Update November 24: The probe has now been successfully launched into space by a Falcon 9 rocket. This is announced by NASA on its website.
Anyone who steers a $ 330 million space probe specifically in the direction of an asteroid, only to have it smashed there, is either crazy or active in science. Lindley Johnson is the latter. At the US space agency NASA, the astrophysicist holds a position with the nice title Planetary Defense Officer. So Johnson is responsible for defending the Blue Planet from dangers from space – and he will have a lot to do in the coming weeks and months. Not because the Earth is under attack, but because Johnson wants to find out how he can better protect it.
The vehicles that NASA and their planetary protection officer have chosen for this is called Dart (short for "Double Asteroid Redirection Test"). If everything runs as planned, the small spacecraft should start on Wednesday, November 24th, from California Vandenberg, the start window opens from 7.21 a.m. CET. If nothing comes in flight, Dart's mission will find an abrupt end after ten months: in the form of a targeted collision with an approximately 160 -meter asteroid. "Hopefully the data collected will give us information on how we can dissuade objects that can be put to earth from their course in the future," says Johnson at an online press conference at NASA in early November.
The DART probe examines a potential diverting action for a genuine risk.
The risk is quite real: astronomers estimate that about five million asteroids with a diameter of at least 25 meters are traveling in orbits that could one day become dangerous to the Earth. Even worse: only 0.4 percent of these fragments, remnants of the birth of the solar system 4.6 billion years ago, have so far been observed and thus detected. No one knows what the remaining estimated 4.98 million asteroids are doing.
Again and again there are surprises, as in February 2013 in Tscheljabinsk, Russia: a probably 20 -meter asteroid had entered the earth's atmosphere at the time and exploded high above the region. More than 1,600 people suffered injuries, the estimated damage was 30 million euros.
The situation looks a little better in the size class from 140 meters. Such asteroids could destroy entire regions if they hit Earth, but almost 40 percent of the estimated 25,000 near-Earth objects are now known – and none of them pose a threat to Earth in the next 100 years. With each new discovery, however, this can change.
The top priority for the planet protectors is therefore to identify threats as early as possible. "The best moment to deflect an asteroid is as far away from Earth as possible," says Lindley Johnson. Then a small push is enough to change the further course of the orbit so much that there will be no collision with the Blue Planet.
This is exactly what DART should try as a kind of feasibility study - with an asteroid that is not a danger to the earth and cannot become one by the thrust. There are other ideas for defense: room probes could fly in parallel to asteroids and throw them out of the track due to their attraction. Or you could shoot the chunks with lasers. These approaches will also be tested in the future. According to Lindley Johnson, the "distraction through kinetic influence" is the technical term, but is the most advanced. Therefore, the method should now be tried out first.
The DART probe engages in automatic asteroid defense exercises.
Nevertheless, it will not be easy. About eleven million kilometers will be between DART and the Earth at the time of the planned crash in September next year. Each radio signal takes almost 90 seconds to reach the probe and until its feedback is back on the ground. A remote control is therefore not to be thought of. DART must therefore independently drive to his destination, a mini-moon of the asteroid Didymos.
The autopilot therefore takes over about four hours before the planned catastrophic end of the mission. In the picture of the darts navigation camera, didymos and its moon appear only a light point among many in the black of the universe. Only an hour before the crash, the probe is still 24,000 kilometers away from the finish, that the mini moon is visible as a single pixel. Dart must now distinguish between the Möndchen, also called Dimorphos, and the asteroid. Under no circumstances may the probe control the larger of the two sky chunks. To prevent this, the researchers at John's Hopkins University, which Dart developed on behalf of NASA, relate to their experience with algorithms for American missile defense. There, too, small, fast objects have to be reliably discovered and hit.
If all this works, DART will almost hit Dimorphos head-on at a speed of about 24,000 kilometers per hour. Ten, maybe 100 tons of dust and rock are thrown out. A crater is formed. Above all, however, the speed of the moon on its orbit around Didymos should change significantly.
True, at the time of the crash, DART will have a mass of only 550 kilograms. However, coupled with the high speed, this should be enough to reduce Dimorphos' previously almost twelve-hour rotation period by at least 73 seconds. Depending on whether DART hits the Moon centrally or rather laterally, whether the probe catches porous rock, a boulder or a sloping slope, the braking effect can also be significantly stronger. According to NASA's promise, DART's camera should run to the end anyway.
Another camera eye will be aimed at the events: Around ten days before the crash, Dart is supposed to emit a tiny satellite, only 14 kilograms of satellites. It was developed in Italy and is supposed to pass the accident site about three minutes after Darts fall into ruin. The researchers hope that his pictures have information about how much material was thrown out in which direction - and what effects this could have on the orbit of the mini moon.
However, the exact extent of the orbital change can only be determined by Earth. By spring 2023, large telescopes will be pointed at Didymos and its moon, even if the duo can only be seen as a tiny point of light on the ground. However, since the minimoon moves between the Earth and the asteroid on its orbit around Didymos, only to disappear behind the boulder half an orbit later, the brightness of the light spot also changes. From these weak but regular fluctuations, researchers hope to be able to draw conclusions about the orbital period of the moon – before and after the collision.
HERA, a European probe, will examine DART's effectiveness.
However, the changed speed alone says little about the success of the experiment. The mass of the mini moon is just as important: was it a lightweight or was it a heavy chunk that was impressed by the 550-kilogram probe? Only with this knowledge can it be estimated how effective the diversion maneuver was. And this is the only way to choose the right caliber, if one day you have to shoot an approaching asteroid.
However, DART and his small satellite can only estimate the mass of the opponent of the accident. In order to measure them exactly, Europe's space agency ESA therefore wanted to send its own probe to Didymos together with DART. She was supposed to observe how strongly the rotating minimoon tugs at the large asteroid due to its centrifugal forces and how violently it eggs around its axis of rotation – the only way to weigh dimorphos before and after the crash.
At a meeting at the end of 2016, however, the ESA member countries knacked with money for their long-planned project, the mission was mothballed. It was only in 2019 that 160 million euros came together, 60 million of them from Germany. Another 130 million euros have to follow by the start. Hera, according to the name of the European space probe, should arrive at Didymos at the earliest four years after Darts. She should look at the scene of the accident, clarify the decisive questions about the course of the collision and thus deliver the missing data for successful asteroid defense.
Until then, only one thing must not happen: a really dangerous asteroid should better not rush towards Earth.
