For the authoritative, current answer, use NASA/JPL’s Center for Near-Earth Object Studies (CNEOS). Its Sentry Impact Risk Page lists confirmed near-Earth objects with possible impacts during the next 100 years; an object’s detail record shows the dates, distances, probabilities and risk scales behind its listing. CNEOS Scout covers unconfirmed candidates for only days to weeks, so its preliminary ratings are not equivalent to a confirmed impact probability.
The services are live. Object rankings, close-approach dates and probabilities can change whenever additional astrometric observations improve an orbit. Always note the page’s update time when you save or share a result.
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What counts as a near-Earth object?
NASA/JPL’s Asteroid Watch definition covers asteroids and comets whose orbits bring them within 120 million miles (195 million kilometers) of the Sun. Most known near-Earth objects (NEOs) are asteroids, ranging from a few meters to nearly 40 kilometers across. “Near-Earth” describes the orbit’s relationship to the Sun; it does not mean that the object is currently close to Earth or is destined to hit it.
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CNEOS determines an orbit by fitting an elliptical path to the object’s available observations. Telescope measurements contain uncertainty, especially when an object has been observed for only a short time. The calculated result is therefore a family of possible orbits rather than a single perfectly known track.
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The quickest reliable way to check an asteroid
- Start at CNEOS. Use NASA/JPL’s Center for Near-Earth Object Studies as the primary source instead of a reposted “asteroid danger” headline. CNEOS computes high-precision NEO orbits and publishes both close approaches and impact-risk assessments.
- Open the Sentry Impact Risk Page. Sentry searches possible future orbits for Earth impacts over the next 100 years. Its live risk page provides a summary list and links to technical records for individual objects.
- Search for the object’s designation. Enter the asteroid’s official designation or use the linked object record. Do not rely on an informal nickname alone; designations can change as observations are consolidated.
- Read the detail record. Record the close-approach date, nominal distance, minimum possible distance, impact probability, Torino Scale value, Palermo Scale value and the page’s last-update time.
- Use Scout only when the object is still unconfirmed. Scout monitors candidates on the Minor Planet Center’s NEO Confirmation Page and evaluates possible trajectories over days to weeks. Once observations confirm an object, it leaves the candidate workflow and long-term monitoring moves to Sentry.
This sequence separates a confirmed object’s long-term risk analysis from Scout’s short-arc screening. It also gives you the context needed to interpret a changing entry instead of treating a headline or a single probability as a prediction.
How to read the Sentry Impact Risk Page
Sentry’s list is not a countdown of objects that are expected to strike Earth. It is a catalogue of objects for which at least one orbit still allowed by the observations intersects Earth during the search interval. The object page explains how many possible solutions were tested and how the resulting probabilities are ranked.
| Item on the record | What it tells you | How to use it |
|---|---|---|
| Object designation | The identifier used to join observations and orbit solutions. | Use it when opening the same object in CNEOS tools, SBDB or Horizons. |
| Impact-probability entry | The calculated chance associated with one or more future encounters in the listed interval. | Read it with the date, uncertainty and update time; it is not a guarantee that an impact will occur. |
| Close-approach date | When the object is expected to pass closest to Earth in the displayed scenario. | Check whether the date is an ordinary close approach or a hypothetical impact solution. |
| Nominal distance | The distance for the best-fit, or nominal, orbit. | Compare it with the minimum possible distance rather than quoting it alone. |
| Minimum possible distance | The closest Earth distance still allowed by the current uncertainty region. | A wide gap between nominal and minimum distances signals that the orbit is not tightly constrained. |
| Torino Scale | A public-facing category combining impact probability and potential severity. | Use it to communicate the level in plain language; it is deliberately coarse. |
| Palermo Scale | A technical comparison with the background impact risk from objects of comparable size over the same time period. | Use it for expert comparisons, not as a simple “percent chance” score. |
| Observation arc and uncertainty | How long and how precisely the object has been measured. | A short arc generally permits a broader set of future orbits than decades of observations. |
| Last-update time | When CNEOS last recomputed or displayed the record. | Save it with any screenshot, quotation or report because the values can change. |
Why an asteroid can appear on Sentry and later disappear
CNEOS states that, “Because orbit computations are based on imperfect data, each asteroid will have a range possible orbits that match the observed sky positions.” If some of those still-allowed orbits intersect Earth, Sentry can list the object even when the best-fit orbit misses by a large margin.
