5 ways we know DART crushed that asteroid (but not literally)

Space experts dove deeper into the dramatic aftermath of NASA's big asteroid-redirection mission.

By Jon Kelvey | Published Mar 2, 2023 2:05 PM EST

Science

DART spacecraft impact on Dimorphos asteroid in a computer model with yellow, gray, and black — When the DART spacecraft slammed into asteroid Dimorphos, the spacecraft body hit between two large boulders while its two solar panels impacted those boulders. The yellow surface is a digital terrain model of the impact site made from DART images, and the rendering of the DART spacecraft depicts its position a few tens of microseconds before impact. The white line extending from the back of the spacecraft shows the spacecraft’s trajectory. NASA/Johns Hopkins APL

On September 26, 2022, NASA’s Double Asteroid Redirection Test (DART) spacecraft slammed into the asteroid moonlet Dimorphos at 13,000 miles per hour, altering the extraterrestrial rock’s orbit around its larger companion asteroid, Didymos. A triumphant success of planning, targeting and autonomous flight that covered 7 million miles, the impact served as the first proof of concept for kinetic impactors—spacecraft that could be used to redirect any future asteroids on a collision course with Earth.

But to understand how a DART-like mission would work in a real apocalyptic scenario, astronomers and national security experts need lots of data and detailed analysis. Data they had almost immediately, as just about every telescope and sensor that could be trained on Dimorphos, was, prior to impact. And now, detailed analyses of what happened are going public, starting with five papers published in the journal Nature on March 1.

1. Kinetic impactors like DART can make a real splash

In a study of Dimophos’s orbit led by Northern Arizona University Astronomer Cristina Thomas, an international team calculated just how much DART’s crash landing changed the asteroid’s orbital period. Using radar and light curves, measured from changes in Dimorphos’s brightness over time, they showed the space rock slowed down in its orbit by 33 minutes, give or take about three minutes.

“To serve as a proof-of-concept for the kinetic impactor technique of planetary defense, DART needed to demonstrate that an asteroid could be targeted during a high-speed encounter and that the target’s orbit could be changed,” Thomas and her colleagues write in the paper. “DART has successfully done both.”

The researchers note, however, that there were probably several reasons why DART was able to slow Dimorphos down by a full half hour. If the only factor were the spacecraft’s mass, the asteroid’s orbit should have changed by no more than seven minutes. Any other explanations would “require modeling beyond the scope of this paper,” they explained.

DART spacecraft impact site on Dimorphos asteroid face — This image depicts the footprint of the DART spacecraft and its two long solar panels over the spot where it impacted asteroid Dimorphos. The largest boulder near the impact site is about 21 feet across. DART took the underlying image three seconds before impact. *NASA/Johns Hopkins APL*

2. DART got a big assist from the asteroid itself

A second paper led by Andy Cheng, chief scientist for planetary defense and the Johns Hopkins Applied Physics Laboratory, dug into why Dimorphos’s orbit shifted so dramatically.

His team’s research found that the “ejecta,” the material shaken loose from Dimorphos by the force of DART’s impact, amplified the transfer of kinetic energy from the spacecraft and the change in the asteroid’s orbit by 2.2 to 4.9 times. In fact, the authors write in the paper, “significantly more momentum was transferred to Dimorphos” from the escaping ejecta than DART itself.

Determining how much momentum a spacecraft can transfer to an asteroid and how that affects the asteroid’s orbit were key questions the DART mission sought to answer, and this study gives scientists the parameters they were waiting for. It illustrates the range of effectiveness kinetic impactors might have on hazardous asteroids given their makeup. Asteroids that respond to a strike with more ejecta may allow a DART-type spacecraft to deflect larger asteroids than it could otherwise, or to deflect an asteroid with less warning time.

3. Planning ahead is key to saving the planet

The key takeaway of the third paper, led by Terik Daly, Carolyn Ernst, and Olivier Barnouin of the Johns Hopkins Applied Physics Laboratory, is that despite DART’s successful strike and the helpful amplification by the impact ejecta, planetary protection remains a game of observation and early warning. “Kinetic impactor technology for asteroid deflection requires having sufficient warning time—at least several years but preferably decades—to prevent an asteroid impact with the Earth,” the researchers write in the paper.

