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By NASA
6 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
This artist’s concept shows the Moon’s hot interior and volcanism about 2 to 3 billion years ago. It is thought that volcanic activity on the lunar near side (the side facing Earth) helped create a landscape dominated by vast plains called mare, which are formed by molten rock that cooled and solidified. NASA/JPL-Caltech Analyzing gravity data collected by spacecraft orbiting other worlds reveals groundbreaking insights about planetary structures without having to land on the surface.
Although the Moon and the asteroid Vesta are very different, two NASA studies use the same technique to reveal new details about the interiors of both.
In the lunar study, published May 14 in the journal Nature, researchers developed a new gravity model of the Moon that includes tiny variations in the celestial body’s gravity during its elliptical orbit around Earth. These fluctuations cause the Moon to flex slightly due to Earth’s tidal force — a process called tidal deformation — which provides critical insights into the Moon’s deep internal structure.
Using their model, the researchers produced the most detailed lunar gravitational map yet, providing future missions an improved way to calculate location and time on the Moon. They accomplished this by analyzing data on the motion of NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission, whose spacecraft, Ebb and Flow, orbited the Moon from Dec. 31, 2011, to Dec. 17, 2012.
These views of the Moon’s near side, left, and far side were put together from observations made by NASA’s Lunar Reconnaissance Orbiter. NASA/JPL-Caltech In a second study, published in the journal Nature Astronomy on April 23, the researchers focused on Vesta, an object in the main asteroid belt between Mars and Jupiter. Using NASA’s Deep Space Network radiometric data and imaging data from the agency’s Dawn spacecraft, which orbited the asteroid from July 16, 2011, to Sept. 5, 2012, they found that instead of having distinct layers as expected, Vesta’s internal structure may be mostly uniform, with a very small iron core or no core at all.
“Gravity is a unique and fundamental property of a planetary body that can be used to explore its deep interior,” said Park. “Our technique doesn’t need data from the surface; we just need to track the motion of the spacecraft very precisely to get a global view of what’s inside.”
Lunar Asymmetry
The lunar study looked at gravitational changes to the Moon’s near and far sides. While the near side is dominated by vast plains — known as mare — formed by molten rock that cooled and solidified billions of years ago, the far side is more rugged, with few plains.
NASA’s Dawn mission obtained this image of the giant asteroid Vesta on July 24, 2011. The spacecraft spent 14 months orbiting the asteroid, capturing more than 30,000 images and fully mapping its surface. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA Both studies were led by Ryan Park, supervisor of the Solar System Dynamics Group at NASA’s Jet Propulsion Laboratory in Southern California, and were years in the making due to their complexity. The team used NASA supercomputers to build a detailed map of how gravity varies across each body. From that, they could better understand what the Moon and Vesta are made of and how planetary bodies across the solar system formed.
Some theories suggest intense volcanism on the near side likely caused these differences. That process would have caused radioactive, heat-generating elements to accumulate deep inside the near side’s mantle, and the new study offers the strongest evidence yet that this is likely the case.
“We found that the Moon’s near side is flexing more than the far side, meaning there’s something fundamentally different about the internal structure of the Moon’s near side compared to its far side,” said Park. “When we first analyzed the data, we were so surprised by the result we didn’t believe it. So we ran the calculations many times to verify the findings. In all, this is a decade of work.”
When comparing their results with other models, Park’s team found a small but greater-than-expected difference in how much the two hemispheres deform. The most likely explanation is that the near side has a warm mantle region, indicating the presence of heat-generating radioactive elements, which is evidence for volcanic activity that shaped the Moon’s near side 2 billion to 3 billion years ago.
Vesta’s Evolution
Park’s team applied a similar approach for their study that focused on Vesta’s rotational properties to learn more about its interior.
“Our technique is sensitive to any changes in the gravitational field of a body in space, whether that gravitational field changes over time, like the tidal flexing of the Moon, or through space, like a wobbling asteroid,” said Park. “Vesta wobbles as it spins, so we could measure its moment of inertia, a characteristic that is highly sensitive to the internal structure of the asteroid.”
Changes in inertia can be seen when an ice skater spins with their arms held outward. As they pull their arms in, bringing more mass toward their center of gravity, their inertia decreases and their spin speeds up. By measuring Vesta’s inertia, scientists can gain a detailed understanding of the distribution of mass inside the asteroid: If its inertia is low, there would be a concentration of mass toward its center; if it’s high, the mass would be more evenly distributed.
