By Jeremy Rehm
After five years of developing and testing a complex particle detection instrument for NASA’s Psyche mission, the world’s first mission to study a potentially metal-rich asteroid, the Psyche team at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, can finally take a breather.
The team’s instrument — a gamma-ray and neutron spectrometer, or GRNS — safely arrived at the Jet Propulsion Laboratory in Pasadena, California, on Aug. 2. There, it will be integrated with the Psyche spacecraft and prepped for launch next year.
David Lawrence, a space physicist at APL and the lead investigator of the Psyche GRNS, has keenly felt those five years. “I’m tired,” he said flatly. “But even though it’s been hard, I try to just take a step back and be grateful that this is really cool stuff.”
At 150 miles (240 kilometers) in diameter, the asteroid Psyche (pronounced SIGH-kee) could be the largest of a class of metal-rich asteroids in our solar system. Scientists suspect these bodies of mostly iron and nickel are shards of early protoplanetary cores like the one inside Earth. But while initial observations indicated Psyche was metal-rich, findings over the last seven years have thrown that into question.
“Psyche [the mission] is exciting because it’s going to one of the few remaining unexplored types of objects in our solar system,” said APL nuclear physicist Patrick Peplowski, a co-investigator on the Psyche GRNS. “We really don’t know what we’re going to find when we get there.”
Key to revealing the asteroid’s composition — and thus planetary history — is the GRNS instrument that Peplowski, Lawrence and the entire Psyche team at APL have been developing.
The GRNS consists of two sensors: a gamma-ray spectrometer (GRS) and neutron spectrometer (NS). Both rely on cosmic-ray protons from the Sun and interstellar space. These protons bombard the asteroid, striking atoms just a few inches beneath its surface. Those collisions can knock neutrons out of the atoms’ nuclei, and those neutrons then either fly into space or hit the nuclei of other atoms, stimulating them to release gamma rays. Both reveal information about the asteroid’s chemical composition.
The gamma rays are like a fingerprint. Every element, whether iron, silicon, oxygen, potassium or anything else on the periodic table, releases gamma rays of a specific energy. As the instrument collects the gamma rays from orbit, it can determine what elements make up the asteroid.
Meanwhile, neutrons provide information about particular elements, such as iron, nickel and hydrogen, that complement the gamma-ray measurements. The NS captures neutrons with three specialized detectors known as helium-3 sensors, which detect different energy ranges of neutrons to reveal unique information about the asteroid’s composition.
“To me, it’s kind of miraculous,” Peplowski said. “We’re orbiting from around 50 miles on average — 10 times higher than a commercial airline flies — and from that distance, we’ll tell you what the rocks on the surface are made of. From 50 miles away! That’s really the power of this technique.”
APL has decades of experience building GRNS instruments, including for the Near Earth Asteroid Rendezvous (NEAR) mission to the asteroid Eros in the late 1990s and for the MESSENGER mission to Mercury, which helped make discoveries about Mercury’s composition that rewrote the books on what we know about our solar system’s formation.
With 15-20 years of technological development since MESSENGER (short for Mercury Surface, Space Environment, Geochemistry and Ranging) launched in 2004, however, the Psyche GRNS team couldn’t just copy-paste the same design. They essentially had to start from scratch.
“Instrument design is a bit of a roller coaster,” said John Goldsten, the Psyche GRNS team’s lead engineer who was also the lead engineer on MESSENGER’s GRNS instrument. He said it can be tedious and depressing, yet still exhilarating and rewarding. “You experience the full panoply of emotions going through it.”
Part of that emotional up-and-down comes from the fact that there is no backup if this instrument fails. Many spacecraft carry a primary and secondary instrument copy just in case one becomes inoperable. But because the GRNS is complex, delicate and expensive, the Psyche orbiter will carry just one.
The GRS is partly made of a specially developed material called AlBeMet, a composite of aluminum and beryllium that minimizes the instrument’s amount of aluminum, which scientists are searching for on the asteroid.
And the gamma-ray detector itself is a half-kilogram, 2-by-2-inch crystal of high-purity germanium — “the purest human-made material on Earth,” Goldsten explained. However, it can’t capture gamma rays unless it’s cooled to near liquid nitrogen temperatures, about minus 320 degrees Fahrenheit (minus 196 degrees Celsius). So the detector is inside a specialized cryostat from Lawrence Livermore National Laboratory — a sort of multimillion-dollar thermos bottle. And it needs 3,000 volts of electricity to operate.
“This is everything you have to do to get this technology to work,” Goldsten said with a chuckle. “But if you can get it to work, it’s the best gamma-ray spectrometer in the world.”
Even before the COVID-19 pandemic, the team was forced to wait around 18 months to get some of these specialized materials. The pandemic just elongated that timeline.
The team also went through three iterations of the instrument, subjecting each one to a rigorous battery of tests to ensure this highly sensitive instrument worked properly and could survive the brutal environment of launch and space.
“So far, the instrument that we built has much, much better performance than MESSENGER’s,” Lawrence said. “And that offers the possibility of getting much better measurements, more precise measurements, just everything.”
Gamma-ray and neutron spectroscopy has a slow data collection process, though. To gather enough gamma rays and neutrons, the spacecraft has to be within 60 miles (100 kilometers) of the surface. It won’t reach that altitude until the tail end of the mission. Even then, because the spacecraft is still high above the surface, and the sensor so small, it takes around 100 days to collect sufficient data.
“These measurements reward the slow and patient,” Lawrence said.
Once collected and combined, though, the data can definitively say what the surface is made of. To Peplowski, that finality — that mystery finally answered at the mission’s very end — will make the GRNS results a sort of crowning achievement of the entire mission.
“We’re excited and sad to see the GRNS go, but we’re really looking forward to having it start the next portion of its journey,” Peplowski said.
The Psyche spacecraft is scheduled to launch in August 2022 from Cape Canaveral Space Force Station, Florida. About two months later, the GRNS will be turned on for the first time in space.
The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.