Why are we launching a probe at a distant space rock?
‘A historic moment with importance for the whole world’, is how Tom Statler called the cosmic bang that will take place in the night from Monday to Tuesday during a press conference. Statler is a researcher at the Planetary Defense Coordination Office, the branch of the American space agency NASA that is tasked with protecting the Earth against cosmic threats.
When the Dart probe bores into the distant asteroid Dimorphos at nearly 22,000 miles per hour—roughly ten times the speed of a gun bullet—it marks the first time humanity has attempted to alter the orbit of another celestial body.
A spectacular first, serving as a cosmic fire drill. The mission must prepare us for the greatest imaginable catastrophe from above: the moment we discover a space rock that is on a collision course with Earth. If we give such a rock a nudge, the hope is, we might be able to adjust its orbit so that it doesn’t ram the Earth.
You should try it now, say the initiators of the Dart, which are affiliated with NASA and the European space agency Esa, among others. They in turn work together with scientists from about a hundred universities and institutes, spread over 27 countries. They also keep an eye on things with telescopes spread over all continents on Earth. Even from space, measuring instruments monitor the impact that the Dart is about to make.
“Planetary defense is a global problem that deserves a global response,” says Statler. “Asteroid impacts have been a threat for as long as Earth has existed, and we think we can prevent them now for the first time.”
It’s not that far yet. The Dart – and the future European space probe Hera, which will be launched in two years to get a closer look at the smashed crater – serves two purposes. First, see whether scientists can even hit a piece of rock that is about 11 billion kilometers from Earth at the time of the impact. And also map out the consequences of such an impact. Whether the orbit of the asteroid is indeed being adjusted, for example. And to calibrate the computer models that predict the effects of such ‘pushings’. So that we have everything in order when it really matters.
What exactly will happen?
From midnight Dutch time, interested parties can watch NASA’s livestream. They will see a new photo every second that spacecraft Dart sends back to Earth. At first you see nothing more than a vague point of light. At the earliest half an hour before the impact, that one dot then changes into two dots. Asteroid Dimorphos, the target, is roughly 160 meters long and orbits a larger asteroid, Dydimos, about 780 meters.
Due to the huge distance, the Dart has to make sure that it hits the right space stone all by itself. He has an operating system on board for this, which runs on algorithms specially written for this mission that determine the correct course based on the photos.
Whether that works is one of the most important tests the Dart performs. “I think the atmosphere will be very tense until the course corrections,” Nancy Chabot, head of the Dart team at the Johns Hopkins Applied Physics Laboratory, said at the same news conference. ‘If all goes well, the first cheers in the control room will rise well before the actual impact.’
After that course correction, the mission has another first in store: the first look at a still unknown celestial body. Because of the enormous distance, no one knows what Dimorphos looks like yet. The shape, the precise composition and even the color are still unknown. Finally, just before impact, the asteroid will fill the entire field of view of the camera. Soon after, the images stop.
We will therefore not see anything of the impact itself. The only images up close are shot shortly after the impact by the mini-satellite Liciacube, but don’t expect anything spectacular. The images only come in after a few hours and the dust plume from the impact is only a few pixels in size due to the distance and speed of the satellite. Only when the European space probe Hera, which will leave at the end of 2024, takes a close look at the crater, will we be able to see the consequences of the impact in genuine HD.
Until then, it’s up to telescopes on and around Earth to see if the impact actually altered Dimorphos’ orbit. That could take a while. Weeks, maybe, the Dart team thinks. The job change will be small: at most one percent.
How often do these kind of space rocks hit?
Look at the history and it is not a question of if, but of when rock from space will crash uncontrollably onto earth. The most famous example is the one that ended the reign of the dinosaurs roughly 65 million years ago, a cosmic monster estimated to be ten kilometers in diameter.
The good news? Such impacts are very rare. Only once every hundred million years does such a giant stone collide. More good news: Scientists estimate that they have discovered about 98 percent of the space rocks in this largest cosmic weight class. None of them are on a collision course with Earth.
But the pockmarked history of our Earth’s crust also reveals smaller impacts. The famous Barringer Crater in the US state of Arizona, for example, a wound in the ground 1,600 meters wide and 120 meters deep. The physical evidence of an impact from a space rock of only 50 meters, about 50,000 years ago.
This category of rocks with a diameter of 50 to 200 meters has, as NASA euphemistically describes it, the potential for ‘regional destruction’. They fall to earth once every few thousand years. And if we are unlucky enough that such an object does not hit an ocean, desert or sparsely populated area, it unceremoniously lays a metropolis in ruins.
But while scientists in this category don’t know of a single example headed for Earth, there’s also bad news: Most space rocks of this type have not yet been mapped. Experts estimate that we have only discovered roughly 40 percent of this type of cosmic rock.
When we discover a piece of rock on a collision course with earth, will the Dart be enough?
That mainly depends on the size of the space rock. And, just as importantly, how far it is from Earth when we discover it. The further away, the smaller the push needed to ensure that such a stone misses the earth. If you see it arriving ten years before the impact, you’ll be fine. And even with a warning one year in advance, it still just works. But if astronomers discover it much sooner in advance, they probably don’t have the time to push the stone away.
Plus, you can’t just move any space rock with a Dart-style probe. Such a cosmic bullet creates what physicists call “impulse transfer.” The momentum – the mass of an object times its speed – is a measure of the move you make in physics. Shoot a marble against a marble and the momentum transfer is large enough to set it in motion. Shoot that same marble at a car and you probably won’t get it to roll. Not even when the car is off the handbrake.
The energy released during the impulse transfer of the Dart – weight: 570 kilograms – is therefore relatively modest on the scale of man-made bangs. By comparison, the nuclear bomb dropped by the Americans on the Japanese city of Hiroshima during World War II is roughly 6,000 times more powerful. NASA and Esa estimate that these types of cosmic bullets are therefore only suitable for space rocks with diameters of 50 to 200 meters, the category with the potential for ‘regional destruction’.
If we have the misfortune that a considerably larger specimen – from 500 meters, to perhaps even a few kilometers – races towards the earth, then other means are needed.
In the scenarios of space organizations, at some point only a nuclear bomb will remain as an option. Not to blast directly on the asteroid, by the way, as one in Hollywood films like Armageddon and Deep Impact did. That is not very smart: the stone can then break into pieces, without being able to predict where those pieces will end up. Instead, it’s better to detonate such a bomb nearby. The bang – the asteroid would be pelted with released neutrons – then provides the energy to give such a space rock a move.
That would be only a last resort, by definition, a first. Shooting nuclear bombs into space is prohibited by international treaties because of the major potential consequences of an accident.
How heavy a probe should be, at what distance from Earth a mission like Dart would still make sense, and from what size and distance from an asteroid an atomic bomb is the only option left: these are the questions that this first test mission must ask. reply. If all goes as planned, they will be clarified with one literal blow from Monday to Tuesday, so that we can calibrate our best collision models for the first time against the merciless reality of the deep cosmos.
