arc: A short arc generally leaves a wider uncertainty region than a long, well-distributed record.
- Orbit uncertainty: Read the object page’s uncertainty description instead of inferring precision from the number of decimal places.
- Last update: Do not compare a newly recalculated record with an older screenshot.
For a report or spreadsheet, make these columns explicit. That prevents a provisional candidate, a close-approach-only entry and a long-term Sentry solution from being treated as the same kind of warning.
Tracking an asteroid’s position and close approaches
Close-approach tables
CNEOS close-approach tables are the quickest way to find when an object passes near Earth and how the service describes that encounter. Use them to locate dates and distances, then open the object record if you need impact probabilities or uncertainty details. A close approach can be millions of kilometers away and still be important for orbit refinement; it is not synonymous with a collision.
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JPL Horizons
Horizons generates ephemerides for selected objects, dates and observing locations. It is useful when you want a predicted right ascension and declination, altitude, rise and set times or other observing quantities. Select the correct object designation, set the observer location and time span, and retain the ephemeris settings with your result.
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Horizons answers “where should I look?” It does not answer “is an impact currently likely?” For that, use the Sentry record and its risk fields.
Small-Body Database
SBDB is useful for checking alternate designations, orbital elements and the observation record associated with an asteroid or comet. If a news report uses a provisional designation that later receives a numbered designation, SBDB can help verify that the names refer to the same object. Use the designation shown by CNEOS when switching between services.
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Manual page checks are adequate for occasional reading. A researcher, observatory or newsroom that checks many objects can use the documented CNEOS and SSD interfaces, provided the requests are limited and purposeful.
Sentry API modes
The Sentry API offers summary data, object-specific queries, virtual-impactor information and removed-object modes. Choose the narrowest mode that answers your task. A summary query is appropriate for a current overview; an object query is better when you already have a designation; a removed-object query helps audit how an entry changed over time.
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- Define the object set. Keep a list of official designations rather than repeatedly downloading every record.
- Choose one service and mode. Use Sentry for confirmed impact-risk data, Scout for unconfirmed candidates and the relevant SSD/CNEOS interface for close approaches, SBDB or Horizons.
- Send one request at a time. CNEOS asks users to make only necessary requests and to avoid concurrent bursts.
- Cache the response. Store the retrieval timestamp, designation, encounter date and returned values so you can identify what changed.
- Validate against the human-readable record. When a value changes materially, open the object page and read the observation and uncertainty context.
- Back off after errors. Do not retry rapidly or create parallel jobs; wait, reduce the request rate and confirm that the query format and designation are valid.
Do not build an alert that treats every small numerical change as an emergency. Orbit refinement commonly changes displayed probabilities without changing the overall interpretation.
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Observing an asteroid yourself
A telescope is optional. Most asteroid-tracking measurements come from large survey telescopes, and consumer equipment does not replace CNEOS orbit determination or Sentry risk analysis. Personal observing is best treated as a way to see or record a predicted position, not as an independent safety assessment.
A practical observing sequence
- Confirm the designation and visibility window in CNEOS or SBDB.
- Generate an ephemeris in JPL Horizons for your location and the dates you can observe.
- Load the coordinates into planetarium or telescope-control software that supports the chosen coordinate format and epoch.
- Plan a time series of exposures rather than expecting a moving asteroid to look like a bright star in one frame.
- Compare successive images for the object’s predicted motion and record the UTC time, location and instrument settings.
- Submit useful astrometry through appropriate professional channels if you are equipped and trained to do so; do not assume an uncalibrated visual sighting changes the official orbit.
Visibility depends on the object’s brightness, phase, angular speed, lunar conditions, altitude and your aperture. An unsuccessful observation does not contradict the CNEOS prediction.
Common problems and how to fix them
“The asteroid is not on Sentry.”
Check the designation, then check whether you are looking at an unconfirmed candidate on Scout or at a close-approach table rather than an impact-risk record. An object can be a NEO and still have no Sentry entry.
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“The probability changed since yesterday.”
Compare the two records’ update times and observation arcs. New astrometry can remove allowed trajectories, add them temporarily or move the possible encounter date. Quote the newer record and preserve the older value only as a time-stamped historical snapshot.
“A social-media post says the asteroid will hit.”
Verify the official designation and open the live CNEOS object page. Look for the date, probability, Torino category, Palermo value and update time. A headline that omits those fields cannot distinguish a possible orbit from the most likely orbit.
“Scout shows a high rating.”
Scout concerns unconfirmed objects and explicitly warns that its information may be unreliable. Wait for confirmation and follow-up observations before treating the result as a long-term impact assessment.
“The API returns an error or stale data.”
Check the designation and query mode, reduce the request rate, send one request at a time and compare the response timestamp with the web page. Avoid parallel polling and do not assume a cached response is current.
“Two services show different distances.”
Confirm that they refer to the same object, encounter date, reference frame and distance unit. Horizons may be giving an observer ephemeris, while a CNEOS risk record may be describing a nominal or minimum possible Earth distance.
A concise checklist for every risk check
- Use CNEOS as the canonical source.
- Decide whether the object is confirmed (Sentry) or unconfirmed (Scout).
- Verify the official designation.
- Open the technical object page rather than relying on a summary headline.
- Capture encounter date, probability, nominal distance, minimum possible distance, Torino, Palermo, uncertainty and update time.
- Interpret a listing as an allowed impact solution, not as the most likely outcome.
- Recheck after new observations and label every quoted value with its retrieval time.
- Use Horizons for observing positions, SBDB for orbit and designation context, and close-approach tables for encounter schedules.
- For automation, select the narrowest API mode, cache responses and make one necessary request at a time.
The practical bottom line
For a confirmed near-Earth asteroid, Sentry is the authoritative place to check NASA’s current impact-risk assessment, and the object page is where the probability and uncertainty become understandable. For an unconfirmed candidate, Scout is an early-warning and follow-up-observation tool, not a dependable impact forecast. Because all of these services update as observations improve the orbit, the responsible way to report a result is to name the object, encounter date, displayed values and retrieval time—and to explain that a listed possibility is not the same thing as a predicted collision.
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