Some theories suggest that over a long period, Vesta gradually formed onion-like layers and a dense core. But the new inertia measurement from Park’s team suggests instead that Vesta is far more homogeneous, with its mass distributed evenly throughout and only a small core of dense material, or no core.
Gravity slowly pulls the heaviest elements to a planet’s center over time, which is how Earth ended up with a dense core of liquid iron. While Vesta has long been considered a differentiated asteroid, a more homogenous structure would suggest that it may not have fully formed layers or may have formed from the debris of another planetary body after a massive impact.
In 2016, Park used the same data types as the Vesta study to focus on Dawn’s second target, the dwarf planet Ceres, and results suggested a partially differentiated interior.
Park and his team recently applied a similar technique to Jupiter’s volcanic moon Io, using data acquired by NASA’s Juno and Galileo spacecraft during their flybys of the Jovian satellite as well as from ground-based observations. By measuring how Io’s gravity changes as it orbits Jupiter, which exerts a powerful tidal force, they revealed that the fiery moon is unlikely to possess a global magma ocean.
“Our technique isn’t restricted just to Io, Ceres, Vesta, or the Moon,” said Park. “There are many opportunities in the future to apply our technique for studying the interiors of intriguing planetary bodies throughout the solar system.”
News Media Contacts
Ian J. O’Neill
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-2649
ian.j.oneill@jpl.nasa.gov
Karen Fox / Molly Wasser
NASA Headquarters, Washington
202-358-1600
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov
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Last Updated May 14, 2025 Related Terms
Vesta Dawn Earth's Moon GRAIL (Gravity Recovery And Interior Laboratory) Jet Propulsion Laboratory Planetary Science Small Bodies of the Solar System The Solar System Explore More
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By NASA
In about 5 billion years, our Sun will run out of fuel and expand, possibly engulfing Earth. These end stages of a star’s life can be utterly beautiful – as is the case with this planetary nebula called the Helix Nebula. Astronomers study these objects by looking at all kinds of light.X-ray: NASA/CXC/SAO/Univ Mexico/S. Estrada-Dorado et al.; Ultraviolet: NASA/JPL; Optical: NASA/ESA/STScI (M. Meixner)/NRAO (T.A. Rector); Infrared: ESO/VISTA/J. Emerson; Image Processing: NASA/CXC/SAO/K. Arcand This image of the Helix Nebula, released on March 4, 2025, shows a potentially destructive white dwarf at the nebula’s center: this star may have destroyed a planet. This has never been seen before – and could explain a mysterious X-ray signal that astronomers have detected from the nebula for over 40 years.
This view combines X-rays from NASA’s Chandra X-ray Observatory (magenta), optical light data from NASA’s Hubble Space Telescope (orange, light blue), infrared data from the European Southern Observatory VISTA telescope (gold, dark blue), and ultraviolet data from GALEX (purple) of the Helix Nebula. Data from Chandra indicates that this white dwarf has destroyed a very closely orbiting planet.
Image credit: X-ray: NASA/CXC/SAO/Univ Mexico/S. Estrada-Dorado et al.; Ultraviolet: NASA/JPL; Optical: NASA/ESA/STScI (M. Meixner)/NRAO (T.A. Rector); Infrared: ESO/VISTA/J. Emerson; Image Processing: NASA/CXC/SAO/K. Arcand
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By European Space Agency
The European Space Agency is releasing the first catalogue of astronomical data from the Euclid space telescope, including three new enormous image mosaics with zoom-ins. Follow the reveal live on Wednesday 19 March at 11:00 BST / 12:00 CET.
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By NASA
X-rays are radiated by matter hotter than one million Kelvin, and high-resolution X-ray spectroscopy can tell us about the composition of the matter and how fast and in what direction it is moving. Quantum calorimeters are opening this new window on the Universe. First promised four decades ago, the quantum-calorimeter era of X-ray astronomy has finally dawned.
