Could we deflect an asteroid if one were on a crash course with our home planet? 

A week ago I wrote about a harmless asteroid. Despite being the size of New York City’s Empire State Building, we knew 2002 NN4 would safely pass by the Earth on June 6 at a distance of approximately 3.2 million miles. 

What I didn’t know at the time was that the previous day a significant asteroid whipped by Earth completely unnoticed. 

OK, so 2020 LD wasn’t quite so scary as 2002 NN4.

2020 LD is about 400 ft./122 meters in diameter, whereas 2002 NN4 is 820-1,870 ft./250-570 meters. 

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However, 2020 LD passed a lot closer. Just a day before 2002 NN4 passed by at a comfortable 13 times the Earth-Moon distance, 2020 LD got to around 80% of the Moon’s distance.

That makes 2020 LD the largest asteroid to have come that close since 2011.

What’s worse is that no-one noticed 2020 LD until June 7 when astronomers at the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope at Mauna Loa, Hawaii saw it and computed its trajectory.

Something similar happened in 2019 with Asteroid 2019 OK

The prospect of a major asteroid impact is scary. It’s something we should plan for, alongside some other Armageddon-esque scenarios:

  • more, and worse pandemics
  • wide-scale droughts
  • devastating earthquakes
  • a blob of hot gas from the Sun

However, planning for an asteroid strike means two things:

  1. you have to find as many asteroids as possible, map their orbits, and zero-in on which ones could be “planet-killers”.
  2. then work out how best to destroy or deflect it.

In short, you need skill and dedication, lots of time, and lots of space hardware.

Finding asteroids

When it comes to asteroids, ATLAS is doing good work; so far it’s discovered 46 threatening “potentially hazardous asteroids” (PHAs), which are defined as being larger than 500 ft./140 meters in diameter.

So 2020 LD is not quite considered a PHA, strictly speaking, but it’s much larger than the asteroid that exploded over Chelyabinsk, Russia, in 2013. At 66 ft./20 meters in diameter, that was the biggest meteor for over a century. 

We’ll find more asteroids by building more sky-survey telescopes; the now-being-built Vera Rubin Observatory in Chile is designed specifically to find PHAs (and anything else in the night sky that moves).

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Deflecting asteroids

How do we deflect an asteroid to prevent if from striking Earth? On average, an asteroid with a diameter of 500 meters can be expected to impact Earth about every 130,000 years or so. MIT researchers—whose work was published in Acta Astronautica—recently attempted to answer that thorny question by considering the orbit and properties of two specific “planet-killer”-sized asteroids—99942 Apophis and 10195 Bennu.

99942 Apophis

Named for the Egyptian god of chaos, near-Earth asteroid 99942 Apophis will get super-close to Earth on April 13, 2029, grazing our geostationary satellites (see image below).

It will be the closest approach by one of the largest asteroids crossing Earth’s orbit in the next decade. It is, therefore, a good asteroid to practice on. It has a mean diameter of 1,100 ft./340 meters.

10195 Bennu

The target of NASA’s OSIRIS-REx mission, Bennu is a near-Earth asteroid that has a 1-in-2,700 chance of striking Earth between 2175 and 2199. Bennu has a mean diameter of 1,610 ft./490 meters.  

Nuclear bomb vs. ‘kinetic impactor’

There are thought to be only two ways of deflecting an asteroid; launch a nuclear bomb into space to blast the asteroid away or send a “kinetic impactor”—a spacecraft or rocket—to collide with the asteroid and push it slightly off course.

The latter is what NASA will try to do at asteroid Psyche in 2026.

“The basic physics principle is sort of like playing billiards,” said Sung Wook Paek, lead author of the study and a former graduate student in MIT’s Department of Aeronautics and Astronautics.

First, get to know your asteroid

However, say the researchers, for the second option to be of any practical use we would have to know, in advance, the following about an asteroid: 

  • mass
  • momentum
  • trajectory
  • surface composition

Otherwise any attempt at deflecting an asteroid would be less than 99.9 percent certain to succeed; and when you’re talking a planet-killer, that’s important. 

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You also need to know:

  • the amount of warning time that scientists have of an impending collision.
  • if the asteroid is close to a gravitational keyhole—a location in Earth’s gravity field that would tug the asteroid’s trajectory so that its next flyby (which may be just a few years later) would likely mean a devastating impact with Earth. 

“A keyhole is like a door—once it’s open, the asteroid will impact Earth soon after, with high probability,” said Paek. Apophis and Bennu are two of only a handful of asteroids for which the locations of their gravitational keyholes are known to astronomers. 

Kinetic impactor: a pre-emptive strike?

“People have mostly considered strategies of last-minute deflection, when the asteroid has already passed through a keyhole and is heading toward a collision with Earth,” said Paek. “I’m interested in preventing keyhole passage well before Earth impact. It’s like a pre-emptive strike, with less mess.”

This is the a kinetic impactor to nudge an asteroid off course. Such a mission could, say the researchers, involve sending a scout to first measure the asteroid, which would determine the kind of projectile that would need to be sent up later. Or it could mean two scouts, one to measure the asteroid and the other to push the asteroid slightly off course before a larger projectile could later be launched to make sure. 

In their simulations with Apophis and Bennu they discovered that if these asteroids passed through a keyhole within one Earth year or less it may be too late to do anything. However, if it was predicted that it would go through the keyhole in five years, all kinds of mission designs are possible. 

“Instead of changing the size of a projectile, we may be able to change the number of launches and send up multiple smaller spacecraft to collide with an asteroid, one by one. Or we could launch projectiles from the Moon or use defunct satellites as kinetic impactors,” said Paek. “We’ve created a decision map which can help in prototyping a mission.”

However, what all this underlines is that we need to know as much as we can about all kinds of asteroids as far in advance as possible. 

Wishing you clear skies and wide eyes.