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Since joining APL in 2016 as a software engineer, much of Peter Ericksen’s time has been consumed by modeling DART’s collision (or complete miss) with Dimorphos — again and again and again.

He works with a program called the renderer and camera emulator, or RCE. It was designed by engineers on the DART G&C team at APL to simulate what the DRACO camera will see during DART’s last four hours.

“It’s a pretty complex simulation,” Eriksen said. It combines a ‘truth model,’ which models the spacecraft’s sensors and physical realities of space — gravity, the Sun’s location, the Didymos system’s movement — with flight software coded specifically for the DART mission.

The simulations are like target practice. Each runs the final four hours, with the Didymos system initially a tiny dot no bigger than a pixel. And as the spacecraft gets closer, that single point of light divides into Didymos and Dimorphos until DART (hopefully) hits.

Of course, building a simulation of DART hitting an asteroid nobody has seen before proved tricky. After all, the shape and texture of each of the system’s bodies could affect whether SMART Nav isolates the correct asteroid or not. But what do you make the asteroid look like?

“We could make the Didymos system look like a donut if we wanted to,” Chen joked. It sounds ridiculous, but she isn’t far off.

Dog bones, squashed potatoes, spheres, even rubber ducks — all have made it into the pantheon of asteroid shapes the team has simulated. Imaged asteroids or binary systems inspired most of them. Asteroid 25143 Itokawa, for example, resembles a potato. 2 Pallas is mostly spherical. 216 Kleopatra, oddly enough, looks like a dog bone. And the rubber duck — well, why not?

And it wasn’t just shape that the team altered. They changed how much the asteroid reflects light, in case that might “distract” the targeting algorithms. And they added fuel properties to estimate how much fuel would be lost — all to see how each property affects SMART Nav’s guidance and targeting algorithms.

Michelle Chen

SMART Nav Technical Lead

I feel like we’ve been so prepared for this. 

Early on, DART frequently missed Dimorphos. “There’s a lot of stuff gone wrong, and usually it was kind of an all-or-nothing deal,” Ericksen said. Either every simulation hit Dimorphos, or they all missed, regardless of what the asteroid looked like.

The team learned problems could arise for all sort of reasons, and had to tweak the algorithms to compensate for each one of them. DRACO, for example, could be oriented so the spacecraft points in the exact wrong direction, or shadowing on Didymos could make it appear as though it were split in two, causing the spacecraft to single out the wrong “blob.” There’s also a slight chance that there’s a third object in the Didymos system — a much smaller, more distant moonlet nobody has detected yet. The team needed to include ways for the algorithms to differentiate such a tiny blob much farther from the Didymos system from the blob it needs to hit.

How long each simulation takes depends on the level of detail it’s striving for and the rate that the team runs the algorithms — once per second or once every two seconds. But with everything running at full fidelity, it can take up to 12 hours to simulate just one four-hour terminal phase. The team usually runs around 300 simulations at a time, Ericksen said. He estimates they’ve run somewhere between 10,000 and 20,000 simulations of just the terminal phase, not to mention the other mission phases up to that point.

With that many simulations under its belt, the team has separated the wheat from the chaff, gradually tweaking each of the parameters in SMART Nav’s algorithms to improve the robustness of DART’s performance. It won’t be until DART launches that the final refinements will be added. Certain things, such as how the spacecraft’s sensors will respond when the thrusters fire, can’t be checked until it’s actually in space. So after running the spacecraft through a series of planned in-flight tests, the team will update their simulations based on what they find and then check whether the algorithm parameters need some final tuning before the real show begins.

Although meant for DART, SMART Nav’s ability to independently guide a spacecraft to a target for hours opens a floodgate of possibilities in space exploration. The idea that you can locate and fly into an asteroid is closely related to locating and flying really close to an asteroid. Instead of hitting the object, you avoid it — something similar to when NASA’s New Horizons mission sped 1 billion miles beyond Pluto to fly by Arrokoth, a bizarre, 22-mile-long Kuiper Belt Object resembling a dirt-covered snowman.

“SMART Nav could be used for a spacecraft to find its own target to fly by like that,” Chen said. “It can be applied to any mission that is bandwidth-limited as well as any small-body mission where you don’t have a priori information.”

Carrelli noted the applications extend even further.

“Truth be told, the missions of the future are going to require more and more autonomy to be successful,” he said. “The problems are getting harder. The distances at which we need to operate are going to get bigger — much bigger.”

Interstellar Probe, for example, is a conceptual APL-led mission that proposes launching a spacecraft hundreds of astronomical units, or Earth-Sun distances, from the Sun in just 50 years. (For scale, Voyager 1, the current record holder for the most-distant spacecraft, is 152 astronomical units from the Sun. It’s been traveling for 44 years.) It would be the fastest, farthest and most ambitious space mission yet. And with a round-trip light time of roughly 11 days at its farthest point, it would certainly need to operate autonomously.

DART, then, is a proving ground not only for capabilities to redirect an asteroid but for the autonomous technology that could define the capabilities of future space missions. DART’s success is largely SMART Nav’s success.

“How do we define success? Success is we hit the asteroid,” Carrelli said. “And secondly, that the scientists can actually observe the effects of hitting the asteroid.”

So long as DART accomplishes the first objective, the latter is pretty much guaranteed. The final 2.5-minute cruising phase will keep images sharp. And the Italian Space Agency’s Light Italian CubeSat for Imaging of Asteroids, or LICIACube, will capture images of the collision from a distance.

But the very slim chance of DART missing the asteroid altogether, or even hitting the wrong one, is not lost on anyone.

David Carrelli

Former DART Guidance, Navigation and Control Lead

How do we define success? Success is we hit the asteroid. 

Those real moments as DART, completely self-directed, treads closer and closer to (hopefully) Dimorphos have people already portending the full range of emotions. Some expect they’ll feel elated, bolstered by the years of preparation leading up to a historic moment. Others are certain they’ll be locked away in a bathroom stall, sick from anxiety.

Chen, for one, is cool and collected about it. “I actually sleep really well,” she said, laughing. She admitted she’s probably strangely calm, especially considering that, in truth, there is no Plan B if DART does miss. There is only Plan A: Hit Dimorphos, and “just sit there and give each other high fives along the way,” she said.

If absolutely necessary, the team can take the reins again up until the last half hour before impact — a sort of ‘point of no return.’ After that, the spacecraft is on its own no matter the scenario.

With so many simulation hours and years of preparation under their belt, though, nobody expects intervening will be necessary.

“I feel like we’ve been so prepared for this,” Chen said.

It’s now just a matter of waiting for that tiny, pixel-sized point of light to come into view and watching SMART Nav work its magic.

Media contact: Jeremy Rehm, 240-592-3997, jeremy.rehm@jhuapl.edu

The Applied Physics Laboratory, a not-for-profit division of The ����ֱ�� University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.