Potentially Hazardous Asteroids
NASA's PHA label is a monitoring category defined by size and orbit geometry. It flags objects worth watching — it is not a prediction that anything will hit Earth.
The definition
An asteroid is classified as potentially hazardous when it meets two purely technical conditions. Its minimum orbit intersection distance — the closest the two orbital paths can ever come, regardless of where the bodies are along them — must be under 0.05 astronomical units, about 7.5 million kilometres or 19.5 lunar distances. And its absolute magnitude must be 22.0 or brighter, which implies a diameter of roughly 140 metres or more.
Both conditions are about the orbit and the object, not about any particular date. An asteroid can carry the PHA label for centuries without ever coming especially close to Earth.
Why the thresholds are set there
The 140-metre size line marks where a regional-scale event becomes physically possible if an impact ever occurred, which is why it also anchors NASA's congressional survey goals. The 0.05 AU line is a deliberately generous margin: orbits drift under planetary perturbations over long timescales, so anything whose path can approach that closely is worth keeping in the monitoring set rather than discarding.
Potentially hazardous asteroids approaching this week
This score ranks how notable the asteroid approach is. It is not an impact prediction.
What the label is not
PHA status is not an impact probability, a risk ranking, or an alert. Impact monitoring is a separate process: NASA's Sentry system continuously screens catalogued objects for any non-negligible impact probability over the coming century, and results there are reported independently of the PHA list. Orbitfall reports the classification because NASA supplies it, and treats it as one input among four in the Orbitfall Index — see the methodology page.
Read more
For the longer explainer, see what a potentially hazardous asteroid is. Related reading: asteroid close approaches, near-Earth objects today, NASA asteroid tracker, and how NASA tracks asteroids. The live list is on the Orbitfall tracker.
Frequently asked questions
- What makes an asteroid potentially hazardous?
- Two criteria: a minimum orbit intersection distance with Earth below 0.05 astronomical units, and an absolute magnitude of 22.0 or brighter, which implies a body roughly 140 metres across or larger.
- Does potentially hazardous mean it will hit Earth?
- No. The label describes orbit geometry and size, not an impact prediction. No catalogued potentially hazardous asteroid is on a collision course with Earth.
- How many potentially hazardous asteroids are there?
- Roughly 2,400 are catalogued, out of more than 35,000 known near-Earth asteroids.
Learn about close approaches
Today's catalogued close approaches, ranked.
What counts as a near-Earth object, and today's list.
What a close approach is and how it's measured.
Where the data comes from and how often it updates.
1 LD = 384,400 km — the yardstick for flybys.
Why a close approach is a schedule, not a warning.
From first detection to orbit refinement.