What are Potentially Hazardous Asteroids?

Asteroids are rocky objects that orbit the sun, left over from the formation of the solar system billions of years ago. Most asteroids reside in the main asteroid belt between Mars and Jupiter. However, some asteroids follow orbits that bring them much closer to Earth. When an asteroid’s orbit crosses Earth’s orbit and the asteroid is large enough to cause significant damage in an impact, it is considered a “potentially hazardous asteroid” (PHA).

More specifically, an asteroid is designated as a PHA if it comes within 4.65 million miles (7.48 million km) of Earth’s orbit and is greater than 460 feet (140 meters) in diameter. For comparison, the moon orbits Earth at an average distance of about 239,000 miles (385,000 km).

The size limit of 460 feet in diameter is not arbitrary. Studies have shown that an asteroid of this size could cause regional devastation if it struck Earth. While the impact of an asteroid that size wouldn’t be a global catastrophe, it could destroy a city or cause a tsunami if it hit an ocean. Smaller asteroids are much more numerous but would likely break up in Earth’s atmosphere, causing little to no damage on the ground.

Tracking and Cataloging PHAs

NASA and other space agencies around the world are actively searching for and tracking PHAs. The goal is to catalog 90% of the estimated population of PHAs so their orbits can be precisely determined. This effort will help identify any PHAs that could pose an impact risk to Earth, ideally with many years or decades of advance warning.

As of 2023, over 30,000 near-Earth asteroids have been discovered, with more than 2,300 of them categorized as potentially hazardous. However, it is estimated that there are around 25,000 near-Earth asteroids larger than 460 feet, so many PHAs remain to be found. New telescopes coming online in the next decade will greatly accelerate the pace of PHA discoveries.

The most comprehensive search for PHAs is NASA’s Near-Earth Object (NEO) Observations Program. This program funds several ground-based telescopes that scan the skies for moving objects that could be asteroids. Some of the most prolific asteroid-hunting projects include the Catalina Sky Survey in Arizona, the Pan-STARRS telescopes in Hawaii, and the NEOWISE space telescope.

When a new near-Earth asteroid is discovered, follow-up observations are made to more precisely determine its orbit. This allows scientists to calculate if and when the asteroid could potentially impact Earth. NASA maintains a public database of these objects called the Sentry Risk Table. It lists the asteroids with a non-zero chance of impacting Earth in the next 100 years. As of 2023, none of the known PHAs pose a significant risk of impact.

Famous PHAs

A few PHAs have become particularly well-known, either because they have made very close approaches to Earth or because spacecraft have visited them:

(99942) Apophis

Asteroid Apophis caused a brief period of concern in 2004 when initial observations indicated a 2.7% chance of an Earth impact in 2029. Additional observations ruled out an impact, but Apophis will still make a very close approach to Earth on April 13, 2029, coming within 19,800 miles (31,900 km) of Earth’s surface. That’s closer than some satellites! Apophis is about 1,100 feet (340 meters) in diameter.

(101955) Bennu

Bennu is one of the most potentially hazardous asteroids known, with a 1-in-2,700 chance of impacting Earth between 2175 and 2199. It is about 1,600 feet (490 meters) in diameter. NASA’s OSIRIS-REx spacecraft visited Bennu in 2018, studied it in detail, and collected a sample that was returned to Earth in 2023. The data from this mission will help refine our understanding of Bennu’s orbit and composition.

(3200) Phaethon

Phaethon has an unusual orbit that brings it very close to the sun, well inside the orbit of Mercury. Its surface gets heated to about 1,390 °F (750 °C) at closest approach. Phaethon is also the parent body of the Geminid meteor shower, which peaks every December. With a diameter of 3.6 miles (5.8 km), Phaethon is one of the largest potentially hazardous asteroids.

Impact Hazards

While no known PHAs pose a significant impact risk in the next century, the potential consequences of an asteroid strike make it a serious concern. The level of hazard depends on the size of the asteroid:

• Asteroids smaller than about 100 feet (30 meters) will most likely burn up in the atmosphere, causing little or no ground damage.
• Asteroids between 100-300 feet (30-100 meters) could cause significant local damage if they struck in a populated area. The shock wave could shatter windows and damage buildings across a city.
• Asteroids larger than about 300 feet (100 meters) could cause regional devastation. The impact would trigger a fireball, unleash a destructive shock wave, and potentially cause an earthquake or tsunami. A strike in a city could level structures for miles.
• Asteroids over 0.6 miles (1 km) in diameter could have global consequences. The dust and ash thrown into the upper atmosphere could block sunlight and disrupt climate for years. Historically, asteroids of this size have caused mass extinctions, like the dinosaur-killing Chicxulub impact 66 million years ago.

Fortunately, the largest asteroids are the easiest to detect, and none of the known asteroids over 0.6 miles in size pose an impact threat. Smaller asteroids are much more numerous but would cause more localized damage. The Chelyabinsk meteor that exploded over Russia in 2013, shattering windows and injuring over 1,600 people, was only about 66 feet (20 meters) across.

Mitigating Impact Risk

If an asteroid on a collision course with Earth was discovered, what could be done? Many ideas for deflecting a hazardous asteroid have been proposed, though none have been fully tested.

The simplest method is a kinetic impactor: crashing a spacecraft into the asteroid to slightly change its velocity. Over time, even a small change in velocity will alter the asteroid’s orbit enough to miss Earth. This technique requires a warning time of several years to decades, depending on the size of the asteroid and how much its orbit needs to be changed.

NASA’s Double Asteroid Redirection Test (DART) mission, launched in 2021, was the first demonstration of the kinetic impactor technique. In 2022, DART intentionally crashed into Dimorphos, the small moon of asteroid Didymos. The impact changed Dimorphos’ orbital period around Didymos by 32 minutes, more than enough to make a difference between an impact and a miss if Dimorphos had been on a collision course with Earth.

Other proposed asteroid deflection techniques include:

• Gravity tractors: a spacecraft hovering near the asteroid for an extended time, using its gravitational pull to gradually tug the asteroid off course.
• Laser ablation: focusing powerful lasers on the asteroid’s surface to vaporize material, creating a jet that acts like a small thruster to push the asteroid.
• Nuclear explosions: detonating a nuclear device near (not on) the asteroid to push it off course. This would only be considered as a last resort.

All of these techniques require years to decades of advance warning to be effective, which is why finding and tracking potentially hazardous asteroids well before they might hit Earth is so important.

Summary

Potentially hazardous asteroids are a real threat to Earth, but one that we can manage with sufficient knowledge and preparation. Continuing to search for and track PHAs is vital to protect against an impact that could cause massive damage and loss of life.

While no known PHAs currently pose a significant threat, it’s only a matter of time before another asteroid like the one that killed the dinosaurs crosses Earth’s path. Unlike the dinosaurs, however, we have the capability to see it coming and do something about it. Studying PHAs and testing asteroid deflection techniques may one day save our planet and our species from a catastrophic impact.