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.
Related: How humanity could deflect a giant killer asteroid
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."
Related: Asteroids in deep space (photos)
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.
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!"
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"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."
That seems extremely inconsistent with the orbital period of Dimorphos, which was 11 hours 55 minutes before DART impact and 11 hours 23 minutes after impact.
Considering that the center of mass is located on a line beween the centers of the 2 bodies, I don't see how they can "orbit" that in 2 different frequencies at the same time. And, if DART changed one frequency, it seems it must change the other, as well.
Edit: Frankly, I was wondering if that sentence should have been about not altering the double asteroid's orbital period around the sun. But, I am seeing numbers like 2.11 years for that.
Looking further, it appears that the 2.26 hour number is the period of rotation for Dimorphos, not its orbital period.
- A human who was not paying attention. Spell check didn't catch it since"orbital" is spelled correctly. Human not submitting their stories for checking over by a chatbot?
- A chatbot who is incorrectly quoting a human who got it wrong? And the journalist does not check chatbot output before publishing?
Nevertheless, would the larger Didymos not take a little time to speed up around the new c.g.? That's a lot of mass to accelerate by little Didymos? Just curious since it looks like a rare example of sudden instability for what was a stable orbital period.
In this case, the smaller object was slowed a little at a point in its circular orbit, so it went into an eliptical orbit with the impact position as its fathest point and a slightly closer point on the opposite sided of its orbit.
To the extent that the lighter body can affect the orbit of the heavier body, the heavier body would also be expected to have its orbit around their center of mass become a bit eliptical and have a corresponging period, which is shortened.
That assumes that the loose ejecta doesn't have any effect on either orbit, once it's ejection momentum is factored into the smaller body's change in its orbit.
However, there was a previous thread where a poster claimed that the "rubble pile" consitencey of the target body was altered such that its optical center was no longer its center of mass. He insisted that was the reason that the orbital period of the target body appeared to be changed by as much as reported then.
More recent reporting seems to indicate that the change in orbital period was 33 minutes instead of 32 minutes. It would be nice to know if that is due to continued observations of the eclipse period, or just refinements of the initial observation data.
There seems to be some debris that was of low enough mass to have been pushed far from the impacted objects by the solar wind, creating a visible tail like that of a comet. So, that is probably not gravitationally bound to the 2 main bodies, at this time.
Do we have any better info on heavier debris that might have stayed in orbits around the two asteroids? It is easy to speculate a lot of scenarios, but what we need is observations to decide which scenario(s) is(are) actually happening.
Edit: Well, here is another Space.com article that gives more info about the behavior of the ejecta: https://www.space.com/hubble-space-telescope-dart-asteroid-collision-dust . It seems that some is in orbits. We really need better resolution pictures.
An analysis of this 500-fold time-compressed video animation provides evidence that the decrease in the asteroid's orbital period declared by the authors of the above articles, which follows from photometric observations of mutual occultations-eclipses and radar data, may have an alternative explanation. Specifically, it could be a consequence of geometric-photometric distortions caused by the essential asymmetric increase of the observed Dimorphos's size, which remained unchanged, while the small-sized component of the wide fan-shaped ejecta continued to move away. Since the brightness and scale of the background star images did not change, the metamorphosis of the asteroid's image cannot be attributed to inaccuracies or errors. This phenomenon is the appearance of an asymmetric and optically dense "cloud" of mini-satellites in orbits around Dimorphos, into which relatively larger fragments ejected at lower velocities turned.
Observations of only two successive occultations-eclipses during Dimorphos's orbital semi-period (Thomas et al., 2023) are able to create the illusion of a shortening of its orbital period due to the displacement of the photometric center of the distorted asteroid image relative to its center of mass. Estimates of the orbital period of the "cloud" of mini-satellites located at heights of several tens of meters above the surface of Dimorphos (according to the video animation) lead to its values being several times larger than the orbital period of the asteroid itself. Therefore, during these events, occurring about 6 hours apart, the "cloud" of mini-satellites will be located on opposite sides of the asteroid most of the time, which is moving in opposite visible directions. Finally, according to estimates, this should manifest as an opposite temporal shift in the positions of the brightness minima. The summarized shift will be close to the declared decrease in the orbital period of Dimorphos. Conversely, when using observational data of the same type occultations-eclipses, occurring only once during its orbital period, such a relative shift is unlikely, despite the presence of a distorting asymmetry in the visible image of the asteroid.
Additionally, the assertion that the ejection was much more efficient in transferring the pushing impulse compared to the actual impact also raises doubts. According to Li et al. (2023) and the above video animation, the mean initial speed of the wide fan-shaped ejection was around several meters per second. Therefore, with an estimated total ejecta mass of up to 1 million kg, its momentum was comparable to the impactor's. Moreover, it is clear that only a small area near the impact direction (i.e., close to the axis of the ejection cone) can effectively act on an asteroid. At the same time, the rest of the ejecta regions significantly compensated for each other's impulse transfer abilities, which is problematic in itself for a totally inelastic collision with a loose rubble pile asteroid.
In summary, at this point, the success of this generally complex and beautiful space experiment can be considered questionable in terms of its main stated goals. The interpretation of the photometric and radar observations in the published articles lacks the consideration of the direct ground-based observations of the collision process and the subsequent ejection of asteroid fragments, which are crucial for understanding the observed effects. The evidence provided by the video animation suggests an alternative explanation for the decrease in the asteroid's orbital period. The assertion that the ejection was much more efficient in transferring the pushing impulse than the actual impact also raises doubts. These issues highlight the need for further investigation and the integration of all available data for a comprehensive understanding of the DART mission's results.
Have you looked for more data to support your theory?
And, do you have any more detailed explanation of how the ejecta should behave? Your description seems to lack orbital mechanics considerations for physically unattached material "in orbit" around Dimorphos, as well as the tidal effects due to the orbit of Dimorphos and its gravity-bound ejecta around Didymos.
I would think that we should be able to predict the times of exlipses as seen from Earth, and look for a slowing over an extended period of time, rather than simply look at the timing for a single pair of eclispes.