How to Build a Spacecraft to Save the World - WIRED

The goal of the Double Asteroid Redirection Test, or DART, is to slam the cube into a small asteroid orbiting a larger asteroid 7 million miles from Earth.

DART’s target, Dimorphos, is at the lower end of that spectrum, and the asteroid it orbits, Didymos, is at the larger end.

There are more than a thousand asteroids with diameters larger than Didymos and Dimorphos combined, and if any of those were to strike Earth, it could lead to mass extinction and the collapse of civilization.

So they built DART, a deep space probe whose primary mission is to destroy itself to prove it can be done.

“Everyone knows it’s possible to hit an asteroid,” says Justin Atchison, a DART mission designer at the Johns Hopkins Applied Physics Laboratory.

“An asteroid impact is not something that freaks me out at all,” he says.

It’s a demo mission meant to prove that it’s possible to move an asteroid and test out some new technologies on the way.

“When I came on the project, one of the first things I saw was that we were making a Christmas tree of new technology, and I said, ‘Oh, we’re not doing that,’” says Elena Adams, DART’s lead engineer, who joined the team after working on NASA missions like the Parker Solar Probe and the Juno mission to Jupiter.

Although mission controllers on Earth can intervene to fly DART until just a few minutes before impact, the spacecraft was designed to complete its mission with minimal human control.

Once the spacecraft has its power source ready to go, it will feed electricity from the panels to an ion drive it’s bringing along for the ride.

So instead of carrying multiple thrusters to use at different stages of a mission, a spacecraft could kick its electric thruster into high gear when it’s close to the sun, where there are plenty of photons to convert to electricity, then throttle it back as it moves farther from the star.

DART’s algorithmic pilot is partially based on systems designed to guide missiles to their targets back on Earth, but it’s been modified to guide the spacecraft to the center of the asteroid.

Although it’s outfitted with a star tracker that will tell it where it is in the solar system using the positions of stars in our galaxy, the spacecraft won’t actually be able to see its target until it’s about a month out.

Even then, it won’t be able to see Dimorphos, only its larger host, Didymos, which will be a single pixel in its frame of view.

The DART team has spent hours upon hours simulating the spacecraft’s approach and teaching the algorithm how to recognize and focus on the asteroid when it’s barely visible.

Building a camera that can handle the rigorous requirements of an asteroid impact mission is a big deal.

The camera must be able to handle a huge range of dynamic conditions, which is all the more challenging because no one on the DART team is entirely sure what the spacecraft will encounter when it arrives.

In fact, when Didymos is close enough for astronomers to resume observations next year, the asteroid will be about 100,000 times fainter than the faintest star you can see with the naked eye on a dark night.

Andy Cheng, now the chief scientist at the Applied Physics Laboratory and one of the lead investigators on the DART mission, was working out one morning shortly after the report was published when he hit on a way to crash into an asteroid on the cheap.

“The idea came to me that we should do this at a binary asteroid, because then you wouldn’t need a second spacecraft to measure the deflection,” says Cheng.

Fewer still are small enough that a spacecraft could make a noticeable difference in their orbit.

Once Didymos was selected as a target, astronomers began observing the asteroid system when it came around every two years.

So how do you plan a mission to crash into an asteroid when you don’t even know what it looks like.

The most important unknowns for the DART team to model before launch are the shape of Dimorphos and its composition, since these factors play a huge role in determining how the spacecraft’s impact will affect its trajectory.

An asteroid shaped like a dog bone, for example, will react differently than a spherical asteroid, and it will also be harder for the spacecraft to identify and hit its exact center.

The size and distribution of these rocks will determine the effects of DART’s impact, since the rocks near the crash site will blow off into space.

The DART team has been working with the planetary defense crew at Lawrence Livermore National Laboratory to simulate the possible impact scenarios using two of the lab’s supercomputers.

Accurately being able to predict how an asteroid will react to an impactor will be critical if we ever need to launch an actual planetary defense mission.

The crash data will be collected by DART’s only payload that isn’t specifically designed to get the spacecraft to its target or relay data back to Earth.

It’s an Italian cubesat called LICIACube that will be ejected just a few minutes before DART slams into the asteroid.

Although DART was originally conceived as a standalone NASA project, Cheng and the mission’s architects soon entered a partnership with the ESA to do a joint mission called the Asteroid Impact and Deflection Assessment.

The idea is to send a small spacecraft, along with two small cubesats, to orbit the Didymos system and observe the aftermath of the DART mission.

These telescopes will begin their observation campaign months before DART reaches its target, and their observations will be critical for determining where the moon is around the asteroid months before the spacecraft arrives.

The last thing the team would want is for Dimorphos to be on the wrong side of Didymos as the craft approaches and for it to crash into the larger asteroid instead.

After all, the entire point of the mission is to determine how a spacecraft can change the trajectory of an asteroid by slamming into it.

The DART crash will only tack about 10 minutes onto the moon’s 12-hour orbit around Didymos.

But it’s enough for Thomas and her team of astronomers on Earth to detect by studying the way the brightness of the asteroid changes as Dimorphos does laps around its host.

Like the images from LICIACube, the data collected from these telescopes will help scientists refine their models of an asteroid impact until Hera can collect more data.

It’s important for the team to maximize the amount of data collected directly after the crash because it’s the closest that the Didymos system will come to Earth for the next 40 years.

In the event of a real asteroid emergency, a crucial factor that would determine whether a spacecraft like DART could save the world would be how far in advance the asteroid is detected.

It took DART nearly a decade to go from concept to a mostly built spacecraft, but Adams says this timeline could be accelerated if there was an asteroid that could wipe out a country heading our way.

Even by ramming into the asteroid at 4 miles per second, it will barely move the rock at all; it’s orbit will change by less than a millimeter per second.

This is what spacecraft engineers refer to as “shake and bake.” The DART team will vibrate it on a large shaker platform up to 3,000 times per second to simulate the stresses of launch and cycle it through a range of extreme temperatures in a chamber that simulates exposure to the vacuum of space.

When it passes this testing, the DART team will do another practice run to make sure everything on the spacecraft is still working properly.

“I have nightmares where the spacecraft gets to the asteroid and is still alive,” he says.

The people who actually needed to be onsite to build spacecraft hardware switched to working in small groups in shifts, and the rest of the team collaborated on simulations remotely

Like a global pandemic, the risk of an asteroid impact is improbable and feels pretty abstract— until it happens