Potentially Hazardous Asteroids: The Watch List for Objects That Could Reach Earth

Two numbers, not a verdict

The most important thing to understand about a Potentially Hazardous Asteroid is that the phrase is defined by arithmetic, not by any assessment that a given rock is coming for us. NASA's Center for Near-Earth Object Studies (CNEOS) uses exactly two cutoffs. First, the asteroid's Earth Minimum Orbit Intersection Distance (MOID) — the closest the two orbits can ever come, regardless of where the bodies happen to be along them — must be 0.05 astronomical units or less. That is about 7,480,000 km, or roughly 19.5 times the Earth–Moon distance. Second, its absolute magnitude H (brightness scaled to a standard distance) must be 22.0 or brighter.

Neither criterion mentions Earth's actual position, the current date, or any probability of impact. A PHA can spend the next thousand years never coming within tens of millions of kilometers of us and still qualify, simply because its orbit's geometry permits a close pass and it is large enough to matter if one ever occurred. That is the whole design philosophy: cast a conservative net now, so nothing dangerous slips in unnoticed later, because orbits drift over centuries through planetary tugs and non-gravitational forces.

The label is therefore a screening tool, not an alarm. Of the roughly 2,500 known PHAs, the number currently rated above the lowest step of the impact-hazard scales is effectively zero for the coming century. "Potentially hazardous" means "worth keeping on the watch list," in the same spirit that an epidemiologist tracks a pathogen's reproduction number before anyone is sick.

Why brightness stands in for size

We almost never measure an asteroid's diameter directly — most are unresolved points of light even in large telescopes. So the size criterion is smuggled in through brightness. Absolute magnitude H is how bright an asteroid would appear if placed 1 au from both the Sun and the observer, at zero phase angle. It captures the total sunlight the object reflects, which depends on both its size and its reflectivity (albedo).

To turn H into a diameter you must assume an albedo. CNEOS uses a typical value of 14%, which converts the H ≤ 22.0 cutoff into a diameter of about 140 m — a bit larger than a soccer stadium. The catch is that albedo genuinely varies: dark, carbon-rich (C-type) asteroids reflect only a few percent, while bright, stony (S-type) or metallic bodies can exceed 30%. Two objects of identical H can therefore differ in diameter by a factor of two or more. This is why the 140 m figure is always quoted as "roughly":

• A very dark object at H = 22 might actually be ~250 m across.
• A shiny one at the same H could be only ~90 m.

The 140 m line is not arbitrary. Objects near and above it are the ones capable of causing regional-scale devastation on the ground rather than a mid-air flash — the threshold below which impacts mostly bruise the sky rather than the surface. Missions such as NASA's now-retired NEOWISE infrared survey refined many albedos by measuring the heat asteroids emit, tightening size estimates that visible light alone leaves ambiguous.

What a close pass actually looks like: Apophis, 2029

The textbook PHA is (99942) Apophis, a stony asteroid about 340–370 m across, discovered in 2004 and briefly infamous for a 2004 estimate that gave it a ~2.7% chance of striking Earth in 2029 — the highest impact probability ever formally assigned to a sizeable object. Follow-up observations and a 2021 radar campaign then eliminated any impact risk for at least the next century. Apophis is the reason the panic-then-refine cycle is now routine.

On Friday, 13 April 2029, Apophis will pass just ~31,600 km above Earth's surface — closer than the belt of geostationary satellites at ~35,786 km altitude, and about one-tenth the distance to the Moon. It will be the closest approach of an object this large that we have ever known about in advance, and for a few hours it will be visible to the naked eye from Europe, Africa, and western Asia, crawling across the sky like a slow star of roughly 3rd magnitude.

Earth's gravity will noticeably bend and reshape Apophis's orbit and may even trigger seismic-scale surface shifting on the asteroid — a natural experiment planetary scientists intend to watch closely. Two spacecraft are lining up for it: ESA's Ramses (Rapid Apophis Mission for Space Safety), aiming to arrive before the flyby, and NASA's OSIRIS-APEX — the OSIRIS-REx probe, redirected after delivering its Bennu sample in 2023 — which will rendezvous shortly after. Crucially, a 31,600 km miss is still a miss by a wide margin; "potentially hazardous" earned Apophis its slot on the list, not a collision.

From bookkeeping to a nudge: DART and planetary defense

Cataloguing threats is only half the job; the other half is being able to do something. On 26 September 2022, NASA's DART (Double Asteroid Redirection Test) spacecraft slammed into Dimorphos, a ~160 m moonlet orbiting the larger asteroid Didymos, at about 6.1 km/s. It was humanity's first deliberate attempt to change the motion of a celestial body — the first real-world test of the kinetic impactor deflection technique.

