WASHINGTON — NASA on Sunday launched a mission to crash-land a spacecraft at an asteroid to study what happens when a spacecraft encounters a rock so close to Earth that it gets virtually no time to correct course.
The impactor, named Bennu, is more than 3 miles across and is described as “somebody’s lunch.” In fact, the would-be impactor will impact this asteroid just after midnight on Dec. 31, 2123. (New Yorkers will be celebrating early, if all goes well.)
Unlike most asteroids, Bennu is not a potential threat to Earth. Bennu is currently located about 32 million miles from our planet. That doesn’t mean the impactor won’t impact it — it just means there will be almost no time for Bennu to get away from us.
This event will take place so quickly that it will occur with such little warning that it’s possible that little by little some of the effects, such as light reflected off the asteroid, could cause Bennu to change course — which could lead to the impactor being ripped apart as it races by.
Preliminary results from the impactor’s first 24 hours show that there are several ways that the impactor could be partially or totally destroyed, depending on how quickly the asteroid shifts course (if any).
The impactor will first travel about 7 miles up from the asteroid’s surface, before entering the impact horizon — between the asteroid’s surface and the sun.
From this point, the spacecraft does not have much room to maneuver. Once the impact horizon begins to flatten out, the spacecraft will begin to tumble. It would fall back into the asteroid, potentially splitting apart in a series of collisions.
That’s not the worst of it.
“We can’t do anything about that at all,” Mark Rober, project scientist for the event at NASA’s Jet Propulsion Laboratory, said in a statement. “But if you can’t stop the spacecraft, your objective is to maximize the damage as soon as possible.”
Rober is working on a variety of algorithms to help the impactor maintain even the tiniest amount of control, otherwise it could hurt itself and pose a threat to Earth.
“If you slow it down sufficiently, that will throw you out to the landing site and away from the initial impact,” Rober said. “If you can’t change the trajectory back to Earth, you are going to explode out to the center of the impact region — and it will go and do damage to itself.”
There are about 17,000 near-Earth asteroids larger than 300 feet in diameter, and Bennu is one of them. But other asteroids of similar size are far more common, and these other asteroids are a lot more likely to survive a collision with Earth.
Rober said that Bennu was actually about 500 times larger than the largest asteroid to approach the Earth in recent history — that was Chicxulub in 1857. There’s a good chance Bennu might be even bigger, Rober said.
In all likelihood, this impactor will miss Bennu by a wide margin. While scientists have the exact collision trajectory for both the spacecraft and the asteroid, they have yet to calculate the asteroid’s center of gravity. There’s a small chance that the spacecraft may strike a major asteroid and somehow be affected by Bennu’s gravity, Rober said. But the goal is to make the best of a bad situation, and crash-landing is the best bet.
Ultimately, NASA thinks the objectives of the mission are still worth the trouble.