How Many Asteroids Are Actually Dangerous?
published during a waxing crescent moon.


The mapping of the near-Earth asteroid Bennu is one of the science goals of NASA’s OSIRIS-REx mission, and an integral part of spacecraft operations. The spacecraft will spend a year surveying Bennu before collecting a sample that will be returned to Earth for analysis. Credit: NASA/Goddard/University of Arizona

Bennu is a dark, ominous force–an asteroid that could possibly get uncomfortably close to Earth in the next century. It obediently answers to the sun’s rays, sponging up their heat and even letting them push its orbit around. Succinctly classified as a “potentially hazardous asteroid”, or PHA, Bennu isn’t exactly an anomaly. There are over 1,720 known PHAs, with tens of thousands more waiting to be charted. The word “potential” doesn’t imply that they will impact Earth: It refers to the possibility of that threat. But Bennu has about a 1 in 2,500 chance of hitting Earth in the 22nd century, according to the team behind NASA’s upcoming sample return mission.

There are a few parameters that qualify an asteroid as potentially hazardous:

1) The asteroid has to be at least 100 meters in size and be of a certain visible brightness on the absolute magnitude scale.

2) A PHA must also have a minimum orbital intersection distance, or MOID — a measurement that helps monitor possible Earth colliders.

3) An asteroid is potentially threatening if it has a MOID that’s 0.05 astronomical units (AU) or less. (One AU is about the distance between the Earth and the sun.)

In simpler terms, the bigger the MOID, the less likely the asteroid will meet Earth in the near future. But if it’s small, it’s something to keep tabs on — and closely. A telescope like the new Large Synoptic Survey Telescope in Chile, for instance, can then give a warning time of a couple of months for smaller objects or eight years for a bigger hazard.


OSIRIS-REx will travel to near-Earth asteroid Bennu on a sample return mission. Credit: NASA

The intrepid OSIRIS-REx spacecraft (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is scheduled to launch next month and rendezvous with Bennu in two years, where it will map it, take its temperature, and even collect up to 2 kilograms of surface material. It’ll drop the samples off on Earth in 2023.

The mission is part of a long legacy of bringing back extraterrestrial matter that dates back to the 1960s. Asteroid samples are particularly desirable because they’re floating fossils of the solar system’s beginnings, which we can use to determine what life-dependent compounds were already on Earth and which ones were delivered by cosmic matter.

In Bennu’s case, insight into its composition and behavior can improve our chances of redirecting it. The asteroid is so dark that it absorbs sunlight and then emits it back as energy as heat, which nudges its orbit over time. This phenomenon is called the Yarkovsky Effect, and Bennu’s sidling migration is in need of new observations because of this effect.

But be wary of exaggerated reports of the Earth’s impending doom: Scientists from the OSIRIS-REx mission soothed agitated worries with reminders that the chances of Bennu reaching Earth are still very, very slender. In an interview with, officials even stressed that Bennu isn’t big enough to even conjure the energy to destroy the planet.

The spacecraft’s journey is to refine those odds, and NASA already has an asteroid redirect mission in motion set to launch in the 2020s. The robotic mission will physically remove a heavy boulder from a near-Earth asteroid and return a sample to us, then redirect the asteroid into a safe orbit around the moon using gravity. In doing so, scientists might be able to take on a space bully with just the powers of their wit and some nimble spacecraft.



The asteroid redirect vehicle demonstrates the “gravity tractor” planetary defense technique on a hazardous-size asteroid. The gravity tractor method leverages the mass of the spacecraft to impart a gravitational force on the asteroid, slowly altering the asteroid’s trajectory. The demonstration is conducted after capturing the boulder and is referred to as the “enhanced gravity tractor” because the additional mass of the boulder enhances the force that can be transmitted to the asteroid. Credit: NASA