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Thursday, May 23, 2024

How to Build a Spacecraft to Save the World

Our best hope of saving the planet from a killer asteroid is a white cube the size of a washing machine that’s currently in pieces in a clean room in Maryland. When I arrived last week at the Johns Hopkins University Applied Physics Laboratory, a sprawling R&D facility where most researchers are working on government projects they can’t talk about, the spacecraft was missing two of its side panels, its ion drive was being cleaned, and its primary camera was in a refrigerator down the hall. Ordinarily, the sterile high bay would be a hive of activity with technicians in white clean suits doting on the spacecraft, but most of them were on the other side of the glass trying to get the half-built cube to talk to a massive radio dish on the other side of the country.

Next summer, that same dish in California will be the spacecraft’s main point of contact with Earth as it blitzes through the solar system on a first-of-its-kind suicide mission for NASA. 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. No one is exactly sure what will happen when the probe impacts its target. We know that the spacecraft will be obliterated. It should be able to change the asteroid’s orbit just enough to be detectable from Earth, demonstrating that this kind of strike could nudge an oncoming threat out of Earth’s way. Beyond that, everything is just an educated guess, which is exactly why NASA needs to punch an asteroid with a robot.

Astronomers have discovered about 16,000 asteroids between 140 and 1,000 meters in diameter lurking in our solar system. DART’s target, Dimorphos, is at the lower end of that spectrum, and the asteroid it orbits, Didymos, is at the larger end. If either of those asteroids were to strike Earth, it would cause a kind of regional death and destruction unparalleled by any natural disaster in history. 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. The odds of this happening are extremely low, but, given the consequences, NASA and other space agencies want to be ready just in case.

The good news is that scientists think it’s possible to divert these killer asteroids if they’re detected far enough in advance. That’s not guaranteed—asteroids sneak up on Earth with distressing regularity—but there have been a lot of proposals floated over the years for how we might go about it. Arguably the most practical ideas involve blowing an asteroid up or crashing into it. But for these strategies to be effective, scientists need a better idea of how an asteroid will react. 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. “But there’s a big step in between saying it can be done and actually doing it. You learn a lot in that process.”

For someone tasked with building a spacecraft to save the world, Andy Rivkin, one of the DART mission’s two lead investigators, is surprisingly nonchalant about it. “An asteroid impact is not something that freaks me out at all,” he says. “We have a pretty good sense of the odds of it being a problem anytime soon. This is mostly building toward a future where eventually people may need to use this and we want to give them the tools to do that.”

On a typical NASA mission, a person in Rivkin’s position would be responsible for wrangling the scientists who would be using the spacecraft for research. But DART’s primary mission isn’t scientific. 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.

Generally speaking, spacecraft engineers want to cut risk wherever possible, which usually means relying on hardware that’s already been proven in space, rather than trying out new tech. Since these spacecraft also must hit really stringent weight requirements, engineers can’t just slap on an extra component to test it out during the primary mission. This makes DART’s design all the more remarkable, because many of its critical technologies will be journeying into deep space for the first time. And because DART’s main goal is to crash rather than gather scientific data, the engineers have a little more breathing room when it comes to making weight, which means it can carry some technologies just to give them a test.

“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. “But it’s only by flying the new technology on a mission and demonstrating it that makes it a real flight article.”

DART’s launch window opens next July, ahead of the asteroid’s closest approach to Earth— a mere 7 million miles—for the next few decades. The probe will be boosted on its way by a SpaceX Falcon 9 rocket and will spend a little over a year clipping through the solar system at around 65,000 mph. 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 it separates from the Falcon 9, DART will unroll its solar panels. The solar cells are embedded in a flexible material that is stretched taut between a pair of booms on either side of the spacecraft. This cuts their weight by a factor of five compared to conventional rigid solar panels. “The solar arrays are going to enable so many missions to the outer planets because they’re incredibly light,” says Adams. “Every kilogram of savings in space is a big deal.”

The solar panel deployment mechanism was tested on the International Space Station in 2017, but this will be the first time that it will be used with actual solar cells. 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. Ion drives use electricity to ionize propellants, which knocks electrons free from the gas. The positively charged gas is repelled by a negatively charged electric field and the ions are thrown out of the engine to push the craft forward.