Photo of the XRISM/Resolve quantum-calorimeter array in its storage container prior to integration into the instrument. The 6×6 array, 5 mm on a side, consists of independent detectors – each one a thermally isolated silicon thermistor with a HgTe absorber. The spectrometer consisting of this detector and other essential technologies separates astrophysical X-ray spectra into about 2400 resolution elements, which can be thought of as X-ray colors.NASA GSFC A quantum calorimeter is a device that makes precise measurements of energy quanta by measuring the temperature change that occurs when a quantum of energy is deposited in an absorber with low heat capacity. The absorber is attached to a thermometer that is somewhat decoupled from a heat sink so that the sensor can heat up and then cool back down again. To reduce thermodynamic noise and the heat capacity of the sensor, operation at temperatures less than 0.1 K is required.
The idea for thermal measurement of small amounts of energy occurred in several places in the world independently when scientists observed pulses in the readout of low-temperature thermometers and infrared detectors. They attributed these spurious signals to passing cosmic-ray particles, and considered optimizing detectors for sensitive measurement of the energy of particles and photons.
The idea to develop such sensors for X-ray astronomy was conceived at Goddard Space Flight Center in 1982 when X-ray astronomers were considering instruments to propose for NASA’s planned Advanced X-ray Astrophysics Facility (AXAF). In a fateful conversation, infrared astronomer Harvey Moseley suggested thermal detection could offer substantial improvement over existing solid-state detectors. Using Goddard internal research and development funding, development advanced sufficiently to justify, just two years later, proposing a quantum-calorimeter X-ray Spectrometer (XRS) for inclusion on AXAF. Despite its technical immaturity at the time, the revolutionary potential of the XRS was acknowledged, and the proposal was accepted.
The AXAF design evolved over the subsequent years, however, and the XRS was eliminated from its complement of instruments. After discussions between NASA and the Japanese Institute of Space and Astronautical Science (ISAS), a new XRS was included in the instrument suite of the Japanese Astro-E X-ray observatory. Astro-E launched in 2000 but did not reach orbit due to an anomaly in the first stage of the rocket. Astro-E2, a rebuild of Astro-E, was successfully placed in orbit in 2005 and renamed Suzaku, but the XRS instrument ceased operation before observations started due to loss of the liquid helium, an essential part of the detector cooling system, caused by a faulty storage system.
A redesigned mission, Astro-H, that included a quantum-calorimeter instrument with a redundant cooling system was successfully launched in 2016 and renamed Hitomi. Hitomi’s Soft X-ray Spectrometer (SXS) obtained high resolution spectra of the Perseus cluster of galaxies and a few other sources before a problem with the attitude control system caused the mission to be lost roughly one month after launch. Even so, Hitomi was the first orbiting observatory to obtain a scientific result using X-ray quantum calorimeters. The spectacular Perseus spectrum generated by the SXS motivated yet another attempt to implement a spaceborne quantum-calorimeter spectrometer.
The X-ray Imaging and Spectroscopy Mission (XRISM) was launched in September 2023, with the spectrometer aboard renamed Resolve to represent not only its function but also the resolve of the U.S./Japan collaboration to study the Universe through the window of this new capability. XRISM has been operating well in orbit for over a year.
Development of the Sensor Technology
Development of the sensor technology employed in Resolve began four decades ago. Note that an X-ray quantum-calorimeter spectrometer requires more than the sensor technology. Other technologies, such as the coolers that provide a
The sensors used from XRS through Resolve were all based on silicon-thermistor thermometers and mercury telluride (HgTe) X-ray absorbers. They used arrays consisting of 32 to 36 pixels, each of which was an independent quantum calorimeter. Between Astro-E and Astro-E2, a new method of making the thermistor was developed that significantly reduced its low-frequency noise. Other fabrication advances made it possible to make reproducible connections between absorbers and thermistors and to fit each thermistor and its thermal isolation under its X-ray absorber, making square arrays feasible.
Through a Small Business Innovation Research (SBIR) contract executed after the Astro-E2 mission, EPIR Technologies Inc. reduced the specific heat of the HgTe absorbers. Additional improvements made to the cooler of the detector heat sink allowed operation at a lower temperature, which further reduced the specific heat. Together, these changes enabled the pixel width to be increased from 0.64 mm to 0.83 mm while still achieving a lower heat capacity, and thus improving the energy resolution. From Astro-E through Astro-H, the energy resolution for X-rays of energy around 6000 eV improved from 11 eV, to 5.5 eV, to 4 eV. No changes to the array design were made between Astro-H and XRISM.
