About 30,000 tonnes of tiny fragments of asteroids and comets collide with Earth every year, but they are usually too small to do any damage. But being hit by an object a few hundred metres in diameter could kill hundreds of thousands of people – and potentially threaten all life on Earth. In autumn 2022, researchers tried for the first time to try to nudge an asteroid off course with a satellite, and now they have published the size of the deflection. The experiment was successful, but we still need a much better overview of what is in outer space to be able to defend ourselves against the threat.
In 1908, an ice comet 50 metres in diameter exploded over central Siberia. The force of the Tunguska event was equivalent to 1,000 Hiroshima bombs, destroyed more than 2,000 km2 of forest and killed three people 150 km away. And although the largest near-Earth asteroids in the past 30 years were only 17 metres in diameter, the threat is real. Thus far, there has been no defence against them, but in autumn 2022, NASA in collaboration with an international research team carried out the first full-scale experiment to determine whether a satellite could save humanity by deflecting objects that are on a collision course with Earth.
“The question is not whether we will collide with an object that will wipe out very large parts of the Earth but when. Now we have crunched the numbers from the collision of the DART satellite with a double asteroid. We influenced its course more than four times as much as expected, so this is very promising. The collision happened 11 million kilometres away from Earth, but using this new weapon requires improved mapping of celestial bodies on a possible collision course with Earth, and this is sensible, because it is not very expensive,” explains co-author Uffe Gråe Jørgensen, Professor in Astrophysics and Planetary Science at the Niels Bohr Institute of the University of Copenhagen.
The Double Asteroid Redirection Test (DART) is not the first experiment involving a collision between a satellite and an asteroid. In 2005, NASA attempted to send the 370-kg Deep Impact towards comet 9P/Temple1. However, this comet is a whopping 6 kilometres in diameter, and the experiment was designed to provide information about the internal structure of a comet.
“It was like a mosquito hitting a plane. This time NASA chose a more equal battle. The Dimorphos asteroid is only 150 metres in diameter, but it orbits around Didymos, a somewhat larger asteroid 780 metres in diameter,” says Uffe Gråe Jørgensen.
The hope was that the collision with NASA’s 580-kg DART would slow down the smaller asteroid’s orbit enough to be detected from Earth.
“Our team participated in determining the effect of the collision by carrying out observations before and after the collision from a Danish telescope we have in Chile. We also captured the only images of Dimorphos’ movements around Didymos before and after the DART collision. They are not images in the true sense but reflections from radar that a computer transformed into images,” explains Uffe Gråe Jørgensen.
Like a rocket engine
The double asteroid was chosen because detecting velocity changes on a single asteroid would be much more difficult. Before Dimorphos and the satellite collided, Didymos and its moon Dimorphos orbited each other every 11 hours, 55 minutes and 17.3 seconds. The hope was to delay the orbital time by about 7 minutes.
“We had calculated that potential delay based on the collision between the satellite and the asteroid being fully inelastic, equivalent to a car hitting a wall. However, the asteroid was far more porous, which we could see when the satellite hit. DART bored deep into the asteroid, melted the asteroid’s icy material and was then propelled back through the tunnel and out through the impact hole like a rocket engine,” says Uffe Gråe Jørgensen.
This effect slowed the asteroid much more than the impact itself could have.
“The satellite thus created a natural rocket engine that slowed Dimorphos. We measured that the orbital period after the impact had instead decreased by a full 33 minutes – more than four times as much – which is a very promising result.”
The most difficult part of the operation itself was getting the satellite to hit the asteroid. According to Uffe Gråe Jørgensen, this was achieved using surplus military equipment previously used for homing missiles. With a speed exceeding 20,000 kilometres per hour, hitting an object only 150 metres in diameter is difficult.
“Especially when the operation takes place 11 million kilometres from Earth, but the satellite hit only 17 metres from the planned target, so this shows that it can be done technically and that it has a sufficient effect. When it takes place so far away from us, all it takes is a tiny change in an asteroid’s course to bring the asteroid far enough away from the collision course on its long journey,” explains Uffe Gråe Jørgensen.
Since the double asteroid was nowhere near a collision course with Earth, the researchers were not concerned about nudging it in the wrong direction.
“If we face a real threat, we must clearly be sure to nudge an asteroid in the right direction. The experiment was just an initial test of how we can learn to deflect cosmic objects that could hit Earth in the future. Although we should learn to become even more skilled at doing this, we should first improve in identifying real threats in good time so that we can deflect them,” says Uffe Gråe Jørgensen.
On average, 1,000 years elapse between episodes of Earth being hit by an asteroid 100 metres in diameter. The interval for asteroids similar to the one that wiped out the dinosaurs 65 million years ago is reckoned to be 100 million years on average.
“If an asteroid the size of Dimorphos were to hit Earth, it would cause regional damage on a scale we have never seen historically, with potentially hundreds of thousands of people losing their lives. But today we have no instruments that can monitor how many such objects exist near Earth, so even if we can get good enough to nudge them, we are not yet good enough to spot them,” explains Uffe Gråe Jørgensen.
Combination of mass and velocity
However, there is reason to hope that it will soon be possible to determine how many cosmic objects of Dimorphos’ size are flying around near the Earth and even map the trajectories of many of them. A large US-international telescope called the Large Synoptic Survey Telescope (LSST), now called the Vera C. Rubin Observatory, has been under construction for some years on a mountaintop in the Chilean Andes.
“When completed in the mid-2020s, the telescope will be able to detect virtually all asteroids and comets in the Earth’s vicinity that can be observed from the Southern Hemisphere and are larger than nearly 200 metres in diameter (about the same size as Dimorphos). We will then know when any of them could potentially be on a direct collision course with Earth,” says Uffe Gråe Jørgensen.
About 10,000 objects of this size are expected to pass close to or cross the Earth’s orbit every year. There are many times more objects that are the size of Dimorphos or less, and about 1,000 are larger than around 1 kilometre in diameter.
“The combination of mass and velocity of a celestial object determines how much damage it causes in a collision – similar to a car hitting a wall, for example. Today, we can only determine the mass of an asteroid that appears to have a specific size with an uncertainty factor of 10, because we generally do not know its composition,” explains Uffe Gråe Jørgensen.
Should we shelter in the basement?
The celestial object that exploded over central Siberia in 1908 was smaller than Dimorphos by a factor of 50 to 100, and with the uncertainty in mass and speed, an object of Dimorphos’ size can therefore easily create destruction many hundreds of times greater than the Tunguska object, which was basically a lump of ice.
“Of course, we could also be lucky if an object somewhat larger than the Tunguska asteroid ends up creating a smaller disaster than the Tunguska event, but we can actually help to investigate the problem more closely in Denmark, and perhaps we could even make some plans for what to do in Denmark under various scenarios,” says Uffe Gråe Jørgensen.
If Earth is hit by an object much larger than Dimorphos, the survival of humanity is at stake. But if the object is similar to Dimorphos, we will still experience a natural disaster that far surpasses climate change, with massive tsunamis and huge forest fires. Even if this scenario is much less likely, we should still invest and be prepared.
“In relation to the potential damage, becoming even better at spotting these asteroids is actually not very expensive. It would be a good investment for humanity, and we should have a plan in place for this type of emergency. If an asteroid of this size hits Berlin, for example, we will probably survive the event if we are well prepared, but it will be a catastrophe of unprecedented dimensions. Should we shelter in the basement? Or what is the plan?” concludes Uffe Gråe Jørgensen.