The normal outcome is removal after more observations narrow the orbit. CNEOS puts it this way: “Whenever a newly discovered NEA 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.” A listing therefore means that an impact solution has not yet been ruled out, not that an impact is the most likely result.
When new measurements arrive, the probability, date, distance, Torino value or Palermo value can move in either direction. Treat those changes as normal orbit refinement. Preserve the previous update time if you are comparing versions, and do not present an old screenshot as the current risk list.
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Sentry and Scout: the difference that matters
| Characteristic | Sentry | Scout |
|---|---|---|
| Objects | Confirmed near-Earth objects with established designations. | Unconfirmed candidates reported on the Minor Planet Center’s NEO Confirmation Page. |
| Time horizon | Future Earth impacts searched over the next 100 years. | Possible trajectories evaluated over days to weeks. |
| Purpose | Long-term impact monitoring and technical risk records. | Rapid triage so observers can decide which new candidates deserve follow-up. |
| Data quality | Uses an orbit based on a confirmed observation set, although uncertainty still exists. | Often relies on a short observation arc and less certain astrometry. |
| Meaning of a rating | A computed impact-risk result that can be investigated on the object page. | An alert about an interesting candidate, not a rigorous probability assessment. |
| What happens next | Continues in CNEOS risk and orbit systems as observations accumulate. | Leaves the candidate workflow after confirmation; long-term analysis then moves to Sentry. |
Scout explicitly warns that its data concern unconfirmed objects and should be treated as potentially unreliable. A high-looking Scout rating is a reason for astronomers to obtain more observations, not a public declaration that an impact is imminent.
Torino and Palermo scales without the confusion
Torino Scale: the public communication category
The Torino Scale is designed to express impact concern in a form that can be communicated to the public. In ordinary cases, a Torino value of zero indicates no unusual concern. CNEOS explains that a zero generally corresponds to a risk below the background level, while higher categories require both a meaningful probability and a potentially significant consequence. The scale is categorical, so it should not be read as a percentage.
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The Palermo Scale compares a particular object’s calculated risk with the background risk expected from the broader population of objects of similar size. CNEOS notes that Palermo values below zero—and, in most cases, Torino zero—indicate risk below that background level. A Palermo value above zero is very rare. Because Palermo is logarithmic and includes the time interval and object severity, it is useful for ranking technical cases but is not intuitive as a standalone public score.
For a responsible explanation, state all three pieces together: the impact probability, the relevant date or time interval, and the Torino and Palermo values as displayed on the current record.
How to compare two objects on the list
A single ranking column cannot tell you which of two entries deserves more attention. Compare the following axes in the same order for both objects:
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- Probability and time horizon: confirm that the percentages refer to comparable dates and encounter windows.
- Nominal and minimum possible distance: a close nominal approach with a narrow uncertainty region means something different from a distant nominal orbit with a broad region that reaches Earth.
- Observation arc: more nights and a longer time baseline usually constrain the orbit more tightly.
- Torino category: use it for a public-facing severity summary.
- Palermo value: use it to compare risk against the background population.
- Last update: compare records captured at similar times; otherwise one object may reflect several additional observing sessions.
A newly discovered object can have a broad uncertainty region because a short arc admits many future paths. An object tracked for decades may show a much tighter orbit even if its nominal close approach is numerically nearer. The numbers only become meaningful when their uncertainty and observation history travel with them.
Use CNEOS’s other tools for repeat tracking
Close-approach tables
CNEOS publishes close-approach tables for browsing upcoming and historical encounters. These tables are useful when you know a date or want to find which NEOs pass within a chosen distance. A close approach is a geometric event, not an impact warning: most listed objects pass safely, and the table does not replace the Sentry risk record.
Small-Body Database (SBDB)
SBDB provides the object’s designation, orbit, physical information where available and links among observations and derived solutions. Use it to verify that you are following the correct object when designations are updated or when several objects have similar names.