The Didymos–Dimorphos pair was chosen precisely because the effect is easy to measure: Dimorphos orbited Didymos every 11 hours 55 minutes, and telescopes on Earth could time that orbit by watching the system's brightness dip. DART's impact shortened the orbit by about 32 minutes immediately, settling near a 33-minute reduction as the plume of debris kept pushing. That was far more than the spacecraft's momentum alone could deliver — the recoil from thousands of tonnes of blasted-off rock did most of the work, a bonus captured by a momentum enhancement factor (β) of roughly 3.6.

The lesson generalizes. A kinetic impactor works best with decades of warning: nudge an asteroid's speed by a few millimeters per second years ahead, and that tiny change compounds into a miss of thousands of kilometers by impact date. ESA's Hera mission, launched in October 2024, is en route to survey the Dimorphos crater in detail so engineers can calibrate exactly how much deflection a given impact buys. This is why the PHA catalogue matters so much: early detection is the entire game. You cannot gently steer what you find only weeks out.

The energy scale, from window-rattlers to civilization-enders

Impact severity climbs steeply with size, because kinetic energy scales with the cube of diameter (mass) and the square of speed. Typical entry velocities run 12–20 km/s. A few benchmarks anchor the scale:

• ~20 m — Chelyabinsk, 15 February 2013. An undetected stony asteroid entered over Russia and detonated in an airburst peaking near ~23–30 km altitude, releasing roughly 400–500 kilotons of TNT-equivalent (~30× the Hiroshima bomb). The shock wave shattered windows across the city; about 1,500 people were injured, almost entirely by flying glass. It was too small to be a PHA — and no one saw it coming.
• ~50–60 m — Tunguska, 30 June 1908. An object exploded a few kilometers above Siberia with an energy usually estimated at 10–15 megatons, flattening about 2,000 km² of forest — some 80 million trees — yet leaving no crater. This is roughly the low end of the PHA size class, and shows why 140 m is a sensible worry line.
• ~140 m and up — the PHA floor. An impact here can devastate a region or, over ocean, drive tsunamis.
• ~10 km — the Chicxulub impactor, ~66 million years ago. Released on the order of 10⁸ megatons and is tied to the extinction of the non-avian dinosaurs.

Small objects (metres across) burn up or airburst harmlessly and strike often; giant ones are civilization-scale but vanishingly rare. The PHA definition deliberately targets the uncomfortable middle: bodies common enough to matter on human timescales, and large enough that we would very much like to see them coming.

How we find them, and what we still miss

PHAs are hunted by wide-field survey telescopes that photograph the sky repeatedly and flag anything that moves against the stars. The workhorses have been Pan-STARRS in Hawaii and the Catalina Sky Survey in Arizona, which together account for most near-Earth discoveries. A newly found object's positions are reported to the Minor Planet Center, and CNEOS computes its orbit — and its MOID — automatically, sorting it onto (or off) the PHA list.

Two automated systems then run continuous impact-monitoring: NASA's Sentry and ESA's Aegis. They project each orbit forward, accounting for planetary perturbations and even the tiny Yarkovsky effect — the asymmetric push from an asteroid re-radiating absorbed sunlight, which for a body of Apophis's size can shift an orbit by hundreds of kilometers per year. When observations narrow, most flagged objects are removed from the risk list, exactly as happened with Apophis. Communicating this back-and-forth is aided by the Torino scale (0–10, colour-coded), which rates public impact hazard.

The blind spots are real and worth stating honestly. Surveys work in reflected sunlight, so objects approaching from the sunward direction are washed out — precisely how Chelyabinsk slipped in. Roughly 95% of the ~1-km-and-larger near-Earth asteroids have been found, but for the 140 m PHA class the completeness is far lower — likely under half. NASA's NEO Surveyor, an infrared space telescope slated to launch around 2027–2028, is designed to close that gap by spotting asteroids by their heat, including many that hide in the Sun's glare. Until then, the honest summary stands: the known PHAs are well-behaved and pose no century-scale threat — but the catalogue is still being completed, which is exactly why the surveys never stop.

Every PHA is a near-Earth object, but not every near-Earth object is a PHA — the two thresholds do the filtering.

Property	Near-Earth Object (NEO)	Potentially Hazardous Asteroid (PHA)
Orbit criterion	Perihelion within 1.3 au of the Sun (comes near Earth's orbit)	Earth MOID ≤ 0.05 au (~7.48 million km) — can actually get close
Size / brightness	Any size, no cutoff	Absolute magnitude H ≤ 22.0, i.e. roughly ≥ 140 m
Approximate count (2025)	~38,000–39,000 near-Earth asteroids known (2025) and rising	~2,500
What the label means	Neighborhood, not threat	Big enough and close-passing enough to warrant tracking
On a collision course?	Almost never	Not necessarily — and none known for this century