NASA launches in the early hours of this Wednesday (24) the first mission to demonstrate the ability to deviate from the route of an asteroid. Dubbed Dart, which stands for Dual Asteroid Redirection Test, the initiative is relatively modest in cost ($324 million) but still historic. Stop to think: humans will try, in a pioneering way, to demonstrate that they can promote the planet’s defense against a celestial bolide that is underway to collide with us. 65 million years ago, dinosaurs didn’t have that luxury.
For starters, let’s get the conspiracy theories out of the way. No, there is not, as far as is known, an asteroid that could seriously threaten us over the next few decades. The population of civilization-killing bolides, 1 km in diameter or more, has already been mapped over 95% (there are about 900), and none of its members pose any danger for the next few decades at least. However, the smaller asteroids, 140 meters or more, are a much larger and lesser known population (it is estimated that there are about 25,000 of them, of which we only know 39%). And while they are incapable of extinguishing humanity, they can wreak considerable local havoc.
Dart represents an attempt to deal with the danger posed by these objects. Its technology is as simple as possible: changing the trajectory of an asteroid simply by colliding with it, a deflection method that scientists call “kinetic impact”. There’s no pump, there’s nothing. It’s just a hit at high speed, with a cosmic traffic accident, that causes the asteroid to undergo a speed change. By changing this, the orbit also changes. And then the asteroid that was supposed to hit us suddenly doesn’t hit anymore.
As the name says, this is a test. It’s to see if it works. The chosen target is an asteroid that will facilitate the measurement of the mission’s effect, but which does not pose any danger to Earth, now or after the collision with Dart. It is the double star Didymus (Didymos) and Dimorfo (Dimorphos). The first, the largest of them, is 780 meters. The second, smaller, is an asteroid moon, measuring 160 meters.
Departing Earth at 3:21 am on Wednesday (GMT), Dart has an appointment with the Dimorpho between September 28 and October 1, 2022. The spacecraft’s impact should, if all goes well, change speed orbital of it. It’s not a huge mass, just over half a ton, but in a collision at 6 km/s – or 21,600 km/h.
Dimorphous has much more mass than the ship, so while it will be entirely vaporized by the encounter (cratering it), it should only undergo a subtle change of course. If the asteroid were wandering alone in orbit around the Sun, the change might be too subtle to be quickly identified. But that’s where the advantage of choosing a double star comes in. Because Dymorphus completes a circle around Didymus in about 12 hours, any change in trajectory will make a detectable change in the orbital period, which astronomers can measure using ground-based telescopes. (The choice of the impact date has to do with this – it will happen when the Earth is close to its minimum distance from Didymus, about 11 million km, making observations easier. The last time there was such close proximity was in 2003, and the next, only in 2062.)
The asteroids will also be studied by the probe itself, during its approach, and it is possible that a small Italian satellite (which travels with it) will record the impact itself. The LICIACube is of the cubesat class (it is shoebox size, 30x20x10cm) and is equipped with two cameras to produce images of the encounter.
So, if all goes well, is the Earth safe? Can we forget about the asteroid threat once and for all, as long as NASA is ready to defend the planet? As you might have suspected, that’s not quite the case.
“The demonstration is very important, but this strategy will only be useful if we have very early warning of a future collision”, explains Cristóvão Jacques, astronomer at the SONEAR Observatory, in Oliveira (MG), and the main Brazilian discoverer of near-Earth asteroids. “If we know that an asteroid of up to 300 meters is on a collision course with us in 10, 15 years, it would give us time to prepare a mission like this, implement and find the object in time to change its orbit so that, with the time passed, it did not reach the planet. But if the notice was 2 or 3 years, this method would not work.”
Hence the importance of the work of astronomers in continuing to identify all members of this vast population of objects potentially threatening to Earth. The sooner we discover an asteroid that has our name taped to it, the greater the chance we can react to avoid the impact.
And, of course, if the asteroid is very large, from that category capable of mass extinctions (like the one that hit the dinosaurs, which had something like 10 km), even with a long warning time, this strategy of simply colliding a spaceship with he would likely be unable to redirect it. “Fortunately, this population is almost completely discovered and we know that it does not pose any danger for the next century”, adds Jacques.
The Dart mission is managed by the Johns Hopkins University APL (Applied Physics Laboratory) and will be launched by a SpaceX Falcon 9 rocket on its first interplanetary mission in the service of NASA. The flight departs from Vandenberg Space Force Base in California, and the weather at the moment gives a 90% probability of good conditions. If there is any violation of the mission criteria, a new attempt can be made on the 25th.
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