In the 1990s, planetary scientists started becoming aware that our planet could become a prime target in the cosmic shooting gallery. There was a growing realization that, over geological timescales, Earth gets hit by large asteroids and comets rather frequently; however, unlike the moon's obvious craters, Earth's atmosphere is very efficient at eroding the evidence of massive impacts.
Scientists had previously identified the infamous Chicxulub Crater buried under the Yucatán Peninsula in Mexico and linked it with the Cretaceous-Tertiary (K-T) boundary – a rocky layer that was created around the time of a mass extinction event that wiped out the dinosaurs 66 million years ago. At the same time, astronomers were discovering more and more large chunks of space rock zooming around our sun. It started to become clear that it isn't a question of if we're going to get hit by a marauding space rock again but rather when.
Inspired by the realization that asteroids could pose a threat, Andy Cheng started pondering the worst-case scenario: If we discovered an incoming asteroid, what could we do to prevent it from hitting Earth?
"For the first 20 years of working on this problem, we had to be really careful. People's reactions on hearing about this were 'are you serious?' We had to overcome the so-called giggle factor, but we're past that now," says Cheng, who works at The Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland.
Cheng devised a concept that uses a kinetic impactor to physically knock an asteroid off course. Kinetic impactors are basically fast-moving spacecraft that use their kinetic energy to smash into an asteroid to slightly modify the space rock's speed and/or direction. No Hollywood-style nuclear warheads are required. So far, they've only been tested in computer simulations, something Cheng hopes to change very soon. Now, he co-leads a NASA mission that will finally test his early work as part of the Asteroid Impact and Deflection Assessment (AIDA) mission.
The AIDA concept consists of two spacecraft: the Double Asteroid Redirection Test (DART) and the Asteroid Impact Mission (AIM). NASA will develop DART, and the European Space Agency (ESA) will develop AIM. In June, NASA approved DART to enter the design phase.
Scientists plan to test this deflection technique on a single asteroid with the help of two spacecraft missions: one is the impactor while the other will rendezvous at the target to measure the orbit change (of the impacted asteroid), Cheng tells HowStuffWorks.
Although DART isn't fully funded yet, Cheng and his colleagues already have identified a very special target. A binary asteroid called Didymos will make a very close flyby of Earth in 2022, coming within 6.8 million miles (11 million kilometers) of our planet, so the researchers hope that both AIDA spacecraft will launch in time to meet up with this target of opportunity.
Didymos consists of two asteroids in a close orbital dance. The larger component, Didymos A, measures approximately half a mile (780 meters) wide, and the smaller asteroid, Didymos B, is about 530 feet (160 meters) wide. As Didymos B is so small, it's often referred to as "Didymoon," and that will be DART's target.
"This binary asteroid Didymos comes very close to Earth. We knew in 2010 that the 2022 approach of Didymos to Earth was really special ... It's the closest approach in many decades; close enough for Earth-based observations by small telescopes and for radar. It's a system that's been well-observed already and known to be a binary asteroid," he adds.
Naturally, there are safety concerns with hitting an asteroid to see how its orbit is modified. Say the mission's team miscalculates and changes the asteroid's trajectory. Would it become a threat to Earth in the future? Fortunately, because Didymos is a binary asteroid, even if DART significantly affects the orbit of Didymoon around Didymon A, it won't pose a threat to Earth. Didymoon is simply too small to significantly change the orbit of the entire binary system.
"We're not changing [the binary asteroid's] orbit around the sun to any measurable degree," says Cheng.
Astronomers also have a good idea about the chemical composition of this well-studied asteroid. The big unknown is how the material of Didymoon is packed – a factor that will greatly influence its reaction to being hit by a speeding spacecraft. Is it solid rock or a loosely packed clump of material known as a "rubble pile"?
"The impact can react very differently depending on what the asteroid is made of," Cheng continues. "It's not specifically the chemical composition, because for many asteroids we think we have a basic idea of what their chemical composition is – based on their spectra and the fact we've had two missions."
NASA's launched its NEAR mission in 1996, spending a year orbiting the near-Earth asteroid Eros. And Japan's Hayabusa mission physically returned a sample of asteroid material from the surface of asteroid Itokawa in 2010. From these missions and spectroscopic analyses of the asteroid, astronomers are confident that Didymos is a silicaceous (or "S-type") asteroid. S-type asteroids are stony space rocks and the second most common asteroids (after carbonaceous, or "C-type," asteroids) known to exist in our solar system, populating the inner asteroid belt between the orbits of Mars and Jupiter. But to get the "ground truth" on how effective a kinetic impactor slamming into an asteroid's surface will be to physically modify its orbit, we need to launch a mission like DART.
"What's NOT known in this type of asteroid is how the material is packed. So, things like the strength and porosity, factors that make a huge difference in response to an impact," Cheng adds, but he's confident that the impactor won't hit it so hard that the asteroid will break up.
"We are working very hard on how to calculate the impact responses by computer simulation ... but the uncertainty comes because when we have a hypervelocity impact on a body, it creates a crater; it spews out crater ejecta back in the direction that you came, but in doing so, those ejecta carry away a lot of momentum and there is a reaction, which can change the amount of deflection on the body – that's the problem and that's a big question."
Cheng points out that the amount of momentum removed from the asteroid can be several times higher than the amount of momentum a kinetic impactor will carry to the asteroid – and it's all down to how much material (impact ejecta) is propelled into space at the moment of impact. And as DART will hit Didymoon at a speed of around 3.7 miles (6 kilometers) per second (that's nine times the speed of a bullet!) and impart a collision energy of "a few tons of TNT equivalent," the only way to understand how this affects the motion of an asteroid in space is to test it.
But there are challenges before DART can make its 2022 collision date. The ESA component of the AIDA mission hasn't yet moved beyond the concept phase and, in December, funds were redirected for the space agency's ExoMars mission. This is one of the reasons why Didymos was selected as a target: The DART mission can still go ahead without its AIM spacecraft buddy. As the binary asteroid makes close approach with Earth, ground-based observatories can observe the effects of the kinetic impactor on Didymoon by timing its orbit. Of course, it's more ideal to have another spacecraft observing the impact up close and carry out science on the impact ejecta, but it wouldn't be the end of the mission if, say, the ESA doesn't eventually launch AIM.
"Decoupling the missions is the secret (to mission success)," Cheng points out.
Sixty-six million years ago, the dinosaurs didn't have a space program that could detect and deflect an incoming asteroid or comet threat. Should something as big as the object that created Chicxulub hit our planet now, the fallout could pose an existential threat to humanity and would undoubtedly destroy civilization as we know it. Testing impact mitigation strategies, as the DART mission proposes to do, could benefit humanity as a whole.