Usually, images from space command admiration, but new photographs of an asteroid surrounded by a swarm of fragments exude a completely different feeling. This cloud of debris that floats around a cosmic rock with a tail similar to that of a comet is not the result of a natural phenomenon, but that of a probe which left Earth to deliberately crash into this asteroid.
Transmitted by the Hubble Space Telescope, these images show us the consequences of the Double Asteroid Redirection Test (DART) mission, the very first attempt to deflect an asteroid undertaken by humanity to practice saving the world from a potentially cataclysmic impact. On September 26, 2022, NASA and the Applied Physics Laboratory at Johns Hopkins University sent a partially autonomous, van-sized space probe crashing into the asteroid Dimorphos at a speed of 22,000 km/h. h, thus modifying its trajectory and its orbit around Didymos, the other body of this binary asteroid system, much more imposing.
The success of the mission shows us that it is possible to deflect asteroids from their collision course with Earth using this technique, provided there is sufficient time. Images such as those assembled by Hubble, the most impressive of which dates from December 2022, confirm that the DART probe exceeded all expectations by projecting a cloud of debris that was still visible in the space surrounding Dimorphos several months after the passage to death of the probe.
“It’s really… breathtaking,” said Megan Bruck Syal, a planetary defense specialist at Lawrence Livermore National Laboratory in California and a member of the DART mission research team. “This gives us an idea of the violence of the event. »
Just like Dimorphos, none of these thirty-seven fragments, some of which still exceed six meters in wingspan, constitutes a danger for the Earth. The Didymos-Dimorphos system was selected for this mission in part because their distant, linked orbit around the Sun poses no threat to our existence, even after their explosive encounter with the DART probe.
The detection of a swarm of rocks is an important step for scientists in understanding the first-ever test of planetary defense. “We have a clearer idea of what happens when we hit an asteroid,” says David Jewitt, an astronomer at the University of California, Los Angeles and lead author of the study that analyzed the Hubble telescope images. .
The haunting nature of these images is a bonus. When I first saw them, Jewitt testifies, “I had a hard time believing they were real. »
However, these surveys have more difficulty detecting “city killer” asteroids, objects approaching 150 meters in size capable of destroying entire metropolises or certain small countries. Of an estimated total of 25,000 NEOs of this size, only 10,500 have been identified. Due to their modest dimensions, they reflect less sunlight, which makes them more stealthy.
Asteroids of this size hit Earth approximately every 20,000 years. The probability of this happening during your lifetime is therefore rather low, but never zero. And if nothing is done to prevent this cataclysm, it will eventually occur. When that day comes, the asteroid may sink harmlessly into the middle of the ocean, but it could also hit a densely populated city.
Determined not to leave it to fate, NASA and the Applied Physics Laboratory built a kinetic impactor, a probe designed to strike an asteroid and deflect it, which they launched on a 10-month odyssey through space aimed at modifying the orbit of the asteroid Dimorphos, with a diameter of 164 meters.
In the weeks following the impact, images provided by various space observatories and several dozen ground-based telescopes revealed that Dimorphos’ orbit around Didymos had decreased from 11 hours and 55 minutes to 11 hours and 23 minutes, more than 25 times the minimum mission success criterion, set at 73 seconds.
The projection of rock debris following the collision is not a surprise. Before the impact, Dimorphos was already suspected of being a gigantic pile of rubble: a collection of massive rocks flying in formation whose precarious cohesion was ensured only by their own fragile force of gravitational attraction. “We knew there was going to be a lot of material coming out,” says Cristina Thomas, a planetary scientist at Northern Arizona University in Flagstaff and director of the DART mission’s observations working group.
Regardless, the images collected in the wake of the impact are no less astonishing. Moments before its fateful encounter with the asteroid, the DART probe deployed LICIACube, a cubic nanosatellite that saw filaments of matter spring from the asteroid and, a few months later, appear a long tail of debris stretching across more than 32,000 kilometers.
For his part, Jewitt conducted observations using the Hubble Space Telescope to try to obtain the most faithful images possible of the chaos. In a study recently published by the journal The Astrophysical Journal Lettersthe scientist external to the DART mission and his co-authors reveal that they detected 37 objects, ranging in size from 90 cm to nearly 7 meters in diameter, gradually moving away from the asteroid.
Objects of this size are in no way a cause for concern. When a 7 meter asteroid crashes into Earth, which happens frequently, it burns up innocently in the atmosphere. “It’s not these particles that worry us,” says Cristina Thomas.
“We have already encountered a multitude of them without even realizing it,” adds Andy Rivkin, planetologist at the Applied Physics Laboratory and leader of the DART mission research team.
On the other hand, this swarm of debris is not without consequences for planetary defense.
There are several ways to defeat a killer asteroid. The ideal is to deflect it: detect it several decades in advance, determine the necessary strike power, send an impactor like DART or a nuclear weapon as a welcoming committee, then use the impact or explosion to change its trajectory.
“If the emergency is imminent, we may have to strike harder,” says Bruck Syal. For example, if the asteroid is particularly large or if detection only occurs a few years before the impact.
Fortunately, the case of Dimorphos suggests that using an impactor to deflect an asteroid of this size and nature is more effective than expected. Last December, the DART mission team revealed that the material shed from the asteroid acted as a https://twitter.com/SquigglyVolcano/status/1603473744965603375thus amplifying the deviation efforts.
Another way to stop an asteroid heading for Earth is to destroy it, to vaporize it almost completely into harmless particles. If there is too little time to organize its deviation, this solution called fragmentation could be our only option, apart from accepting our fate while trying to limit the damage. According to simulations involving nuclear weapons, this technique could work, but an early diversion remains the preferred option of specialists.
What must be avoided at all costs is transforming a single projectile into a multitude of near-Earth objects. That’s not what happened with Dimorphos, but this scenario could arise if humanity hits another asteroid of the same nature a little too aggressively. “It’s a bit like shooting a bunch of grapes,” says Jewitt.
Successful and safe deflection is not simply a matter of hitting as hard as possible, but rather getting to know your enemy by studying the composition of the asteroid and its mechanical properties, if possible up close. using a recognition probe, to then operate with precision. “Each asteroid will be slightly different,” explains Cristina Thomas.
Over the next few years, we will discover a multitude of new, potentially dangerous asteroids. Currently under construction in Chile, the next-generation Vera C. Rubin Observatory is expected to identify many more Dimorphos-sized objects, as is the Near-Earth Object Surveyor project, an infrared space telescope specifically designed to track down the most elusive asteroids.
Furthermore, the images captured by Hubble will not be the last we see of Dimorphos. In October, the European Space Agency plans to launch the Hera probe which should reach the asteroid in 2026 to observe it up close. In the meantime, other astronomers continue to point their telescopes at the asteroid’s remains, hoping to uncover new hidden details.
The images collected by the members of the DART mission are already more spectacular than they had dared to imagine. “But we would never have been able to do all this,” says Cristina Thomas, gesturing, looking fascinated, in front of the photos taken by Hubble. “It is a real pleasure that others have been able to join our efforts. »