Although they don’t produce much thrust, ion drives are extremely efficient compared to rocket engines that rely on combustion. DART will use 12 small conventional chemical thrusters to correct its path and change its orientation, but it will also test out a commercial variant of the NASA Evolutionary Xenon Thruster along the way. The NEXT-C ion drive has been in development for nearly two decades but has yet to be tested in space. It operates at a power level three times higher than the other ion drives NASA has used on deep space missions, and is about 10 times more efficient than conventional chemical propulsion systems.

But the real potential of the NEXT-C drive, says Atchison, is its ability to throttle between a wide range of power levels, since most ion drives have to stick within a narrow band. 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.

NEXT-C will only be used for short tests on DART and is effectively a backup to the primary propulsion system. But the important thing is to prove the technology in space after so much testing in the lab. During the probe’s transit, the ion drive will only be used to correct DART’s course or for short demos that involve slightly altering the probe’s trajectory and then putting it back on course. “Once it’s demonstrated, it will open up a lot of different missions,” says Atchison. “As a technology, it’s really exciting.”

The solar panels will also provide power to DART’s radio antenna, which is also being tested in space for the first time. The circular antenna is flat, making it easier to haul to space compared to the large parabolic dishes that spacecraft usually need to phone home. All the data it sends back to Earth will be processed by the craft’s field programmable gate arrays, or FPGAs. Unlike general purpose computers, FPGAs are specially designed to efficiently handle specific tasks. This is critical for DART, which will have to do a lot of precision computing to hit its target.

As DART makes its final approach, it will be streaming images from its camera back to Earth until just a few seconds before impact. At the same time, another computer has to process those images and feed them to the spacecraft’s bespoke autonomous navigation system, Smart Nav. 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. “Smart Nav is our number key technology that allows us to hit the asteroid,” says Adams.

For most of the cruise phase of the mission, DART will effectively be flying blind. 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. Dimorphos won’t come into view until the spacecraft is only an hour away from crashing.

“Draco will be constantly streaming images to us once a second,” says Adams, referring to DART’s onboard camera. “It’s kind of like getting a very boring video feed of one pixel. It’s incredible because you really have to zoom in on your screen to be able to see it, but by then the guidance system will have started to point at it and lock on to it.”

At that point, it’s too late for mission controllers on Earth to do any major corrective maneuvers. The success of the mission will come down to the ability of DART’s Smart Nav algorithms to keep the tiny asteroid in the center of view and guide the craft to its target. 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. This can be an excruciatingly dull way to pass the time, but it’s absolutely critical to the mission’s success. Unless the probe knows how to identify its target, it could, say, mistake a speck of dust on its lens for the asteroid, or set its sights on the main asteroid instead of its moon.

Building a camera that can handle the rigorous requirements of an asteroid impact mission is a big deal. Draco is first and foremost a navigation tool, which means that its photographs have to be extraordinarily precise. The problem is that optical devices are very sensitive to temperature change. “When you go cold, everything moves,” says Zach Fletcher, Draco’s system engineer. Even the smallest change in Draco’s optical apparatus—a mere micron of change between its primary and secondary cameras—can throw the camera completely out of focus and cause DART to go blind. So the camera’s optics use a special type of glass that resists temperature distortions. “It’s really different,” says Fletcher. “You wouldn’t ever use this glass on the ground.”

Once Draco is fully assembled, Fletcher and his team will spend weeks working through the tedious process of fine-tuning the camera to get it ready for launch. They’ll use extremely precise laser systems called interferometers to measure submicron distortions in Draco’s optics when it’s ensconced in a chamber replicating the frigid temperatures it will encounter in the vacuum of space. The camera must be perfectly tuned to detect the faint Didymos system from millions of miles away. But it also needs to be able to relay crisp images of the space rocks back to Earth. “We want to try to get as much signal as possible so we can see regions on the asteroid that aren’t very bright,” says Fletcher. 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.

One of the most unique aspects of the DART mission is how little its architects know about their target. Didymos was discovered in 1996 and astronomers suspected it might have a moon, but it wasn’t until 2003 that they confirmed a satellite’s existence. Didymos is about a half mile in diameter and dwarfs its moon, Dimorphos, which is about the size of a professional sports arena. Dimorphos is too dim to be seen directly with Earthbound telescopes, and most of the time so is the main asteroid. 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.

The little we already know about Didymos and Dimorphos is thanks to observations done by ground-based optical and radio telescopes. In fact, the only way astronomers can tell Didymos even has a moon is because its brightness dims at regular intervals, suggesting that there is an object in orbit around it. “Much of what we know about the Didymos system comes from observations in 2003,” says Cristina Thomas, an astronomer at Northern Arizona University and the leader of DART’s observation working group. “The Didymos system has an observing window approximately every two years, and once DART was an idea, we started observing Didymos regularly.”