Resolve detector scientist Caroline Kilbourne installing the flight Resolve quantum-calorimeter array into the assembly that provides its electrical, thermal, and mechanical interfaces.NASA GSFC Over the same period, other approaches to quantum-calorimeter arrays optimized for the needs of future missions were developed. The use of superconducting transition-edge sensors (TES) instead of silicon (Si) thermistors led to improved energy resolution, more pixels per array, and multiplexing (a technique that allows multiple signals to be carried on a single wire). Quantum-calorimeter arrays with thousands of pixels are now standard, such as in the NASA contribution to the future European New Advanced Telescope for High-ENergy Astrophysics (newAthena) mission. And quantum calorimeters using paramagnetic thermometers — which unlike TES and Si thermistors require no dissipation of heat in the thermometer for it to be read out — combined with high-density wiring are a promising route for realizing even larger arrays. (See Astrophysics Technology Highlight on these latest developments.)
The Resolve instrument aboard XRISM (X-ray Imaging and Spectroscopy Mission) captured data from the center of galaxy NGC 4151, where a supermassive black hole is slowly consuming material from the surrounding accretion disk. The resulting spectrum reveals the presence of iron in the peak around 6.5 keV and the dips around 7 keV, light thousands of times more energetic that what our eyes can see. Background: An image of NGC 4151 constructed from a combination of X-ray, optical, and radio light.Spectrum: JAXA/NASA/XRISM Resolve. Background: X-rays, NASA/CXC/CfA/J.Wang et al.; optical, Isaac Newton Group of Telescopes, La Palma/Jacobus Kapteyn Telescope; radio, NSF/NRAO/VLA Results from Resolve
So, what is Resolve revealing about the Universe? Through spectroscopy alone, Resolve allows us to construct images of complex environments where collections of gas and dust with various attributes exist, emitting and absorbing X-rays at energies characteristic of their various compositions, velocities, and temperatures. For example, in the middle of the galaxy known as NCG 4151 (see figure above), matter spiraling into the central massive black hole forms a circular structure that is flat near the black hole, more donut-shaped further out, and, according to the Resolve data, a bit lumpy. Matter near the black hole is heated up to X-ray-emitting temperatures and irradiates the matter in the circular structure. The Resolve spectrum has a bright narrow emission line (peak) from neutral iron atoms that must be coming from colder matter in the circular structure, because hotter material would be ionized, and would have a different emission signature. Nonetheless, the shape of the iron line needs three components to describe it, each coming from a different lump in the circular structure. The presence of absorption lines (dips) in the spectrum provides further detail about the structure of the infalling matter.
A second example is the detection of X-ray emission by Resolve from the debris of stars that have exploded, such as N132D (see figure below), that will improve our understanding of the explosion mechanism and how the elements produced in stars get distributed, and allow us to infer the type of star each was before ending in a supernova. Elements are identified by their characteristic emission lines, and shifts of those lines via the Doppler effect tell us how fast the material is moving.
XRISM’s Resolve instrument captured data from supernova remnant N132D in the Large Magellanic Cloud to create the most detailed X-ray spectrum of the object ever made. The spectrum reveals peaks associated with silicon, sulfur, argon, calcium, and iron. Inset at right is an image of N132D captured by XRISM’s Xtend instrument.JAXA/NASA/XRISM Resolve and Xtend These results are just the beginning. The rich Resolve data sets are identifying complex velocity structures, rare elements, and multiple temperature components in a diverse ensemble of cosmic objects. Welcome to the quantum calorimeter era! Stay tuned for more revelations!
Project Leads: Dr. Caroline Kilbourne, NASA Goddard Space Flight Center (GSFC), for silicon-thermistor quantum calorimeter development from Astro-E2 through XRISM and early TES development. Foundational and other essential leadership provided by Dr. Harvey Moseley, Dr. John Mather, Dr. Richard Kelley, Dr. Andrew Szymkowiak, Mr. Brent Mott, Dr. F. Scott Porter, Ms. Christine Jhabvala, Dr. James Chervenak (GSFC at the time of the work) and Dr. Dan McCammon (U. Wisconsin).
Sponsoring Organizations and Programs: The NASA Headquarters Astrophysics Division sponsored the projects, missions, and other efforts that culminated in the development of the Resolve instrument.
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