DART's epic asteroid crash: What NASA has learned 5 months later

NASA’s Double Asteroid Redirection Test (DART) mission had two main goals: to show that an asteroid could be targeted in a high-speed encounter, and to demonstrate that the target’s orbit could be changed — a technique astronomers hope to use for planetary defense should a dangerous space rock come our way.

DART has successfully done both,” astronomers report in a new study (opens in new tab). The mission’s resounding success shows that a “kinetic impactor” like DART is a “viable technique to potentially defend Earth if necessary,” researchers note in another (opens in new tab) new study.

Those two studies are part of a raft of five DART papers published online Wednesday (March 1) in the journal Nature. In the five studies, astronomers shared additional findings from the mission using data the probe sent home up in the leadup to its colllision with Dimorphos, a moon of the 2,560-foot-wide (780 meters) asteroid Didymos, on Sept. 26, 2022, and in the crash’s aftermath.

Related: Behold the 1st images of DART’s wild asteroid crash!

The latest results focus on reconstructing DART’s final moments; precise calculations of how much the spacecraft changed the orbit of its target; Dimorphos’ puzzling twin tails; and key mission moments captured by a network of citizen science telescopes worldwide.

Although researchers are still studying the DART data, they are already developing a sequel mission: the European Space Agency’s Hera spacecraft, which is scheduled to launch in October 2024 and reach Didymos two years later. Hera is expected to study the Didymos-Dimorphos system in detail, including the crater formed by DART’s plunge.

“This situation is rare for planetary exploration, and is very exciting!” Carolyn Ernst, a planetary scientist at The Johns Hopkins University Applied Physics Laboratory (APL) and a co-author of one of the studies (opens in new tab), told Space.com in an email.

DART’s final moments in detail

A month prior to its impact, the DART probe began sending home pictures once every five hours, which were processed by a ground optical navigation team, researchers report in the new papers.

About four hours before impact, researchers handed over control to DART and allowed it to navigate itself using its autonomous SMART Nav system, which also processed images onboard to first identify Didymos and later Dimorphos.

The mission team already knew that Dimorphos would be hidden from the spacecraft’s view for much of this time, so they kept DART moving toward Didymos until it was able to detect Dimorphos, the smaller and dimmer of the two — which it did 73 minutes prior to slamming into it, researchers say.

“It was amazing to see it for the first time — no one had ever acquired a resolved image of Dimorphos before,” Ernst said. That image showed Dimorphos’ surface to be strewn with boulders, similar to rubble-pile asteroids like Ikotawa, Bennu or Ryugu.

About 2.5 minutes prior to crashing, the DART probe stopped maneuvering to settle down and reduce jitters and smear in its final images, researchers noted in one (opens in new tab) of the five new studies. Throughout this phase, the spacecraft clicked one image every second, including a picture of its impact site, a patch of Dimorphos covering 9,472 square feet (880 square m) — the last thing it sent home 1.8 seconds before plunging between two large boulders on the asteroid as planned.

DART approached Dimorphos at a 73-degree angle and had its solar arrays slightly slanted, so the probe ended up grazing one of the boulders just before impact. Before this mission, researchers had little idea how Dimorphos looked — it could have been anything from a collection of rubble to a single large rock.

Using DART’s data, researchers modeled the asteroid’s shape with the help of a technique called stereophotoclinometry, which is often used to model the shapes of small bodies. They found Dimorphos is an oblate spheroid, like a rugby ball, with a diameter of 580 feet (177 m).

“Clearly, it looks like a collection of rocks!” Ernst said, adding that she was surprised how ellipsoidal the asteroid looks.

Ernst said her team is working on new models and experiments to better understand what exactly happened during DART’s impact and how the event changed the asteroid’s orbit and spin, all of which will be essential for applying this kinetic impact technique for planetary defense.

“There are many, many more things to be learned,” she said.

How the impact changed Dimorphos’ orbit so dramatically

When DART slammed into Dimorphos, the spacecraft hit as designed on Dimorphos’ leading hemisphere, the one facing forward as the rock travels around the sun. Researchers had planned the impact in this way so as to maximize momentum transfer from the spacecraft to the asteroid, helping push it closer to Didymos. 

Previously, the asteroid circled Didymos every 11 hours and 55 minutes. Astronomers announced in October 2022 that DART had successfully shortened the orbit of Dimorphos by 32 minutes, which one of the new studies tweaks to 33 minutes.

Imagine the impact to be like playing billiards in space: a solid spacecraft crashes into a solid asteroid, and no material is ejected. In this scenario, researchers expected that DART would shave off seven minutes from Dimorphos’ orbit. (DART had only to reduce the orbital period by 73 seconds to be hailed a success.)

If the asteroid turned out to be a relatively loose pile of rocks, however, researchers estimated a much higher orbit change, up to 40 minutes.

When DART plunged into Dimorphos, it confirmed the latter scenario: at least 2.2 million pounds (1 million kilograms) of blasted-out material provided an extra boost of momentum, which was key to shortening the asteroid’s orbital period by 33 minutes, one of the new studies found.

“I was, like many of us on the team, surprised to find such a large momentum transfer,” Andrew Cheng, the lead author of the study (opens in new tab) that measured DART’s momentum transfer to Dimorphos, told Space.com in an email. 

Researchers had predicted that this might happen, so it was not a total surprise — but it was nevertheless exciting. The blasted material acted similar to a triggered gun: it kicked back against Dimorphos because of recoil, increasing the momentum transferred to the asteroid above and beyond what DART’s mass and velocity alone could have contributed.

