One day a large meteorite will fall on Earth, capable of causing a planetary catastrophe. It is a certainty. What we don’t know is when. DART (Double Asteroid Redirection Test o Double Asteroid Redirection Test) is the mission led by NASA to test the possibilities of altering the trajectory of these celestial bodies as a planetary defense. It has its scheduled departure for this November 24, at seven in the morning, Spanish peninsular time. The spacecraft that will carry out this historic mission, baptized with the same name as the mission, arrived last October at the Vandenberg Base (California) before being launched to test an asteroid deviation method called “kinetic impact” .

More than 66 million years ago the impact on Chicxulub changed the course of evolution by causing the extinction of the large reptiles. Several automated observatories scan the sky each night, looking for visitors who may pose a similar hazard well in advance. Because the secret of a good defense consists of an early detection, when there is still time to adopt some measure.

So far about 20,000 asteroids have been discovered whose orbit can bring them closer to our planet. NASA considers dangerous anyone that exceeds 140 meters in diameter and can approach less than ten million kilometers, perhaps 5,000 in total. Although at the moment there is no danger for the next 50 years, the fall of a mass of that size could cause serious damage if it occurs over inhabited areas. The recent case of the Chelyabinsk meteorite, even without causing fatalities (but hundreds injured by the shock wave and glass fragments), is an example of the destructive power of rocks falling from the sky.

Faced with a possible impact, there are two possible courses of action: destroy the dangerous asteroid or alter its course to prevent it from colliding with Earth. The former has been the subject of many catastrophic films, especially Armageddon (1998) where a heroic Bruce Willis and his team of oil drillers faced an intruder “the size of Texas.”

Armageddon holds the dubious record of accumulating the maximum number of scientific errors in just over two hours of projection. From the roar of explosions in the vacuum of space (a classic of all these films) to the monstrous size attributed to the meteorite, to the detonation of a nuclear device so modest that it would hardly tickle the asteroid. Some have calculated that to break a body of this size, 10,000 million bombs such as the one would have been needed. Zar Soviet 50-megaton, the most powerful ever tested.

Most of the asteroids that could represent a real danger are of much smaller dimensions. The one that killed the dinosaurs, about ten kilometers in diameter, was a true giant; most of those that have been cataloged do not exceed a few hundred meters. This suggests that in the future, with the appropriate technology, it would be possible to divert them from their route. Naturally, if they are discovered at a sufficient distance; at least beyond the orbit of Jupiter.

First essay

In 2005, NASA already made a first test: to crash a spacecraft of almost 400 kilos against the nucleus of Comet Tempel 1. It is a huge target, a kind of giant potato, 15 kilometers long. It is no wonder, then, that he hardly noticed the collision. But it did have some effect: its speed was reduced by half a millimeter per hour with the consequent alteration of its trajectory. Now Tempel 1 is approaching the Sun ten meters closer than before the collision (if since then, the pull of Jupiter has not made it change course, which is more than likely). Nobody measured it, of course; they are mere theoretical calculations.

Now, the American and Italian space agencies are about to repeat the experiment. Very appropriately, the name of the project is DART (dart in English). This time, asteroid 65803 Dídymo has been chosen as a target, a boulder 700 meters in diameter that from time to time approaches us, although without danger of collision.

Dídymo is one of the few binary asteroids that have been cataloged, although not the only one. In fact, his name means twin. Around it an even smaller satellite – Dimorphs – revolves, with only about 100 meters from side to side. In 2003, during its approach to Earth, the Arecibo radio telescope obtained a sequence of radar images, in which the ballet of the strange couple is very well appreciated.

Dídymo spins very quickly. His day lasts just two and a half hours. And Dimorfos completes an orbit around it every 12 hours. It is almost a stopwatch in space. Since Dimorfos has very little mass, it is perfect as a target for this experiment. When the DART probe crashes into that tiny moon, it will alter its speed by half a millimeter per second, enough to change its rotation period as well, perhaps up to ten minutes. It is a perfectly measurable difference from Earth by radar or by analyzing their luminosity variations in a technique similar to that used to detect exoplanets.

A quick mission

For what interplanetary missions usually last (years, if not decades), this will be very fast. The impact will occur in October next year. In part, because the asteroid is relatively close to Earth (about 11 million kilometers at the time of the encounter) and in part because the spacecraft will accelerate for most of its journey, thanks to an ion engine. To maximize the effect, the collision will be head-on, that is, encountering Dídymo “in reverse”, at a combined speed of almost 7 kilometers per second.

Ion engines like the one equipped with the DART probe produce very little impulse (just a few tenths of Newtons) but for a long time. The result is that they accelerate almost imperceptibly to very high speeds. And with minimal fuel consumption, since what it expels through its nozzle are Xenon ions driven by a powerful electric field. In its tanks it carries about 60 kilos, but probably in the whole mission it will not consume more than ten. That is, about 30 grams per day.

In this case, what it does consume is electrical energy, necessary to accelerate the ions to speeds close to 140,000 km / hour. But that’s free, thanks to two large panels of photoelectric cells: About 22 square meters, capable of producing almost 4 kilowatts, more or less, the installed power of a normal home. Those panels are thrown rolled up like blinds so that they unfold once in space. They are so huge that it was difficult to test them on the ground, so the prototype had to be sent to the space station to be able to open it wide in weightless conditions.

DART carries a partner who will be in charge of photographing the moment of impact. It is a minisatellite built by the Italian space agency, equipped with a television camera. It will separate from the main ship a few days before the crash and, in fact, it will be the only thing that survives the experiment, since the half ton of the ship is destined to merge with the asteroid, in a spectacular fireworks display.

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