Before NASA’s planetary defense probe DART self-destructs next week by crashing into the asteroid Dimorphos, it will only offer views of the sixth asteroid we’ve ever seen up close.
Scientists are excited to get their hands on these images as they admit we know extremely little about the potentially threatening space rocks Earth.
missions to asteroids are full of surprises. Almost two years before September 26th ARROW collision, NASA learned firsthand how unpredictable these space rocks can be when the The OSIRIS-REx mission touched down briefly on the asteroid Bennu to collect a sample. Contrary to expectations, the 0.5-kilometer-wide asteroid’s boulder-strewn surface was so soft that it nearly swallowed the probe, sending shivers down the spines of the spacecraft controllers and throwing a huge wall of debris into space. With DART (short for “Double Asteroid Redirection Test”), NASA has sent a spacecraft to change the orbit of an asteroid that scientists know as little about as Bennu was before OSIRIS-REx‘s encounter.
“DART will be our first mission to study a binary asteroid system up close,” says Terik Daly, associate instrument scientist at DART’s Didymos Reconnaissance and Asteroid Camera for Optical Navigation (DRACO) and planetary scientist at the Johns Hopkins Applied Physics Laboratory, who is responsible for the DART mission managed for NASA, versus Space.com.
Related: NASA’s DART mission to hit an asteroid will be a key test of planetary defenses
The sixth space rock ever seen in detail
DART targets the 520-foot-wide (160-meter) asteroid Dimorphos, which orbits a larger, 2,560-foot-wide (780-meter) asteroid named Didymos. From ground-based measurements, scientists know the speed at which Dimorphos Didymos orbits and have a rough idea of the chemical composition of the larger asteroid. However, Dimorphos, the ultimate target of DART, is completely unknown.
“Dimorphos is small enough that it hasn’t really been studied in detail separately from Didymos,” Daly said. “We know it’s a separate body, but we know very little about the shape. We do not know whether Dimorphos is oblong or spherical; we don’t know if it’s a single rock or a pile of boulders.”
Thanks to the DART mission, Dimorphos will become one of the best-studied asteroids in the world universejoining the target asteroid OSIRIS-REx Bennuthe Itokawa and Ryugu Asteroids visited by the Japanese missions Hayabusa 1 and Hayabusa 2 and the Eros Asteroid explored by NASA’s NEAR Shoemaker spacecraft in the early 2000s. Dimorphos and Didymos will become only the sixth and seventh space rocks ever seen up close by a spacecraft, out of more than 26,000 asteroids currently known to periodically approach Earth orbit. In addition to the four above, the Asteroid Toutatis was briefly visited by the Chinese Chang’e 2 Lunar probe that took several pictures of it in 2012.
Unpredictable collision effects
Before DART impacts Dimorphos’ surface at an incredible speed of 13,680 mph (22,015 km/h), the spacecraft will transmit images of the asteroid captured by its DRACO camera at a rate of one per second. First the camera sees both asteroids, then focuses on Dimorphos and guides DART towards it. As DART hurtles towards the smaller space rock, the views become more detailed until the transmission stops abruptly – at the moment of the collision.
An Italian-built CubeSat called LICIACubewho traveled as a passenger on DART but was released 11 days before the impact, will observe the crash from a safe distance of 600 miles (1,000 km) and then zoom in on the freshly scarred surface to examine the impact in detail.
Because scientists know so little about Dimorphos, they have no idea how the rock will react to DART’s attack. Will the asteroid be as soft as Bennu, engulfing DART like a swamp, or will it be a solid chunk of rock that completely flattens the van-sized DART? Asteroids are so small and their heaviness so faint that even seeing the rock from above does not help predict the impact effect.
“Images can be deceiving unless you touch them [the asteroid]They don’t know,” envisioned Patrick Michel, the lead researcher at the European Space Agency (ESA). Hera missionwhich will visit Dimorphos and Didymos in 2027 to complete the study of the consequences of the impact, said in an ESA press conference on September 15.
“The reason is that you are in a very, very low gravity environment,” added Michel, a planetary scientist at the University of the Côte d’Azur in France. “And the surface reaction is sometimes completely counterintuitive because our intuition is based on what we experience on Earth.”
What is dimorphos made of?
Based on how Didymos, the larger of the two rocks, reflects light, astronomers believe the asteroid is made up mostly of silicate-rich rocks, in contrast to Bennu, which is made up of a less dense carbon-rich material.
If Dimorphos is made of the same material as its larger sidekick and the assumptions are correct, then DART’s collision will be less chaotic and potentially less efficient at altering Dimorphos’ orbit than if the asteroid were softer, Daly said. However, to know for sure, we have to wait for the data from LICIACube.
The Cubesat will also make a flyby of Dimorphos and look at the entire asteroid so scientists can reconstruct its shape. However, it will take weeks to months to download all the data and unravel the mysteries of Dimorphos.
How are binary asteroids formed?
Because the Didymos-Dimorphos duo is the first binary asteroid to be studied in detail, scientists hope to learn something about how these space-rock pairs form, Daly said. Estimates suggest that about 16% of near-Earth asteroids wider than 200 m could be binary stars. Even asteroid triplets are known. According to some theories, such families of asteroids could form when a larger rock starts spinning very quickly, shedding some of its material in the process, Daly said. Other theories suggest that the binaries and triplets can be created upon collisions.
“And one of the things we can do with the DART mission is see what Didymos looks like in pictures and what Dimorphos looks like in pictures,” Daly said. “And if they look similar – if their brightness is very similar, if they have similar types of morphologies – that would suggest that maybe Didymos and Dimorphos have split up somehow.” If it turns out that we’re looking at Didymos and it looks more like Bennu, but Dimorphos looks like a single rock in space, then maybe that splitting approach doesn’t make sense.”
Asteroids are full of surprises, and seeing one doesn’t mean we can predict how all the others will behave. But learning as much as possible about Didymos and Dimorphos will help scientists make better guesses about other asteroids, Daly said. And the more we know, the better our chances of getting it right when a dangerous space rock sets its sights on Earth.
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