Psyched for launch

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At Nasa’s Jet Propulsion Laboratory, engineers integrate the gamma ray and neutron spectrometer into the Psyche spacecraft. (Image: NASA/JPL-Caltech)

A gamma ray and neutron spectrometer was recently installed on the ‘Psyche’ spacecraft, which will launch next year to probe a distant asteroid and uncover clues about the origins of our solar system

In August 2022, Nasa’s Jet Propulsion Laboratory, in collaboration with partners including SpaceX, Arizona State University, MIT, Yale University, the Johns Hopkins Applied Physics Laboratory (APL), and the Lawrence Livermore National Laboratory, will launch the Psyche mission. 

Psyche will investigate a metal-rich asteroid of the same name, which lies in the main asteroid belt between Mars and Jupiter. The partners believe the asteroid could be part or all of the iron-rich interior of an early planetary building block that was stripped of its outer rocky shell as it repeatedly collided with other large bodies during the early formation of the solar system. 

Alternatively, the asteroid, which measures around 173 miles (280km) at its widest point, could be the leftover piece of a completely different kind of iron-rich body that formed from metal-rich material somewhere in the solar system. 

Previous ground-based observations led scientists to believe that the asteroid was as much as 90 per cent metal. However, recent research1 using updated density measurements estimated the asteroid is more likely between 30 and 60 per cent metal. 

‘There are a lot of basic questions about Psyche that are unanswered,’ said the mission’s principal investigator Lindy Elkins-Tanton, of Arizona State University. ‘And with every detail that gets added from data we can collect from Earth, it just becomes harder to make a sensible story. We really don’t know what we’re going to see until we visit, and we’re going to be surprised.’ 

Scientists are also puzzled as to why Psyche appears to be low in iron oxides, which are present on Mars, Mercury, Venus and Earth. ‘So, if we’re correct that Psyche is a mixture of metal and rock, and the rock has very little iron oxide, then there’s got to be a strange story about how it was created – because it doesn’t fit the standard stories of planetary creations,’ Elkins-Tanton added. 

The Psyche spacecraft will therefore orbit the asteroid for two years to find answers, taking pictures, mapping the surface, and looking for evidence of an ancient magnetic field. 

Cosmic collisions

To help determine the asteroid’s origins, scientists will rely on remote instrumentation to conduct analysis without having to touch or dig into its surface: a magnetometer, a multispectral imager, and a gamma-ray and neutron spectrometer. 

Scientists know that the asteroid doesn’t generate a magnetic field in the same way as Earth, but if Psyche had a magnetic field in the past, it could still be recorded in the asteroid’s material today. The magnetometer will therefore determine whether Psyche is still magnetised. If so, that would confirm the asteroid is part of the core of an early planetesimal – the building block of an early planet. 

Meanwhile, the multispectral imager, consisting of a pair of colour cameras using filters in the ultraviolet and near-infrared wavelengths, will help determine the mineralogy of any rocky material that may exist on Psyche’s surface. 

The third key instrument for this mission will be the gamma-ray and neutron spectrometer (GRNS), which was integrated into the Psyche spacecraft in August. It will detect, measure and globally map the elemental composition of the asteroid. The instrument, made up of two separate pieces of sensor hardware, is mounted on a 2m boom to distance its sensors from background radiation created by energetic particles interacting with the spacecraft and to provide an unobstructed field of view. 

The Psyche spacecraft will use the GRNS to study neutrons and gamma rays coming from the asteroid, generated by cosmic rays and high-energy particles impacting its surface.

Cosmic rays impacting the surface of the Psyche will release gamma rays and neutrons with varying energies, revealing details of the asteroid’s elemental composition. (Image: John Hopkins APL)

‘Galactic cosmic rays (very fast protons) collide with a planetary surface and break apart the atoms in that surface,’ said David Lawrence, Psyche GRNS investigation lead at John Hopkins University’s Applied Physics Laboratory. ‘This causes the neutrons of those atoms to “rattle around”, which emits both high-energy (fast) neutrons and gamma rays. These gamma rays have different energies that act as a fingerprint and can be analysed to indicate whether, for example, iron, nickel or silicon are present. The quantity of  gamma rays then indicates how much of that element is there.

'As has been demonstrated on many prior planetary missions, combined gamma-ray and neutron measurements provide comprehensive elemental composition information for airless, or nearly airless, planetary bodies.’ 

A combined approach 

The GRNS consists of a high-purity germanium (HPGe) gamma-ray spectrometer (GRS), surrounded by a borated plastic anticoincidence shield, and a helium-3 gas-proportional-sensor-based neutron spectrometer2

The exceptional gamma-ray energy resolution and sensitivity provided by the HPGe sensor enables the GRS to measure all the required gamma-ray lines. The borated plastic scintillator and helium-3 sensors provide robust measurements for neutrons ranging in energies from <0.1eV up through to a few MeV. 