DART traces its origins to Don Quijote, an asteroid impactor proposed by the European Space Agency in the early 2000s. The idea was to send out two spacecraft—one to hit an asteroid while the other watched—and study how the strike changed the asteroid’s trajectory around the sun. ESA officials ultimately determined that the mission would be too expensive and killed the idea. But a few years later, the National Academies of Science, Engineering, and Medicine, which sets priorities for various scientific disciplines, published a report that strongly recommended an impactor mission. The question was how to lower the cost.

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. “You could do it from Earth with ground-based telescopes.”

All that was needed was a target. There aren’t many double asteroids floating around, and only a few of those pass close enough to Earth to be observed by ground-based telescopes while a spacecraft rams into them. Fewer still are small enough that a spacecraft could make a noticeable difference in their orbit. By the time Cheng and his crew had whittled down the list of possible targets, there were only two viable options—and one of them was Didymos. “It was by far the best choice,” says Cheng. So he and a small group put together a proposal and pitched the idea to NASA in late 2011. It didn’t take long for the agency to bite. By 2012, DART was officially on the books.

Once Didymos was selected as a target, astronomers began observing the asteroid system when it came around every two years. “We realized that we needed to understand the pre-impact system as well as we could before we changed it forever,” says Rivkin. The first Didymos observation campaign since 2003 began in 2015 and has occurred every two years since.

Based on previous observations, astronomers know that Dimorphos orbits Didymos about once every 12 hours and is about 500 feet wide. But beyond that, it’s a mystery. Before Didymos became the DART target, there just wasn’t that much of a reason to keep an eye on it, because it didn’t pose much of a threat to Earth—at least not for the foreseeable future. “We don’t know what Dimorphos looks like at all,” says Adams. “We’ve only seen Didymos.”

So how do you plan a mission to crash into an asteroid when you don’t even know what it looks like? Simulations—and lots of them. 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. Evidence suggests that many asteroids aren’t solid but are actually big rubble piles held together by the gravity of their individual rocks. 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. When they push off the asteroid, they will further increase the change in the asteroid’s trajectory.

Modeling a bunch of different possible shapes will help DART autonomously make decisions about where it should aim to crash on the surface. And by modeling the effects of different shapes and compositions of the asteroid, scientists can compare the results of their simulations with actual data from the collision. 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. These sorts of scenarios aren’t out of the ordinary for the national lab, which also simulates how to blow up asteroids with nuclear weapons. By studying the way that ejecta is thrown off the asteroid, they’ll be able to get a better idea of what it is made of and how this composition will affect the trajectory change. 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. Shortly afterward, LICIACube will fly by the asteroid and take pictures of the aftermath. These photos will be useful for helping scientists back on Earth validate their models. The cubesat will be pretty far away from the asteroid while it shoots these pictures, so the images won’t be very detailed. But they’ll be better than nothing, which is almost what NASA got after the European Space Agency pulled out the mission in 2016.

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 plan was for the Europeans to build a probe called AIM that would launch before DART, scouting out the asteroid for a few months before the impactor arrived. When DART rammed into the surface, AIM would be around to watch it happen.

Despite strong support for the AIM mission from many of the ESA’s member states, things fell apart in 2016 after those states didn’t vote to give the program the funds it would need to continue. “There’s a long history of missions that start as collaborations between NASA and ESA, and for various reasons one side can’t do their part and the whole thing falls apart,” says Cheng. “We proposed keeping the two missions independent so that they are each worth doing if the other partner doesn’t show up.” It proved to be a prudent choice.

Until 2018, it looked like DART was going to have to go it alone. Then, the Italian Space Agency approached NASA with a proposition to send one of their cubesats they had developed for a moon mission along for the ride. NASA officials embraced the idea and LICIACube was added to the mission. Not long after, the ESA rolled out a followup to AIM called Hera. 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. Although the ESA’s new probe won’t be there for the main event—it won’t be ready to launch until 2024—when it arrives it will be able to map the crater created by DART and take detailed measurements of Dimorphos to understand how the impactor affected it.

In the meantime, a network of telescopes will keep an eye on the Didymos system from Earth. 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. By the time DART is close enough to determine the orbit of the moon on its own, it would be too late to tap the brakes to adjust the timing. Rivkin says the final observation campaign before launch, which begins this spring, should be sufficient to pin down the moon’s orbit with enough accuracy that Dimorphos will be in the right place at the right time.

