September 26, 2022

— A NASA robotic probe has slammed into an asteroid and been destroyed. The spacecraft's sacrifice was in not vain, though — its loss may someday help save life on Earth.

The Double Asteroid Redirection Test (DART) completed its mission on Monday (Sept. 26), ending its 10-month journey by crashing into Dimorphos, a "moonlet" of the asteroid Didymos. Its impact marked the the first time that a spacecraft autonomously navigated to a target asteroid and intentionally collided with it in a try at changing the rock's motion in a way that could be measured by telescopes back on Earth.

"Today we're taking a giant step in planetary defense, that is, protecting our home planet Earth," said NASA Administrator Bill Nelson in a video statement shared on the space agency's social media channels. "[DART is the] first test to help determine our response if we really do see an asteroid that's out there threatening to hit Earth."

If it was successful, the $330 million DART mission will serve as a model for one of the ways that NASA could protect Earth from an impending asteroid impact. Were a rocky body of similar size to Dimorphos (which is 525 feet or 160 meters in diameter) be found on a collision course with Earth, the agency could launch a DART-like spacecraft to change its trajectory.

"We're doing this test when we don't need to, on an asteroid that isn't a danger, just in case we ever do need to and we discover an asteroid that is a danger," said Tom Statler, NASA program scientist for the DART mission, in a press conference previewing the impact. "At the end of the day the real question is, how effectively did we move the asteroid?"

Now you see me...

Targeting, let alone hitting, an asteroid that is about the same size as one of the pyramids in Egypt at a distance of about 7 million miles (11 million km) from Earth is not easy.

"We have never seen it up close. We don't know what it looks like, we don't know what the shape is, and that's just one of the things that leads to the technical challenges of DART," said Statler.

DART's approach to Dimorphos. Click to enlarge video in a new pop-up window. (NASA)

DART's navigation camera, DRACO (or Didymos Reconnaissance and Asteroid Camera for Optical Navigation) did not get its first look at its destination until four hours before it impacted, when it was still 56,000 miles (90,000 km) from the binary asteroid system. Even at that distance, though, the two asteroids appeared as one.

It was not until the spacecraft was 14,000 miles (22,000 km) away — an hour out from impact — did Dimorphos become observable. At that point, DRACO could only resolve its target as a 1.5-pixel-wide dot (Didymos, by comparison, appeared 7 pixels wide).

Four minutes out, at a distance of 920 miles (1,500 km), Dimorphos had grown in size to 22 pixels wide. Two minutes and 460 miles (740 km) closer, DART had made its final maneuvers and was drifting at 14,000 miles per hour (22,000 k/ph) to its end.

It was only at 11 miles (18 km) — three seconds before hitting Dimorphos — that DART was able to capture and return a high-resolution image of the site where it was about to impact.

...now you don't

At 7:14 p.m. EDT (2314 GMT), the 1,260-pound (570-kilogram) DART spacecraft plunged into Dimorphos, giving the asteroid a healthy shove and, depending on the conditions at the time, possibly excavating a crater and blasting somewhere between 22,000 and 220,000 pounds of asteroid surface material into space. The recoil "kick" from these ejecta could rival, or even exceed, the direct push from the 3.9 by 4.3 by 4.3-foot (1.2 by 1.3 by 1.3-meter) box-shaped DART spacecraft.

"If the surface of the asteroid is very solid, we'll get a smaller crater. If it's friction free ... we might get a very, very large crater or a large section of the asteroid being reshaped," Statler said. "That's a big part of why we needed to do the test, because every time we go to an asteroid, it's a surprise."

The impact was set to be recorded by cameras on a small satellite released from DART 15 days ago. The Light Italian CubeSat for Imaging of Asteroids, or LICIACube, which was contributed and operated by the Italian Space Agency, was on a course to fly past Dimorphos about three minutes after the collision and take images of the kinetic impact, the ejecta plume and possibly the impact crater.

The Didymos binary asteroid system was chosen for the DART mission because, from the vantage point of Earth, Dimorphos regularly passes in front of and behind Didymos as it orbits. Before the impact, the orbital period of Dimorphos around Didymos was 11 hours and 55 minutes.

"Just like if you dropped your wristwatch and damaged it, it's not going to keep necessarily the same time anymore," said Statler. "We are expecting that clock — that orbit — to run just a little bit faster. You might not notice it right away, but in the weeks and days to follow, you would notice that your watch is running fast and we will notice that the binary asteroid system is running fast."

Scientists at NASA and the Johns Hopkins University Applied Physics Laboratory expect the DART impact will quicken Dimorphos' orbit by about 10 minutes, though an increase of at least 73 seconds would be considered a successful deflection of an asteroid.

The test's results will not be known until October, after ground-based telescopes can precisely measure Dimorphos' new orbit.