The GRS can be seen in figure 1. The instrument is based on the GRS of a previous mission, ‘MESSENGER’, which between 2011 and 2015 orbited Mercury and studied its chemical composition, geology and magnetic field. Modifications to the GRS design have since been made, however, derived from updated developments and lessons learned from the MESSENGER mission. 

Figure 1: The Psyche gamma-ray spectrometer uses a high-purity germanium crystal to capture gamma rays emitted by asteroid Psyche. These gamma rays can reveal what elements the surface is made of. (Image: Johns Hopkins APL/Ed Whitman)

The gamma-ray sensor is a 5cm diameter by 5cm long cylindrical germanium crystal. It is housed within a standalone cryostat that thermally isolates the cryogenically cooled germanium crystal from its surrounding environment. The cryostat is surrounded by the anticoincidence shield that both rejects charged particle background in the germanium sensor and measures fast neutrons. The germanium sensor is cooled to cryogenic  temperatures of around 90K using a long-life pulsed tube cryocooler. 

The orientation of the GRS is arranged such that the asteroid will be viewed through the anticoincidence shield instead of through the top of the germanium crystal, as was done on MESSENGER. This offers multiple benefits, including a simplified arrangement of connector ports and a more uniform angular response for rotations around the spacecraft’s z-axis. The latter reduces the magnitude of corrections for measurement geometry and attenuation, which ultimately improves systematic uncertainties. 

The neutron spectrometer can be seen in figure 2. The design features three helium-3 neutron sensors placed on a flat plate, which compared to a previous arrangement used on the Lunar Prospector mission – which comprised two cantilevered neutron sensors – offers both mass savings and a simplified mechanical design. 

Figure 2: The Psyche neutron spectrometer uses three cylinders filled with a gas of helium-3, an isotope of helium that will rapidly grab free neutrons emitted by asteroid Psyche. (Image: Johns Hopkins APL/Ed Whitman)

The addition of the third sensor also benefits the anticipated neutron measurements of the Psyche mission.

‘The expected metal-rich nature of Psyche’s composition will significantly modify its equilibrium neutron flux compared to rocky silicate-rich bodies, such as the moon,’ explained Lawrence. ‘Specifically, the thermal neutron flux will be greatly suppressed if high concentrations of iron and nickel are present, which are both neutron absorbers. In addition, the flux of high-energy epithermal neutrons (>100keV) will be enhanced in comparison to silicate bodies.’ 

A clean measurement of epithermal neutrons is also important for quantifying hydrogen abundances on the Psyche asteroid, which may be present with concentrations up to hundreds of ppm, Lawrence added. ‘While such measurements are not required to meet specific mission goals, if hydrogen is present and spatially variable on the asteroid, mapping it will reduce the systematic uncertainties of other gamma-ray and neutron measurements,’ he said. 

Two of the neutron sensors will measure thermal and low-energy epithermal neutrons using bare and cadmium-covered sensors respectively, while the third sensor, covered in 1cm-thick polyethylene, will provide a clean and independent measurement of high-energy epithermal neutrons. 

‘The four neutron-energy bands of the GRNS (three helium-3 sensor energy bands and one AC shield energy band) thus provide an optimal set of measurements for characterising the neutron environment around a metal-rich body like Psyche,’ concluded Lawrence.

Secrets of the solar system 

With the aid of the magnetometer and multispectral camera, the GRNS will enable the Psyche mission to characterise Psyche’s topography and determine: whether the asteroid is a planetesimal core, or if it is unmelted material; the relative ages of regions of Psyche’s surface; whether small metal bodies incorporate the same light elements as are expected in the Earth’s high-pressure core; and whether Psyche was formed under conditions more oxidising or more reducing than Earth’s core.

Despite launch being planned next year, the Psyche spacecraft won’t start orbiting the asteroid until 2026-2027. It must first undertake an approximately four-year journey, during which it will have to fly past mars and perform a gravity-assist manoeuvre.

References

[1] L. T. Elkins-Tanton et al. ‘Observations, Meteorites, and Models: A Preflight Assessment of the Composition and Formation of (16) Psyche’: JGR Planets, Volume 125, Issue 3: https://doi.org/10.1029/2019JE006296

[2] D. Lawrence et al. ‘The Psyche Gamma-Ray and Neutron Spectrometer: Update on instrument design and measurement capabilities.’ 50th Lunar and Planetary Science Conference (2019): https://www.hou.usra.edu/meetings/lpsc2019/pdf/1554.pdf

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