Early warning, thankfully, is something NASA has been investing in since long before the DART mission. The NASA Authorization Act of 2005 directed the space agency to catalog 90 percent of all near-Earth asteroids of 460 feet in diameter or greater, a task that is now complete. NASA is now building an infrared space telescope scheduled for launch in 2028 that will help scan the skies for unseen asteroids.

“NEO Surveyor represents the next generation for NASA’s ability to quickly detect, track, and characterize potentially hazardous near-Earth objects,” Lindley Johnson, NASA’s planetary protection officer, said in a statement.

Dimorphos asteroid moving through night sky with tail after DART impact — This GIF is constructed of images taken on November 30, 2022 by astronomers at Magdalena Ridge Observatory in New Mexico, USA. It shows the motion of the Didymos system across the sky over the course of roughly 80 minutes, and features a long, linear tail stretching to the right from the asteroid system to the edge of the frame. The animation is roughly 32,000 kilometers across the field of view at the distance of Didymos. *Magdalena Ridge Observatory/NM Tech*

4. DART was also secretly a planetary-science mission

Dimorphos’s ejecta not only affected the orbit of the asteroid, they gave it a dust tail that strutted more than 900 miles from the asteroid within three hours of the impact, according to a fourth study led Jian-Yang Li, a senior scientist at the Planetary Science Institute.

Thought comets are better known for their brilliant tails, asteroids can also become “active,” as scientists put it, and form a little train on their backsides. It’s thought that this happens after some kind of impact, though the idea has never been put to the test.

The September mission gave scientists a “detailed characterization” of the ejecta-to-tail-making process serving double duty as a planetary-protection and a planetary-science mission. “DART will continue to be the model for studies of newly discovered asteroids that show activity caused by natural impacts,” the researchers write.

Dimorphos asteroid ejecta seen in blue and red after DART impact — Astronomers using the NSF’s NOIRLab’s SOAR telescope in Chile captured the vast plume of dust and debris blasted from the surface of the asteroid Dimorphos by NASA’s DART spacecraft when it impacted on 26 September 2022. In this image, the more than 10,000 kilometer long dust trail—the ejecta that has been pushed away by the Sun’s radiation pressure, not unlike the tail of a comet—can be seen stretching from the center to the right-hand edge of the field of view. *CTIO/NOIRLab/SOAR/NSF/AURA/T. Kareta (Lowell Observatory), M. Knight (US Naval Academy)*

5. DART really lit Dimorphos up

The last paper also falls into the planetary-science bucket with a close look at Dimorphos in its post-DART hangover. A study with ground-based telescopes in Africa and an Indian Ocean island led by SETI Institute astronomer Ariel Graykowski found it took the asteroid more than 23 days to return to its pre-impact levels of brightness in the night sky.

The analysis also found that ejecta appeared reddish at the time of impact, which is somewhat mysterious. “Typically, active bodies appear bluer in color on average than their inactive counterparts,” the researchers write in the paper, giving the examples of active comets versus inactive Kuiper Belt objects. “Some of these redder observed surface colors may be due to irradiation of organics,” they add, noting that lab experiments have shown space radiation can cause redden some of the same minerals probably found in asteroids like Dimorphos.

The five studies are just the first wave of an ongoing campaign to analyze the DART mission from different angles. The European Space Agency’s HERA mission, for instance, will rendezvous with Dimorphos sometime in 2026 to better assess the aftermath of DART’s impact in detail. Until then, NASA and other collaborators can continue to celebrate a major milestone in humanity’s relationship with the space around us.

“I cheered when DART slammed head on into the asteroid for the world’s first planetary defense technology demonstration, and that was just the start,” NASA administrator for its Science Mission Directorate, Nicola Fox, said in a statement on March 1. ”These findings add to our fundamental understanding of asteroids and build a foundation for how humanity can defend Earth.”

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