Astronomers and geologists maintain permanent watch lists for events that could reshape life on Earth. Most entries carry vanishingly small annual odds, yet the consequences are large enough that telescopes, satellites and ground sensors track them continuously. Eleven of those watched hazards, and the monitoring that keeps tabs on each, appear below.
1. Asteroid Bennu: The Highest Impact Odds in the Catalog
Bennu is a carbon-rich asteroid roughly 490 meters across, sampled by the OSIRIS-REx spacecraft and tracked closely by NASA’s planetary defense office. Its orbit is mapped well enough to single out one date: September 24, 2182, when the chance of impact sits near one in 2,700.
Framed the other way, those numbers describe a better than 99.9 percent probability of a miss, more than a century and a half out. The returned sample matters more for chemistry than for defense, though refined orbit data from the mission has already narrowed the uncertainty window considerably.
2. A Carrington-Level Solar Storm: The 1859 Benchmark No Modern Grid Was Built For
In September 1859 a geomagnetic storm now named for astronomer Richard Carrington sparked telegraph lines and drove auroras toward the tropics, an event catalogued across NASA solar system pages. Modern grids carry far more long conductors, and transformer damage rather than fire is the modern concern.
Utilities and space-weather forecasters now watch the Sun continuously, and warning of an incoming storm arrives hours before the plasma does. That margin allows operators to shed load and disconnect vulnerable transformers. Repair timelines, not the storm itself, drive the months-long outage estimates that appear in grid resilience studies.
3. Kessler Syndrome: The Debris Cascade That Could Close Low Earth Orbit
Donald Kessler’s 1978 paper described a runaway sequence: a single collision produces fragments, those fragments raise the odds of the next collision, and low Earth orbit slowly fills with shrapnel. Agencies publish running hazard feeds for such risks, much as USGS volcano updates do for ground hazards.
Roughly 40,000 objects are tracked today, with millions of untrackable fragments below one centimeter. Nothing about the cascade is instantaneous; models describe decades of degradation, not a single bad afternoon. Deorbit rules, collision-avoidance maneuvers and active removal demonstrations exist precisely because the trend, rather than any one crash, is the problem.
4. Near-Earth Comets: Long-Period Visitors That Arrive With Little Warning
Asteroids dominate the risk catalogues because their orbits repeat and can be projected centuries ahead. Long-period comets behave differently: they fall in from the outer solar system on orbits lasting thousands of years, which is why planetary defense monitoring treats them as a separate category with far shorter lead times.
Detection often comes months, not decades, before closest approach. Comets also strike faster than asteroids, carrying more energy per kilogram of mass. Their share of the overall impact risk stays small, perhaps a few percent, but the short warning window is the reason survey telescopes keep scanning wide swaths of sky every clear night.
5. Coronal Mass Ejections: Billion-Ton Plasma Clouds That Reach Earth in Days
A coronal mass ejection hurls a billion tons of magnetized plasma off the Sun at speeds up to several million miles per hour. Solar observatories tracked by NASA solar system research spot the eruption immediately, while the cloud itself needs one to three days to cross the distance to Earth.
Most ejections miss entirely or graze the magnetosphere, producing little beyond bright auroras. The damaging cases induce currents in long transmission lines and can degrade satellite electronics. Airlines reroute polar flights and satellite operators safe their spacecraft on forecast alone, which is why detection hours ahead has real operational value.
6. Betelgeuse: A Red Supergiant on Supernova Watch
Betelgeuse, the orange-red shoulder star of Orion, is a supergiant several hundred times the Sun’s diameter and roughly 500 to 700 light-years away. Astronomers watch its brightness the way agencies watch active ground hazards through USGS monitoring updates, because the star sits near the end of its fuel supply.
The 2019 dimming episode turned out to be a dust cloud, not an imminent collapse. Best estimates put the explosion anywhere within the next 100,000 years, a blink in stellar terms and an eternity in human ones. At that distance the supernova would outshine the full Moon without endangering the planet.
7. Rogue Black Holes: Stellar-Mass Drifters That Emit No Light
Stellar-mass black holes drift through the galaxy with no companion star to give them away, emitting nothing on their own. The wide-field sky surveys that feed NASA’s planetary defense pages catch them only by accident, through gravitational microlensing, a brief brightening when one crosses in front of a background star.
One confirmed free-floating example, detected in 2022, lies about 5,000 light-years away. The galaxy likely holds a hundred million of them, spread across a volume so large that a close encounter with the solar system is statistically negligible. Cataloguing them mainly refines estimates of how many massive stars have already died.
8. Gamma-Ray Bursts: The Brightest Explosions in the Known Universe
A gamma-ray burst releases more energy in a few seconds than the Sun will emit across its entire life, beamed into narrow jets visible from billions of light-years away. Orbiting detectors, whose findings are published through NASA’s public science pages, register roughly one burst somewhere in the sky each day.
Every burst recorded so far originated in a distant galaxy. A damaging event would require a nearby massive star with a jet aimed almost exactly at the solar system, a geometry that makes the odds vanishingly small. Detection value lies in stellar physics rather than in any practical warning.
9. Solar Superflares: What Kepler Found on Other Sun-Like Stars
Kepler and TESS observations turned up sun-like stars firing off flares hundreds of times stronger than anything measured from the Sun. Long-baseline records, kept with the same patience as USGS hazard bulletins, show the pattern is not purely alien: tree rings and ice cores mark sharp radiocarbon spikes in 774 and 993 AD.
Those spikes point to solar particle events far larger than the 1859 storm, though still short of the stellar extremes. Estimated recurrence sits in the range of once every thousand years. The practical upshot is a higher assumed ceiling for space-weather planning, not a forecast of any particular event.
10. Interstellar Objects: Visitors Like 3I/ATLAS on One-Way Paths
Three confirmed interstellar visitors have crossed the solar system since 2017: ‘Oumuamua, comet 2I/Borisov, and 3I/ATLAS, spotted in 2025. Each arrived on a hyperbolic path, meaning it entered from outside and will leave for good, and each was flagged by the survey network behind the planetary defense program.
None came close to Earth, with 3I/ATLAS passing no nearer than roughly 1.8 astronomical units. Speed is what makes the category interesting: these objects move fast enough that detection typically happens weeks before closest approach. Better survey coverage, including the Rubin Observatory, should push discoveries from rare to routine.
11. Supervolcano Ash Clouds: Eruptions Big Enough to Cool the Planet
Toba’s eruption roughly 74,000 years ago ejected about 2,800 cubic kilometers of material and dusted South Asia in ash. Comparable systems, including Yellowstone and Campi Flegrei, are wired for ground deformation, gas output and earthquake swarms; the resulting bulletins are as public as a NASA asteroid overview.
Eruptions at that scale occur on the order of once every 50,000 to 100,000 years, and none of the monitored systems currently shows signs of moving toward one. Smaller eruptions matter more in practice: Pinatubo in 1991 cooled global temperatures by about half a degree Celsius for two years.
