NASA’s Dart (Double Asteroid Redirection Test) spacecraft will make a collision with the asteroid Dimorpho, from approximately 160 meters, this Monday (26). It, in turn, orbits another asteroid, Didymus, at 780 meters. It is the first time that humanity has deliberately attempted to divert a celestial bolide from its path, in a test that could save the planet from real dangers in the future. The impact is expected at 20:14 (Brasília time), and the Sidereal Messenger broadcasts live from 7pm.
Although the duo is on the list of so-called NEAs, near-Earth asteroids, and belongs to the category of potentially dangerous, it does not pose any immediate risk (understood, for at least a few centuries) to our planet. But because it is a double star, it is the ideal target for a test of the kinetic impact deflection technique. It translates from scientific as “hit with everything”. There’s no detonation of a warhead or anything like that. It’s just a cosmic crash.
In the same way that traffic accidents normally cause the vehicles involved to change speed and trajectory, NASA expects the collision to have an effect on the asteroid’s path. As the mass of the giant rock is much greater than that of the spacecraft, the expected change in velocity would be less than 1%. But it would lead to a change in the Dimorph’s orbit around Didymus, accompanied by a change in the orbital period – the time it takes for the moon asteroid to complete one revolution around the larger star.
That change, in turn, will have to be measured over the next few days and weeks, as astronomers use telescopes on the ground and in space to see what the dynamic impact was on the asteroid. It will be the first time that a method is tested that could be used if, in the future, we discover an asteroid on a collision course with Earth.
There are models that suggest what the post-crash change will look like, but nothing can replace the actual test.
Also because, apart from the existence of Dimorph, its orbital period (11h 55min) and a rough estimate of its size, we know nothing about it. Astronomers know it’s there because they can measure the change in brightness of the cluster as the moon passes in front of or behind Didymus, with its periodic pattern, but they’ve never had even an image of Dimorph as a single pixel.
Only during the final approach of Dart will it be possible to have a view of the said whose, revealing its shape and appearance. “The test will also provide information about the internal structure of the body. For example, just the amount of debris that is thrown out due to the impact already provides information about the internal part”, says Othon Winter, a physicist at Unesp (Universidade Estadual Paulista) who is not involved in the mission, but studies asteroids and planetary defense strategies.
Aside from basic scientific curiosity, it is very important to know more about the internal structure of asteroids, because this directly affects the outcome of trying to deflect it.
“The denser the asteroid, the more material it will eject. If it is a very porous body, it will eject very little material – the impact would be assimilated by the Dimorph, it would eject very little material and, if it ejects little material, it is because a lot was damped there. So the momentum transfer [grosso modo, a intensidade da força aplicada] is very low and then it will change the orbit very little”, explains Winter.
“If, on the other hand, it ejects a lot of material, it means that it is denser, less porous, and consequently it will produce a greater momentum exchange, causing a greater change in the Dimorph’s orbit.”
What to expect? Scientists have recently been surprised to discover that both the asteroid Ryugu, visited by the Japanese probe Hayabusa2, and Bennu, explored by the American Osiris-REx, were much more porous than imagined, almost as if they were immense piles of weakly pebbles. held together by the body’s weak gravity.
However, they were C-type (carbonaceous) asteroids, and Didymus (as well as, presumably, Dimorph) is of another category (Xk), which is sort of an umbrella for more exotic types. The “light signature” (spectrum, in the jargon) suggests a silicate (rock) composition, and the rapid rotation indicates higher density than Ryugu and Bennu.
What it really will be, we will only know after the impact.
An interesting aspect of the mission, which cost $324 million for a spacecraft with a mass of 550 kg, is that it will broadcast as “live” as possible the final approach phase, all the way to collision.
The only instrument on board is the Draco camera, which will record approximately one image per second, sending it back to Earth as quickly as possible. Between the time of capture, processing, transmission, travel through space (via radio, at the speed of light, covering the little more than 11 million km that separate Earth from the asteroid) and receiving it by mission control, about 50 seconds. That is, it is a mini-film broadcast less than a minute later than the events that took place in deep space.
It is critical that this be so, as there is no second chance – when impacting against the surface at about 6 km/s (or 21,600 km/h), it will be instantly vaporized. An Italian nanosatellite, LICIACube, travels close behind and intends to record the moment of impact, as well as the plume of material ejected from Dimorph’s surface. But its transmission capacity is much more modest, and it is expected that two images a day can be downloaded through it.
It’s not an easy or risk-free mission. At this point, the spacecraft already sees Didymus, the largest of the asteroids, in the Draco images. “But we still don’t see Dimorph, and we won’t see it until an hour before impact,” says Elena Adams, the mission’s systems engineer. “And four hours before impact the spacecraft becomes fully autonomous.”
This means that from that point until the collision, unless something goes wrong, there will be no interference from mission control on Dart, which will use its auto-guidance software to “find” Dimorph in the images and then adjust course. to impact him.
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