Researchers used DART’s DRACO instrument to record positions of Didymos and Dimorphos relative to each other as the probe approached the asteroid system. These images, which include DART’s final moments, were “a fantastic addition” for the analysis, Cristina Thomas, an astronomer at Northern Arizona University and a co-author of one of the latest studies, told Space.com in an email.

Using data from telescopes on all seven continents, the team calculated the 33-minute change in Dimorphos’ orbit “despite the presence of ejecta in all of our observations,” it noted in the study. The team also found that DART’s crash did not change Didymos’ orbital period around the asteroid duo’s center of mass, which is still 2.26 hours.

This test is the first and so far the only one that shows we can use kinetic impactors like DART to deflect asteroids. As many asteroids are similar piles of rocky debris, researchers say material blasted out by impacts from spacecraft like DART can lead to significant additional momentum and, as a result, a greater deflection of the targets.

“This means that we could change an asteroid’s path with less warning time,” Thomas said. “This fact would be so incredibly important if we needed to deflect an actual target.”

Mysterious twin tails remain unexplained

We know of a dozen or so active asteroids, which are space rocks that look like asteroids but behave like comets, with tails that sometimes stretch 1 million miles (1.6 million kilometers). While astronomers think asteroid collisions likely led to such features, they have never observed the process directly.

So when DART crashed into Dimorphos, researchers had a rare front-row seat to watch the ejected debris from the moment it blasted out of the asteroid. The team used the Hubble Space Telescope to image the ejecta for 18.5 days, beginning 15 minutes after the impact, according to one of the new studies (opens in new tab).

Soon after the impact, the ejected material morphed into a cone-like shape with rock clumps of various sizes flying as far as 310 miles (500 km) from the asteroid. These non-uniform ejecta show that Dimorphos likely has a bouldery surface but a rubble-pile interior, researchers say.

Three hours after the collision, the first dust tail emerged in a direction opposite to the ejecta cone, and radiation from the sun stretched it more than 930 miles (1,500 km) — so much so that it “exceeded the spatial coverage of our images,” researchers note in the study. 

They watched a second tail form between Oct. 2 and Oct. 5, and the increase in scattered dust decreased the Didymos system’s overall brightness. The team tracked the tail until it faded away two and a half weeks later. While astronomers know of a few asteroids with twin tails, they had not expected Dimorphos to flaunt them.

“When I first see those images,” Jian-Yang Li, an astronomer at the Planetary Science Institute in Tucson, Arizona, and the study’s lead author, told Space.com in an email, “I thought my eyes are tricking me, or there might be some problems with the images.”

Although researchers do not yet know how the double tail formed, Li said it could be explained by either a few blasted rocks re-impacting Dimorphos or Didymos, or larger rocks colliding and then disintegrating into small pieces. 

The smaller ejected particles spanning a few centimeters will likely hover in the Didymos-Dimorphos system for a few months, while the larger ones could be around for even longer, as long as they don’t hit either Didymos or Dimorphos, or get too close to them, Li said.

Related stories:

— Asteroid impact: Here’s the last thing NASA’s DART spacecraft saw before it crashed

— DART asteroid crash seen by James Webb, Hubble space telescopes (photos)

— DART impact gave asteroid Dimorphos a debris tail thousands of miles long (stunning photo) 

Citizen astronomers capture key moments of DART’s crash

Although this mission was one of the few relying on ground-based observations for its success, there were very few places on Earth where the Didymos system was visible at the moment of DART’s crash. 

So, despite the mission’s importance, “astronomers couldn’t just turn some of their best telescopes (like Keck in Hawaii) to watch it, because they were not in the right place at the right time,” Ariel Graykowski, an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, told Space.com in an email.

Astronomers also worried that Dimorphos would move too fast for Hubble or even the mighty James Webb Space Telescope to capture good images. Luckily, both telescopes worked in sync and recorded valuable data. But their observations were delayed by at least 15 minutes and as a result did not include images at the time of impact.

So citizen astronomers in Reunion Island in the Indian Ocean and Nairobi, Kenya, used the Unistellar eVscope, among the smallest telescopes that observed the Didymos system during DART’s crash. From the resulting data, the team estimated the mass of the dust cloud to be 0.3% to 0.5% that of Dimorphos. 

“This network of telescopes was the best tool, and perhaps even a necessary tool to accomplish this,” said Graykowski, the lead author of a study (opens in new tab) that reported observations of DART’s impact (opens in new tab) using citizen science telescopes.

Her team also found that the impact spiked the system’s brightness to a magnitude of 2.29, or by nearly 10 times — so much so that it led to some speculation that Dimorphos broke apart, Graykowski said. The asteroid returned to its original brightness a little over two weeks later, and the study’s findings confirm that Dimorphos is safe and sound, albeit with a lesser mass.

“This is good, because the goal was to deflect the asteroid, not destroy it!” Graykowski said.

In addition to the increased brightness of the system, her team also noticed that Didymos reddened slightly for a bit shortly after DART’s plunge. This color shift could be because of either our viewing angle of the thick dust cloud or its irradiated material, Graykowski said. Researchers saw a similar reddening effect in the thick dust cloud caused by NASA’s Deep Impact spacecraft when it crashed into comet Tempel 1, whose color returned to normal once the dust cloud faded.

Graykowski said the new study was a collaboration between eight SETI Institute astronomers and many citizen scientists, ranging from hobbyists to physics and astronomy professors, who voluntarily shared their observations of DART’s impact.

“The Unistellar citizen astronomers are absolutely the driving force behind this work,” Graykowski said. “Upon acceptance [of the paper], we all agreed that we would celebrate with a slice of cheesecake in our respective parts of the world!”

Source: Space.com

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