Thomas says there’s a chance that ground telescopes might even be able to see the impact from Earth. “If we do get that opportunity, it will likely appear to be a brief flash of light,” she says. “It will be incredibly exciting.”

But even if the telescopes don’t pick up a crash flash, they’ll still have an important role to play in observing the aftermath. 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.

NASA is leading the DART mission, but planetary defense is, by its very nature, a global effort. In 2016, NASA established a Planetary Defense Coordination Office at its headquarters in Washington, DC, to collaborate with sister programs at the world’s space agencies. So far, most planetary defense work has involved a coordinated campaign with observatories around the world to track down potentially hazardous asteroids and plot their trajectories. “The reason that people are keen on searching for asteroids is: The earlier you find something, the more time you have to do something,” says Rivkin.

Following a relatively close brush with a civilization-ending asteroid in the late 1980s, Congress tasked NASA with figuring out exactly how much of a threat asteroids pose to life on Earth. The agency’s official report to Congress painted a dire picture and made the case for allocating funds to address the issue, starting with a comprehensive effort to locate all the potentially killer asteroids in the solar system. “Although the annual probability of the Earth being struck by a large asteroid or comet is extremely small,” the report noted, “the consequences of such a collision are so catastrophic that it is prudent to assess the nature of the threat and prepare to deal with it.”

Two years later, Congress directed NASA to find 90 percent of the asteroids in the solar system larger than 1 kilometer in diameter; they would almost certainly lead to mass extinction if one crashed into us. In 1998, the agency officially began its search and by 2010 had fulfilled its goal. But asteroids significantly smaller than 1 kilometer can also be catastrophic on a regional scale. So in 2005, Congress expanded NASA’s mandate and tasked the agency with finding 90 percent of asteroids greater than 140 meters in diameter—about the height of the Washington Monument—by the end of 2020.

Still, even if the agency meets that goal, the remaining 10 percent could represent hundreds of uncharted asteroids. And finding the killer space rocks lurking in our solar system is only half the battle. Even though NASA has identified many of them, it can still take years to work out their orbits. So not only are there plenty of big asteroids out there we don’t know about, but even the ones we are aware of could still pose a threat until we can accurately predict their trajectories.

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. This is important for a few reasons. First, it takes a lot of time to get a spacecraft ready for launch. 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. “If you’re trying to defend Earth, you probably would not fly so much new technology,” she says. “There were so many lessons learned that I feel like we could do it faster next time around.”

The other factor has to do with how much a spacecraft can realistically change an asteroid’s orbit. As far as asteroids go, Dimorphos isn’t that big, but neither is DART. 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. “Depending on how much warning time you have, that could be plenty, or it could be not nearly enough,” says Rivkin. When it comes to planetary defense, timing is everything.

The team at the Applied Physics Laboratory has a lot left to do before the craft is ready for launch next summer. After the team verifies that DART is able to send and receive data with NASA’s Deep Space Network, the next step is a thorough practice run of the launch sequence using the craft and a computer simulation. They’ll practice things like discharging the spacecraft’s batteries to prepare for launch and monitoring the solar panels as they unroll.

The goal is to get a baseline of the spacecraft’s performance before it’s subjected to environmental testing. 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. If it all looks good, the spacecraft will be shipped out to Vandenberg Air Force Base in California next May, where it will undergo its final check out before SpaceX technicians load it into the rocket for launch.

It’s not unusual for spacecraft engineers to grow attached to their creation; after all, they’ve often spent years working on the project, and some of them will spend several more years studying the data it beams back home. But everyone I spoke with on the DART team was enthusiastic about destroying their intrepid robot. “There's a part of me that finds it thrilling whenever something is being smashed or blown up,” says Cheng. Fletcher agrees. “I have nightmares where the spacecraft gets to the asteroid and is still alive,” he says. “That’s total failure. I can’t wait for it to be destroyed.”

It’s remarkable that the team has managed to stay on its launch schedule during the pandemic, but Adams says they quickly found workarounds. 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. Things will get a bit trickier this winter and in the spring, when the entire crew needs to be onsite for the simulations, but the team is already starting to plan how to make it work with social distancing protocols.

Like a global pandemic, the risk of an asteroid impact is improbable and feels pretty abstract— until it happens. The key is knowing how to react quickly and decisively even in the face of overwhelming odds. That’s what the DART mission is all about. “Through Covid, through everything, we're not stopping,” says Adams. “We have one goal and we’re going to get it.”

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