After asteroid collision: can we defend our Earth?

  • It worked: The DART spacecraft hit the asteroid Dimorphos at high speed.
  • But what does that mean and what now?
  • There are still many questions to be answered before the Earth can be reliably defended.

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It was a great moment for NASA. After a ten-month flight, the Double Asteroid Redirect Test, or DART for short, space probe approached the asteroid Dimorphos – a fairly small piece at 160 meters, which along with the much larger asteroid Didymos orbits slightly outside the Earth’s orbit. .

DART spotted him, corrected his course completely autonomously in the closing minutes and reached the target at 01:14 AM (CEST) as planned. According to engineer in charge Mark Jensenius of the Applied Physics Laboratory in Laurel, the target flight’s accuracy was 17 meters. Given the size of the asteroid, that was a bull’s eye.

The DART mission was to test several technologies, including roll-out solar cells and a highly efficient ion engine, to keep the probe on course. This mainly included procedures to reliably hit the extremely small Dimorphos: at the time of the approach, there was 10.8 million kilometers between the spacecraft and Earth. Each signal took 36 seconds to travel between transmitter and receiver.

The primary goal of the $330 million mission was to test a technique to repel asteroids that could pose a threat to Earth in the future. This risk does not exist with Didymos and Dimorphos. Its orbit extends slightly beyond the Earth’s orbit around the sun. NASA was in the process of slightly altering the orbital period of the small Dimorphos around the larger Didymos. How strong this deflection will be after the impact of the DART is not yet clear so soon after the impact. That will only become clear when the data collected by telescopes in the coming months is evaluated.

Nobody knew what would happen

DART flew at a fast pace against the target Dimorphos, at six kilometers per second, more than 21,000 kilometers per hour. From a physical point of view, however, it was just the opposite: The orbital energy of the twin asteroid is higher, so the pair of rocks flew toward the probe, as ESA mission analyst Michael Khan points out. To hit such an object with pinpoint accuracy took some preparation, but based on NASA’s years of experience in optical navigation methods for flybys, it was just another step in precision.

Less certain was what would happen at the impact itself. The mission’s greatest unknown was the asteroid Dimorphos itself: “You just have to sum up that we know very little about the nature of such bodies,” said geophysicist Kai Wünnemann of the Berlin Museum of Natural History before the impact.

Just a few meters from his office is one of the largest collections of meteorites in the world. 6,000 rocks from asteroids and comets that once fell to Earth from space. But all these chunks help little: No meteorite looks the same on the ground as it once traveled through space as an asteroid. Such meteorites have passed through the atmosphere, become hot, melted or crumbled. The physical nature of asteroids in space is therefore still unclear today. “These are properties like the porosity or the internal structure,” says Wünnemann.

First countdown, then silence, then cheers

The probe hit Dimorphos at 1:14 as scheduled. The asteroid pair was first visible to the DART camera about an hour before the impact as a bright spot. Soon a large light gray and a small dark gray point of light emerged from the glow. At around 12:55 PM, DART entered the so-called precision lockthe fully automatic mode in which all maneuvers were decided independently by the computer on board the probe.

Technicians on the ground could only intervene in an emergency. From 01:09 this was no longer possible: the people in the control room leaned back because the signal running time had prevented them from doing anything. Then, at exactly 1:14 a.m., the last images from the probe came very close to the surface, strewn with what appeared to be boulders and fine dust. After that, the probe remained silent.

The DART spacecraft sent these images back to Earth in real time from its DRACO camera as it approached the asteroid. Here you see the last five and a half minutes before the collision. © YouTube

The impact was observed by telescopes on every continent, as well as the Hubble Telescope and the new James Webb Telescope. One of the first images released was a sequence from the Asteroid Terrestrial-Impact Last Alert System (ATLAS) in Hawaii. It shows Didymos and Dimorphos as a single point of light that wanders against the background of stars and then becomes significantly brighter for a few moments. Then a semicircular dust cloud spreads in the opposite direction of the (apparent) movement.

The impact was also observed up close: as early as September 11, 2022, DART deployed the Italian-built small satellite LICIACube, a 14-kilogram satellite that weighs 40 times less than the parent probe DART. LICIACube only passed Dimorphos three minutes after the collision and was able to observe him from a safe distance.

New answers, new questions

For the engineers at NASA and the responsible Applied Physics Laboratory at Johns Hopkins University, the mission is over for the time being. The $330 million probe is believed to have broken or even melted. In contrast, the research work has only just begun: in the coming weeks, telescopes will determine how the orbital period from Dimorphos to Didymos has changed.

Existing footage is already causing excitement: Planetary researcher Philip Metzger of the University of Central Florida commented on the first images from LICIACube: “I’m shocked that material was ejected there in ribbons. We don’t see anything like that in normally grainy impact experiments,” he writes. on Twitter.

The processes surrounding impacts cannot be simulated on Earth in the same way as on the surface of asteroids, where gravity is extremely low. The data should now occupy Metzger and many of his colleagues: “That’s exactly why we do experiments (like the crash of a space probe on an asteroid), because we never know in advance what we’re going to see.”

ESA’s Hera mission arrives in 2026

The researchers will not know exactly what traces the impact of the probe on the small asteroid has left behind for four years. In December 2026, ESA’s Hera spacecraft will arrive at Dimorphos to assess the change on the asteroid. It’s about the shape and size of the resulting crater, as well as any other changes on the asteroid’s surface. Hera carries a radar and can scan the inside of the body.

Most researchers firmly believe that Hera will find a crater between 5 and 14 meters wide. After all, the impact with cosmic speed should have set several thousand tons of material in motion. However, those involved were not really sure. Some studies had suggested that the asteroid could be so soft that it would contract like a rubber ball on impact. And that, according to Kai Wünnemann, would be bad news for asteroid defenses: “If it’s very porous and soft, the energy from the impact can melt away.”

Fighting off asteroids: somehow?

Such a direct impact is just one of many methods in the planetary geologists’ toolbox. There is also the idea of ​​a so-called gravity tractor: a probe is stationed next to a dangerous asteroid for years, with the result that it is deflected piece by piece over a long period of time. The ability to reflect light, the so-called albedo of an asteroid, can also be manipulated to infer it: “If you were to change the albedo, say painting a bright body black, the effect of the light pressure would change,” Planetary researcher Stefan Ulamec says the sun’s force would deflect it in its orbit, but all of these methods require the ability to work continuously on the asteroid for years or, better yet, decades before the impact.

The ultima ratio would be the use of nuclear weapons. Unlike the movie “Armageddon,” where the explosives are sunk into drilled holes, they should actually be detonated close to the surface. “The heat wave would vaporize the rock and this would cause a thrust,” says Stefan Ulamec. But it would hardly be possible to test this method in advance, because nuclear experiments in space are prohibited under the space law. “That is currently not under discussion and is also not politically justifiable,” says Stefan Ulamec.

This message comes from the journalism portal RiffReporter. About 100 independent journalists report together on current affairs and background information on riffreporter.de. The RiffReporter received the Grimme Online Award for their offering.


A NASA probe successfully collided with an asteroid on Monday. The goal of the “Dart” mission: to change the trajectory of two celestial bodies. The US space agency wants to be prepared for critical scenarios in the future.

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