An asteroid headline usually gives you one alarming number and leaves out the context. The useful questions are more specific: when is the closest approach, how far away is it, how well is the distance constrained, and is the quoted size measured or estimated? Asteroid Watch brings those fields together inside Celestial Lab, but each field needs a different interpretation.
In this guide
- Read an asteroid close approach in the right order
- Why the diameter is an estimate
- “Close & bright” is not the official PHA flag
- Keep approach, impact-risk and fireball records separate
- What the 3D distance comparison actually represents
- A useful workflow for checking an alarming headline
- Frequently asked questions
Quick answer: read the Board for the object’s designation, UTC approach time, miss distance and relative velocity. Treat the diameter as a brightness-based estimate. The Close & bright screen is the app’s descriptive filter, not NASA’s official potentially hazardous asteroid classification. The Risk & fireballs view contains different datasets and should not be read as one combined warning list.
Read an asteroid close approach in the right order
Start with the designation and the Date (UTC). UTC avoids the ambiguity of a local date around midnight: an approach occurring early on Saturday in UTC can still be Friday evening elsewhere. Match both the designation and date when checking a news story; one object can have multiple listed encounters.
Next read Miss distance, then its units. A lunar distance, or LD, uses the mean Earth–Moon distance of about 384,400 kilometres. A hypothetical 2 LD encounter is therefore roughly 768,800 km away. This is a worked unit conversion, not a claim that a specific asteroid currently has that encounter.
Read Velocity separately. The app reports kilometres per second. A fast object can pass far away; a slow object can pass relatively close. Neither column alone tells you an impact probability. Closest-approach distance is generally a center-to-center distance in the source data, not the asteroid’s height above Earth’s surface.
JPL’s close-approach field documentation describes the encounter time, nominal and bounded distances and relative-velocity fields. It is the reference to consult when two reports use different definitions for what looks like the same number.
Why the diameter is an estimate
Asteroids are usually unresolved points of light. Brightness depends on both size and reflectivity: a small bright object can look like a larger dark object. Absolute magnitude H is therefore useful, but it does not uniquely determine diameter.
This app’s H-based diameter calculation assumes a geometric albedo of 0.14. The estimate is expressed in metres. That assumption must travel with the number. It is not a measured width, and the current app does not turn a single assumed-albedo result into a measured diameter range.
NASA’s asteroid size estimator illustrates why changing albedo changes an H-based size estimate. When a source supplies a radar or other independently measured size, that is a different kind of evidence and should be identified explicitly rather than silently substituted into the same narrative.
“Close & bright” is not the official PHA flag
The app’s screen selects encounters with a nominal distance no greater than 0.05 AU and an absolute magnitude H no greater than 22. It helps you find objects worth inspecting within the current approach list.
An official potentially hazardous asteroid, or PHA, classification uses orbital criteria, including the minimum orbit intersection distance. That is not the same as the distance of a particular encounter. A close-approach board cannot reproduce the official classification merely by applying a threshold to today’s flyby distance.
Do not interpret the descriptive screen as an imminent-danger warning. Conversely, absence from this screen does not establish an object’s long-term safety. For an actual impact-risk question, use the official risk system and its object-specific orbital solution rather than substituting a simple brightness-and-distance filter.
Keep approach, impact-risk and fireball records separate
Close approaches describe predicted encounters of catalogued objects. Sentry monitors possible future impact solutions as orbits and uncertainties evolve. Fireball records describe observed atmospheric events. They answer different questions and have different timestamps and measurements.
The app’s Sentry panel is a selected risk watch, limited to the 25 entries chosen by its source ranking. It is not the entire official Sentry catalog. A row’s presence may change as more observations improve the orbit; removal from a risk list commonly means that earlier possible solutions have been ruled out.
Fireball energy, velocity and position fields can be missing. A missing value means it was unavailable in that record. It should not become zero, a guessed trajectory or a decorative estimate. Read the source record when you need the precise interpretation of an event.
What the 3D distance comparison actually represents
The Distance comparison view is a quantitative atlas. Its axes encode UTC encounter time, logarithmic miss distance in lunar distances, and relative velocity in km/s. A point’s position helps compare encounters across those three variables.
It is not a map of the Solar System, and it does not show which direction an asteroid approaches Earth from. The close-approach feed does not supply the directional information needed to draw that geography. The logarithmic distance axis spreads very different distances across the scene; equal visual spacing does not mean equal additional kilometres.
Hover over a point to read its actual values rather than estimating distance from perspective. Drag to change the viewing angle, zoom to separate nearby points and reset the camera if the axes become difficult to follow. The view draws a bounded recent subset of the active encounter list, so its point count can be smaller than the total Board count.
A useful workflow for checking an alarming headline
- Find the exact object designation and encounter date in Board.
- Convert LD to kilometres only after checking the displayed unit.
- Read the size field with its assumed albedo; do not present it as a measured diameter.
- Compare the nominal encounter with any available distance bounds.
- Use the official risk record for an impact-probability claim.
- Check the source timestamp and whether the story describes the same orbital solution.
This gives you a defensible explanation: “the listed encounter is distant, the size is estimated, and the public approach row is not an impact forecast.” It is more useful than repeating either “dangerous asteroid” or “nothing to worry about” without examining the data.
Frequently asked questions
Does passing inside the Moon’s orbit mean an impact is likely? No. A nominal close approach and a probabilistic impact solution are different quantities.
Why did a size estimate change? Updated brightness, a different albedo assumption or a better measurement can change the estimate. Check which input changed before treating the change as a discovery about growth or fragmentation.
Why is an object in the table but absent from the 3D scene? The scene uses a bounded display subset and requires usable numeric fields. The table remains the better place to inspect the full supplied encounter list.
Are the 3D paths real trajectories? This view displays points on quantitative axes. It deliberately does not invent orbital trajectories from a feed that lacks their geometry.
