The massive tail, created by the collision of a spacecraft and an asteroid earlier this year, is revealing important information about space rocks – and how to deal with such rocks that could one day threaten Earth.
NASA Double Asteroid Redirection Test (ARROW) mission hit a small space rock called Dimorphos in late September to prepare for the possibility that humans might one day try to distract you asteroid on a collision course with Earth. Within weeks of impact, the DART team announced that the impact was 32 minutes from Dimorphos’ orbit around its larger companion. Didymos – in the high range of the team’s prelaunch estimates. Scientists are now sharing additional findings on the impact during the American Geophysical Union’s annual conference, taking place this week in Chicago and online
“DART was a tremendous success,” said Tom Stadtler, program scientist for the DART mission, during a press conference held in conjunction with the meeting on Thursday (December 15). “I’ve seen those results, I know they’re extremely cool.”
Related: Check out the first images of DART’s wild asteroid crash!
Many of the new findings focus on the stunning, comet-like tail created by debris from impact. Mission scientists weren’t sure in advance how much debris DART’s collision would cause, but the impact didn’t disappoint.
And scientists had thanks to the Italian hitchhiker of the DART spacecraft, Light Italian Cubesat for Imaging of Asteroids (LICIACube) equipped with two cameras and deployed by DART 15 days before impact, allowing it to fly past Dimorphos just three minutes after impact. Photos of the tiny spacecraft show quite a cosmic mess, with clouds of material bursting from the space rock.
“The images were indeed impressive,” said Alessandro Rossi, member of the LICIACube science team and scientist at the Instituto di Fisica Applicata Nella Carrara in Italy, during the press conference. “We weren’t expecting some of the features we’re seeing.”
Scientists are still analyzing the data from LICIACube, but images captured by it two cameras can provide a sense of how big certain debris is, how fast it’s traveling, and more, Rossi said. Researchers even think they can see the debris casting a shadow on the orbit of the larger Dimorphos asteroid, Didymos.
The debris gives some idea of the asteroid’s structure, as an asteroid made out of solid rock would produce much less ejecta than an asteroid made out of clumped boulders — imagine bouncing a tennis ball off the sidewalk instead of throwing it into a sandbox .
In addition, the ejection solved a key riddle about Dimorphos and Didymos. Scientists suspected that the two space rocks would be made of similar material, but had no way to test that theory, either while the spacecraft sped toward its destination or with ground-based telescopes, neither of which are powerful enough to see Dimorphos directly.
Before the impact, scientists were able to use the light they saw from the system to analyze the composition of the space rock pair as a whole, knowing that almost all of the light came from Didymos. But in similar data taken shortly after impact, it’s the debris flying away from Dimorphos that accounts for most of the light.
Comparing the two light signatures showed that the material between the two asteroids appears to be quite similar, although there are some slight differences. “We are very pleased to see that these two objects do indeed have a similar composition,” said Cristina Thomas, a planetary scientist at Northern Arizona University who leads the DART observing working group, during the press conference.
Scientists will study The fresh tail of Dimorphos for quite a while, including examining observations made in the days after the collision, collecting new data to see how the cloud is changing over time, and comparing observations from different angles.
“We have a vision of the ejecta cloud up close, we have a vision of the ground, we have a vision of Hubble Space Telescopeof the James Webb Space Telescope‘ said Rossi. “So we have many different geometries to compare against, and this allows us to clearly characterize the ejecta cloud from many angles.”
crunching the numbers
During the press conference, the scientists also shared two key figures that they have calculated since the collision.
First, they’ve begun to estimate how much debris flew off the asteroid: at least 2.2 million pounds (1 million kilograms) and possibly as much as 22 million pounds (10 million kg). Given Dimorphos’ total mass of perhaps 11 billion pounds (5 billion kg), the rock may have lost only 0.2% of its material, even if the higher estimate proves correct.
“We’re talking a tiny, tiny fraction,” Andy Rivkin, a planetary scientist at the Johns Hopkins Applied Physics Laboratory and co-lead of DART, said at the press conference.
The second number gets to the heart of the purpose of the DART mission. DART wasn’t about looking inside an asteroid, it was about that planetary defense. This includes looking for asteroids in orbits that intersect Earth and calculating whether the two bodies could ever be in the same place at the same time.
If scientists ever spot a sizable asteroid that poses a real threat, humans might try to intervene by accelerating the asteroid’s orbit Sun so it misses its appointment with Earth. DART tested a technique called kinetic impact – a fancy name for hitting the asteroid with a heavy, fast-moving object.
However, scientists do not have a sufficient sense of how the properties of an asteroid and a collision might interact to produce a specific change in the momentum of rocks in space, making it difficult to know, for example, how large spacecraft to launch.
Scientists use a crucial number called the “Impulse Transfer Factor” or betato describe how effective an asteroid impact is. If a starship hits an asteroid head-on in a collision that doesn’t cause debris, the space rock picks up exactly the momentum the starship had when it crashed, a beta of 1.
A variety of properties can affect the beta factor — for example, whether the spacecraft hits a smooth patch or a large boulder, the asteroid’s internal structure, and what the asteroid is made of — but let’s leave those out for simplicity aside.
Debris shooting off the asteroid into space adds momentum to the asteroid and gradually increases the impact’s beta factor. And scientists have now calculated the beta factor of the effect of DART to be 3.6. This value means that the asteroid picked up more than three times the momentum of a clean impact, and that the debris from the impact affected the asteroid even more than the spacecraft itself.
“This is very good news for the kinetic impact technique,” said Andy Cheng, leader of the DART investigation team at the Johns Hopkins Applied Physics Laboratory, during the press conference. “At least in the case of DART, the kinetic impact on the target was really efficient in changing the target’s orbit.”
The calculation also provides scientists with much-needed real-world data to understand how an asteroid’s properties affect momentum transfer — data critical to determining how massive a kinetic impact spacecraft should be to avert catastrophe . The successor to DART, the European Space Agency Hera spaceshipcurrently scheduled for 2024, will also play a key role after arriving (much softer) at the asteroid pair to examine Dimorphos and Didymos up close.
“What we’re trying to do is be able to observe an asteroid, both from the ground or maybe with a reconnaissance mission, and infer what the reaction will be if we use a kinetic impactor against it,” Stadtler said.
Despite the fascinating findings, both in terms of science and the defense of the planet, the mission team emphasized that the project is far from finished.
“Now from here we actually get to our dream list, where we can start thinking about the really complicated predicted dynamic effects that we weren’t sure we could observe because we’d never have done that before.” said Thomas. “We look forward to more observations that will allow us to study things in great detail and I think this is a really exciting place.”