JPL Horizons
Horizons produces ephemerides—predicted positions and related observing quantities—for a selected object, observer location and time span. It is the appropriate tool for planning where an asteroid should appear in the sky. An ephemeris is not an impact-risk assessment; pair it with the Sentry record when your question is safety rather than visibility.
APIs
The Sentry service offers summary, object-specific, virtual-impactor and removed-object modes. The broader SSD/CNEOS service also exposes close-approach, SBDB, Horizons and Scout interfaces. For a script or dashboard, request only the data you need, store the retrieval timestamp and handle the possibility that an object’s result changes between requests.
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CNEOS asks users to follow fair-use guidance: make only necessary requests and submit one API request at a time. A polling loop that repeatedly downloads the entire list is unnecessary and can burden a public service. Cache results, use a sensible interval and refresh immediately only when your application has a genuine reason.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A repeatable tracking workflow for a spreadsheet or alert system
- Define the question. Decide whether you need long-term impact risk, an upcoming close approach or a sky position. Choose Sentry, the close-approach table or Horizons accordingly.
- Save the identifier. Store the official designation, not just a headline’s nickname.
- Capture the full context. Keep probability, date, distances, Torino, Palermo, observation arc and update time in separate fields.
- Separate confirmed and unconfirmed data. Label Scout candidates as preliminary and do not merge them into a confirmed-object risk chart.
- Refresh conservatively. Use the API’s summary or object-specific mode, obey one-request-at-a-time guidance and cache unchanged responses.
- Flag changes for review. A changed value is expected after new observations. Compare the new and old update times before describing it as a significant development.
- Link readers to the live record. Because list membership and probabilities are dynamic, show when your copy was retrieved and direct readers to the current CNEOS page.
What a telescope can—and cannot—do
You do not need a telescope to check Sentry or Scout. Most asteroid-tracking observations are collected by large survey telescopes, and CNEOS uses those measurements to determine orbits and calculate risk.
A consumer telescope can help you observe or record an asteroid’s apparent position if you have an accurate ephemeris, a suitable camera and enough time to collect multiple frames. Comparing the object’s measured position with a predicted position can contribute useful follow-up observations when submitted through the appropriate professional observing channels. It does not, by itself, replace CNEOS orbit determination or Sentry’s impact analysis.
For visual observing, obtain the object’s ephemeris from Horizons, check the observing location and time zone, and account for the object’s brightness and the Moon. A telescope listing or a point of light in one image is not evidence that an object is dangerous.
Common mistakes and how to correct them
Mistake: treating every Sentry row as an expected impact
Correction: Explain that at least one orbit allowed by current observations intersects Earth. Read the probability and uncertainty on the detail page; the most likely outcome for a newly listed object is removal after more observations.
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Mistake: quoting a stale screenshot
Correction: Include the record’s last-update time and check the live page again before publication, posting or making a decision.
Mistake: confusing Scout with Sentry
Correction: Label Scout information as preliminary and tied to an unconfirmed candidate. Do not present its rating as a settled long-term probability.
Mistake: using the nominal distance alone
Correction: Report both nominal and minimum possible distances, then look at the observation arc and uncertainty.
Mistake: treating Torino or Palermo as percentages
Correction: Describe Torino as a communication category and Palermo as a technical comparison with background risk. Quote the actual probability separately.
Mistake: asking an API for more data than needed
Correction: Use the relevant summary or object-specific mode, cache responses and make one request at a time in accordance with CNEOS fair-use guidance.
A compact checklist for any asteroid story
- Is the object confirmed, or is it still a Scout candidate?
- What is the official designation?
- What future date or dates are being assessed?
- What are the nominal and minimum possible Earth distances?
- What impact probability is shown, and over what interval?
- What are the Torino and Palermo values?
- How long is the observation arc, and how large is the orbit uncertainty?
- When was the CNEOS page last updated?
- Are you using a live CNEOS record rather than an undated repost?
Following that checklist keeps the distinction clear between an object that is merely near Earth in orbital terms, a candidate that needs more observations and a confirmed object whose possible future paths are being tested for impact.
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