At a Planetary Defense Conference in April 2015, a group of aerospace experts was given this fictional scenario to solve:
The hypothetical asteroid 2015 PDC, "discovered" on April 13, 2015, is on a collision course with Earth. Between 500 and 1,300 feet in diameter, the asteroid could crash into Earth as early as September 2022. What do we do? 1
Although the scenario is completely hypothetical, the conference organizers tried to make it as realistic as possible, including all the unknowns scientists would face, such as the asteroid's size, what it's made of, and where exactly it might hit Earth.
At the conference, attendees explored a strategy of crashing spacecraft into the asteroid to change its course. Now, a recent paper investigates a second way to avoid catastrophe if this situation came up in real life.
What to do if an asteroid is on a warpath
First, the team used the available data to calculate if and where the fictional asteroid might strike. The areas of highest risk covered a swath from Turkey to India, including heavily populated areas such as Tehran, New Delhi, and Dhaka.
There was some chance the fictitious asteroid would fall into the middle of ocean without cause much damage. (An impact just off the coast, on the other hand, could be pretty disastrous.) For the sake of this study, the researchers assumed the worst: that the imaginary asteroid was headed for those heavily populated city centers. Which raises the next question:
How do we stop it from killing a bunch of people?
Although everyone from Armageddon director Michael Bay to NASA has suggested we could blow up an incoming asteroid, the idea is controversial. The Outer Space Treaty technically bans the use of nuclear weapons in space. Plus, some scientists think that the shrapnel from an asteroid explosion could potentially cause just as much if not more damage than the original asteroid.
“My position is that nuclear deflection should be used only when all the other options are doomed to fail,” says Claudio Bombardelli, a space dynamics researcher at the Technical University of Madrid and lead author on the paper.
"Nuclear deflection should be used only when all the other options are doomed to fail."
Instead, Bombardelli's team supports using an “ion beam shepherd”--a concept their group and others have been working on for a few years now--to nudge an incoming asteroid out of harm's way.
How ion deflection works
Bombardelli and his colleagues evaluated a plan to shoot a beam of ions (charged particles) at the hypothetical asteroid. The ions would travel at about 19 miles per second, transmitting their momentum to the asteroid when they crash into it, knocking it ever so slightly off its expected path.
“It's a very tiny force,” says Bombardelli, “and the only way this force could work is if it is applied for a very long period of time.”
For this paper, the team assumed the hypothetical asteroid weighed about 20 million tons, and by steadily applying about as much force as the weight of a strawberry for about two years, they were able to change its course by hundreds of miles.
The ion beam could be generated by a spacecraft that uses ion engines, similar to the Dawn spacecraft that recently arrived at the dwarf planet Ceres in the asteroid belt.
Initially, these ion engines would drive the spacecraft to the asteroid. Then, when it gets to the asteroid, it could turn around to direct its ion exhaust at the asteroid to push it. So the engines would be dual-use, says Bombardelli, which cuts down on the weight of the spacecraft.
Like the Dawn spacecraft, this hypothetical spacecraft would make its ion beams using electricity from solar panels to break xenon gas into ions, which the spacecraft charges up until they jettison out of the spacecraft's rear end.
The team calculates that the asteroid-deflecting spacecraft would need about 11 kilowatts of power to do its job, which is right in line with what Dawn achieved.
"We're confident ion beam deflection would work [in some scenarios]," says Paul Chodas from NASA's Near Earth Object program. Chodas was key in designing this hypothetical scenario for the conference, but was not involved in Bombardelli's study.
It might even be possible to devise a much more powerful ion spacecraft than the one this paper calls for, says Chodas. NASA's Asteroid Redirect Mission will test out a 40 kW ion engine.
The exact amount of force an ion beam could exert on an asteroid is still somewhat fuzzy, but Bombardelli and his team are testing the strategy in a plasma vacuum chamber at the German Space Agency. So far their results are in line with what they've estimated.
