October 21, 2021
The Path to Psyche Starts at Mercury and Moves through Long Island
By David Lawrence and Patrick Peplowski
Building a flight instrument for our asteroid mission
Everyone has their story for how they made their way to the Psyche mission. For some of us on the Gamma-Ray and Neutron Spectrometer (GRNS) team, our path to Psyche started out nearly two decades ago, at the planet Mercury, or more specifically with NASA’s MESSENGER mission (https://messenger.jhuapl.edu/). MESSENGER’s name is a creatively descriptive (but hard to remember) acronym that means MErcury Surface, Space ENvironment, GEochemistry, and Ranging. MESSENGER sought to perform the first-ever orbital investigation of an unexplored world with an anomalously high density. Interestingly, this goal applies equally to Psyche.
The MESSENGER spacecraft, which launched in 2004 and orbited Mercury from 2011 to 2015, carried a GRNS just like Psyche. Both the MESSENGER and Psyche instruments use a technique known as gamma-ray and neutron spectroscopy to measure the elemental composition of airless (or nearly airless) planetary bodies (see “Tools of the Trade” at https://www.jpl.nasa.gov/news/science-of-psyche-unique-asteroid-holds-clues-to-early-solar-system). Four of us who work on Psyche — Morgan Burks, John Goldsten, and the two of us — we got our start working with each other on MESSENGER.
MESSENGER’s GRNS measured Mercury’s elemental composition, and those data were key to discovering that Mercury had a high content of volatile materials (i.e., materials that evaporate at relatively low temperatures), and confirming the existence of water-ice deposits at Mercury’s North Pole. These results were named the top two science results coming out of the MESSENGER mission (see here: https://messenger.jhuapl.edu/index.html#top10). MESSENGER’s successes at Mercury were part of the reason that the GRNS team was invited to join the Psyche project in April 2014.
When it came time to propose the Psyche GRNS instrument, MESSENGER provided a starting point but it was clear to us that improvements to the gamma-ray spectrometer (GRS) were needed before the next GRS should fly on a future mission. This included the need to update various obsolete parts from the almost 15-year-old MESSENGER design. The most important metric for GRS performance is energy resolution, which is the ability of a GRS to distinguish element-diagnostic gamma-ray peaks of slightly different energies (see Fig. 1). While the Psyche GRS was designed to accomplish its measurements with MESSENGER-like energy resolution (5 keV), our goal for Psyche is a significantly better performance of 2 keV, which is the performance that is typical for laboratory measurements using this technology.
The most important challenge for using our gamma-ray sensor in space is that high-speed protons (or cosmic rays, https://www.swpc.noaa.gov/phenomena/galactic-cosmic-rays), which travel throughout the solar system, slam into the sensor (a crystal lattice of Ge atoms) and damage its lattice by dislodging the Ge atoms. The result of this damage is that the energy resolution of the sensor can be significantly degraded (Fig. 2). Fortunately, this damage inflicted by the cosmic rays can be reversed by heating the crystal to a high temperature in a process known as annealing. Essentially, this annealing process uses energy from the high temperature to “jiggle” the crystal lattice so that the dislodged Ge atoms fall back into their proper locations.
While annealing restores performance, at the time of MESSENGER the details of this process were uncomfortably ill-defined. We didn’t know basic things like: “How hot can we safely heat the Ge crystal?” and “How long does it need to be heated?” To answer these questions, GRNS team members (Morgan, John, Patrick, myself, and Zachary Yokley, who joined APL after the completion of MESSENGER) took a Ge sensor to Long Island, NY (Brookhaven National Lab) to carry out an experiment at a proton accelerator. The goal of this experiment was to intentionally damage Ge sensors with the high-velocity protons, and then answer the questions “How hot?” and “How long?” This experiment was fully successful, as we developed an annealing procedure that we will use on the Psyche mission (Fig. 2). And the answers to our questions? 105 degrees Celsius (How hot?). Up to 10 days (How long?).
With this information in hand, the full GRNS team started to build the flight model hardware for Psyche. The process to design, build, and test the GRNS was challenging, and made particularly so since most of the flight-model work was done under restrictions put into place to mitigate the transmission of the COVID-19 virus. The final GRS was completed and tested this summer, and not only performed with excellence, but is beautiful to look at (Fig. 3).
As the next step on its path to Psyche, the GRNS instruments arrived at JPL on August 2, 2021, and over the next month and a half were installed on the Psyche spacecraft. Its first big performance test started the week of September 20, 2021. When we got the first results on the Thursday of that week, we were astounded to find out that its performance was better than we could have hoped for — an energy resolution of 1.92 keV! The four of us who started this journey at Mercury, and traveled through Long Island, finally reached Pasadena in JPL’s High Bay 2 with big smiles on our faces (though you can’t see them under the masks, Fig. 4). The journey for the GRS, and its Ge crystal, is of course, not yet over. But it has a great start, and we all look forward to finding out what it will tell us about the enigmatic asteroid Psyche.