For years, Lindy Elkins-Tanton had been looking forward to blasting a probe to the metallic asteroid Psyche, which is orbiting about 230 million miles away and may be the remains of a protoplanet whose surface was smashed to bits eons ago. She and her team designed and assembled the NASA spacecraft—also called Psyche—and booked a ride for it on a SpaceX Falcon Heavy rocket. Launch day was supposed to come sometime between August and early October.
But Psyche’s software derailed their plan. While engineers at NASA’s Jet Propulsion Laboratory have completed the guidance, navigation, and control system, they and their colleagues at Maxar Technologies haven’t been able to thoroughly test and validate it. That means they’ll have to miss their crucial 2022 launch period. “We have no known problems with the [guidance] software, but we just haven’t been able to test it,” Elkins-Tanton, a planetary scientist at Arizona State University and head of the mission, said during a virtual press conference in June announcing the delay. “We have today a beautiful functioning spacecraft, built and ready, but there was that one challenge we couldn’t overcome in time to launch in 2022 with confidence.”
NASA’s new plan is to launch in July 2023 at the earliest, or perhaps in 2024. But this postponement won’t mean that Psyche will arrive at its destination just a year or two later than planned. In fact, what was once a four-year journey will now likely extend into a six-year one, and the probe won’t get to its eponymous space rock until 2029 or 2030. Understanding why requires learning a bit about the intricacies of orbital mechanics.
Flying to Psyche, a massive member of the asteroid belt between Mars and Jupiter, is no simple matter. It requires lots of energy to voyage that far, so NASA had originally planned for the probe to attempt a Mars flyby—a convenient halfway point—to provide an extra power boost some call a “gravity whip.” The move would turn gravitational energy into kinetic energy, speeding up the craft. (The maneuver in effect steals energy from the planet, slowing Mars’ orbit, but by a negligible amount.)
Timing is also key: NASA scientists have to coordinate the destination’s orbit with those of Earth and any intermediate bodies that could provide gravity boosts. “For planetary missions, we have to align everything just right, so that when we leave Earth when the planets are aligned, we can get to the destination with a reasonable amount of fuel,” says Jeff Parker, lead mission designer for EscaPADE, an orbiter mission to study Mars’s atmosphere that was planned to launch along with Psyche. He’s also the chief technology officer at Advanced Space, an aerospace company based in Westminster, Colorado.
And therein lies the problem: Heavenly bodies are always in motion. By the time the next launch window opens, Earth, Mars, and the asteroid will no longer be in exactly the same alignment they would have been this fall. If researchers miss the best possible alignment, they usually can’t wait decades for the orbits to optimally sync up again. Taking the shot in 2023 or 2024 will require new calculations about the energy it will cost Psyche to reach its destination. The energy-boosting Mars maneuver may no longer be feasible. And the journey will be two years longer, mainly because the orbital positions of Earth and Psyche will make for a longer distance to cover.
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NASA and the Psyche team declined interview requests until an independent review of the mission delay has been completed. Agency officials will make a decision about next steps based on that review in the coming months, said Lori Glaze, head of NASA’s planetary science division, at last month’s press conference. But WIRED spoke with other experts about options for sending a probe deep into the solar system, even if you can’t whip it past Mars.
Parker, for example, thinks it might be feasible for Psyche to reach its asteroid by relying more on the spacecraft’s solar-electric propulsion system. This system has solar arrays that will unfold to the size of a tennis court, and they’ll convert sunlight into electricity to power Psyche’s Hall thrusters, efficient and long-lasting devices that emit a blue glow.
Parker says that using the Falcon Heavy for launch is another advantage, because it will give the spacecraft more kinetic energy to start with than a smaller rocket would, which means it has to come up with less solar energy en route. Focusing on power from the liftoff and from the onboard propulsion system would give the mission planners some flexibility about launch times, he thinks, potentially allowing them to make the journey without counting on an alignment with Mars.
Another option for a spacecraft that needs a speed boost is to fling itself past Earth. This was the option chosen for the European Space Agency’s Rosetta spacecraft, which launched in 2004 on a comet-bound mission, says Andrea Accomazzo, the head of the agency’s solar system and exploration missions division. During the probe’s 10-year voyage, it gained speed through three Earth flybys and then swung by Mars before making a beeline for the Churyumov-Gerasimenko comet and deploying the Philae lander to it.
Rosetta’s team faced two additional challenges: The comet had a swooping elliptical orbit rather than a more circular one like most asteroids, which made it hard to match its speed and velocity. And the researchers wanted to plan the trek so that Rosetta and its lander sidekick would rendezvous with the comet when it wasn't very close to the sun, where it would be more active, ejecting bits of ice and dust and complicating a landing that already would be tricky to pull off.
Engineers design spacecraft with launch and trajectory options in mind, and in this case, a few trips around Earth was the best path. “You start from the target, and then you work backward,” says Accomazzo. “You have three sources of energy: the initial energy of the rocket, the energy in the propellant tanks of the spacecraft, and the energy you can get from planetary swing-bys. It’s a bit of handcrafted work of my colleagues who tried to find the optimal solution.”
Parker points out that the utility of planetary swing-bys depends on the geometry of the spacecraft’s trajectory, so they’re not always an option. But he agrees they can be beneficial, especially when the destination’s far away. “Those main-belt asteroid missions are hard, and they take a lot of fuel,” he says. “Psyche could have launched straight to its target with a bigger launch vehicle or a smaller spacecraft or different engine,” but that could have increased costs or reduced the scientific exploration that could be accomplished once the spacecraft arrives. NASA has been planning for the probe to orbit the asteroid for at least 21 months while it images it and uses a magnetometer to search for remains of a magnetic field, which might indicate it was originally a planet’s core.
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And there’s always a third option: terminating a mission. A very long delay can make the science motivation of a mission, or the technologies involved, obsolete. A big delay can also be detrimental if there’s a little competition between missions with similar goals—one can end up eclipsing the other.
Psyche’s postponement has consequences for other, smaller missions. To keep costs down, NASA often has sets of spacecraft launch together on the same rocket, sort of like a rideshare. The EscaPADE Mars mission that Parker was working on was initially supposed to ride with Psyche, but the originally planned launch period didn’t work out for his team. (They’re looking for another rideshare opportunity in early 2024.)
So researchers lined up a different mission to co-launch with Psyche, called Janus. This one calls for launching a pair of spacecraft to a binary asteroid, completing two Earth flybys on the way. The Janus team will now have to wait to see if they can launch along with Psyche on a later date. If not, they’ll have to find another ride to space—and make new calculations for their crafts’ trajectory.
No matter what happens with Psyche, space fans won’t have to wait until 2029 or 2030 for a close-up look at an asteroid. NASA’s OSIRIS-REx probe, which rendezvoused with the asteroid Bennu in 2020, will deliver a sample to Earth in 2023.
And after NASA’s DART spacecraft deliberately crashes into Dimorphos later this fall—a practice test of asteroid deflection—the European Space Agency plans to send a follow-up spacecraft called Hera, which will arrive in 2026 to scope out the aftermath.
Update 6-5-2023 4:30 PM: Following a positive assessment by NASA's independent review board, Psyche is now on track for an October 2023 launch.