Launch and rendezvous
The researchers launched their imaginary spacecraft on May 28, 2017, and it arrived at the imaginary asteroid on September 30, 2019.
With only three years left to deflect the asteroid, and assuming it turned out to be 820 feet in diameter with the density of the average asteroid (2 g/cm3), there wasn't enough time to push it far enough to bypass Earth entirely. But it might be possible to move it onto a less destructive path. So the next big question was:
Where do you put it?
In one of the worst-case scenarios, the researchers imagined the asteroid was heading for New Delhi, where more than 16 million people live. They calculate that, with 22 months of applying the ion beam, the asteroid could be deflected to the rural Paktika province of Afghanistan, reducing casualties by two orders of magnitude, and virtually eliminating infrastructure damage.
If the spacecraft only has 15 months to work on the asteroid, however, that would only be enough time to send it to the Punjab province of Pakistan. This area is more developed than the Afghan desert, but it would still reduce casualties by an order of magnitude.
The political challenges of either strategy would probably be more difficult to overcome than the scientific challenges. Is it fair for India to deflect the asteroid away from New Delhi and send it over some heavily populated areas of Pakistan and into the deserts of Afghanistan? Obviously, countries would need to work out an agreement to compensate the countries and regions that take the hit.
If the asteroid were bound for Dhaka, the capital of Bangladesh, instead of Delhi, the researchers calculate that 13 months of ion beam deflection could redirect the asteroid into Myanmar, to reduce casualties and infrastructure damage by two orders of magnitude.
And an asteroid hell-bent on hitting Tehran would actually be the easiest to deflect, since the capital of Iran is surrounded by desert. With as few as one or two months of pressure from the ion beam, the asteroid could be redirected to reduce casualties and damage by more than two orders of magnitude, and without requiring as much political maneuvering.
To deflect the fictional asteroid so that it avoids Earth altogether would require five of the ion beaming spacecraft, according to the scientists' calculations, and it would take 33 months. And to fully deter an asteroid that's 1300 feet wide would require 20 spacecraft. Building and launching so many spacecraft could become very expensive.
"Size matters a great deal," says Chodas, "and that's something we don't know very well ahead of time."
In some scenarios, ion beam deflection wouldn't work at all. It takes the ions months or years move an asteroid significantly, so the method can only be deployed if humanity gets an early warning that the asteroid is headed for Earth.
"Size matters a great deal."
If there's not a lot of lead time, it could be that the conference's kinetic impactor solution would work better. This method would crash spacecraft into the asteroid to change its course. Since these spacecraft are relatively simple, they could be built more quickly and easily than an ion beam spacecraft.
However, one disadvantage to the kinetic impactor method is that it can only move the asteroid in one direction. The method works by slowing down the asteroid; scientists place the spacecraft in the path of the asteroid, and when it runs into the spacecraft the asteroid slows down. Because of orbital mechanics, slowing down makes the asteroid's orbit smaller, which changes the place at which it will cross paths with Earth.
In the scenario from the conference, the kinetic impactor could only move the asteroid westward, whereas ion beam deflection could move the asteroid in any direction. That's potentially useful if the best we can do is move it to an area where it will harm fewer people.
NASA's Asteroid Redirect Mission will explore a third strategy of moving asteroids: the spacecraft will travel to an asteroid, pick up a boulder, and use its increased mass as a sort of gravity tractor beam to slightly push the asteroid off course. This is another "slow push" method, but it's not as simple as the ion beam deflection method.
Finally, for very large and very close Earth-threatening asteroids, there's one more option:
"Nuking it is a last-ditch option it seems to me," says Chodas. "It's more unpredictable, but it's certainly valuable to consider it in our toolbox. And we seriously discussed it at the conference. We think it could have been effective for this scenario."
In the end, the best asteroid deflection method will very much depend on the situation.
"If will vary from case to case," says Chodas, "depending on the warning time, the orbit of the asteroid, and the size of the asteroid."
The scenario was actually much more detailed than this. Click here for the full description.