Space sample retrieval is having a moment. On 24 September 2023, in a triumph of complex engineering, a US National Aeronautics and Space Administration (NASA) mission, OSIRUS-REx (short for origins, spectral interpretation, resource identification, and security-regolith explorer; regolith is the surface layer of unconsolidated rocks and dust/soil covering bedrock), successfully delivered to Earth a capsule containing the largest sample of asteroid material ever collected, captured from Bennu, a 500-meter-diameter asteroid (
Fig. 1) [
1⇓-
3]. Several months earlier, the China National Space Administration (CNSA) for the first time extended a broad invitation to international researchers to apply for access to the regolith sample returned from the Moon in 2020 by its Chang’e-5 mission [
4,
5]. And in a development affecting its flagship Mars Sample Return (MSR) mission, major criticism and funding issues have NASA scrambling to reformulate its approach to keep the mission alive [
6].
Seven years after launch, and two and a half years after leaving Bennu, the OSIRIS-REx mission’s sample capsule touched down in the desert landscape of Utah carrying
of the asteroid’s regolith, more than twice its
target (
Figs. 2 and
3) [
7]. That it arrived at all was fortunate. The collection procedure at Bennu, involving a container on the end of a robotic arm, was fraught, said Sara Russell, an OSIRIS-REx team member and professor of planetary sciences and leader of the planetary materials group at the Natural History Museum in London, UK. "The arm touched the surface, but then just kept sinking and sinking into Bennu. We were lucky the craft’s thrusters allowed it to escape."
The pristine Bennu regolith is thought to represent the oldest building blocks of the solar system, with its analysis expected to provide critical insight into Earth’s early history. More than 4.5 billion years ago, the newly formed Earth was hot and dry [
8]. But about 4 billion years ago, during a period known as the Late Heavy Bombardment, scientists believe showers of asteroids and comets delivered large amounts of water and other materials to Earth and the other inner planets of the Solar System. The initial results of the Bennu sample analysis appear to support this hypothesis, that objects like Bennu’s parent body could have-in the extremely distant past-helped set the stage for life on Earth [
9].
In a recent interview [
10], Dante Lauretta, principal investigator for OSIRIS-REx and professor of planetary science at the University of Arizona (Tucson, AZ, USA), said that early examinations of the Bennu sample show it consists primarily of water-bearing clay-more specifically, minerals called serpentines. "Serpentines, at least on Earth, form when rocks from the mantle are forced upwards into the seabed and react with water, in a reaction that releases heat; it was probably a similar process on Bennu’s parent asteroid," Lauretta said. "We clearly have a set of rocks that were interacting with a carbonated fluid-that is a huge result." Lauretta’s working hypothesis is that Bennu’s rocks originated from an ancient ocean world that subsequently broke apart [
10].
Yet there is also evidence that Bennu consists of materials from at least two sources, because some minerals in the sample originated without the involvement of water. "The sample also contains non-hydrous silicates, such as olivine, which come from another source," said Guy Libourel, professor of cosmochemistry at the Université Côte d’Azur in Nice, France, whose lab received 100 of the Bennu sample. "Clearly, Bennu is a mixture. One question is, where and how did this object initially form?"
A more immediate concern is cataloguing the entirety of Bennu’s constituent parts. Scientists already knew from close-range observations by the OSIRIS-REx spacecraft’s instruments that Bennu is a carbon-rich body [
11]. Early analyses of the sample by multiple teams of scientists have revealed a great deal more. "Bennu has an immense chemo-diversity, including many of the building blocks of life," said Philippe Schmitt-Kopplin, professor of analytical chemistry and director of the Research Unit Analytical BioGeoChemistry at the Helmholtz Center in Munich, Germany. His lab received
of the Bennu sample. "We found amino acids, fatty acids, and other molecules that are critical to life," he said. "These molecules are very easily generated by abiotic chemical reactions."
Schmitt-Kopplin said the wide range of molecules found in the Bennu sample indicate an origination process involving a lot of water, and hydrothermalism at a relatively low temperature. The analysis techniques his team uses include ultrahigh resolution mass spectrometry, which can differentiate two molecules with the mass difference of an electron. "It is amazing to have access to such a time capsule," said Schmitt-Kopplin. "But our analysis techniques are destructive-it breaks my heart to crush the sample!”
A big surprise for everyone, according to Russell, was the amount of magnesium phosphate in the sample, which appears as a white component of Bennu’s black rocks [
12]. "At first, we thought there was some sort of contaminant, but then we realised the phosphate was actually in the rocks," she said. "It is incredibly rare as a geological mineral on Earth."
Lauretta, Libourel, Russell, and Schmitt-Kopplin are part of an international consortium of hundreds of scientists already working together to analyse the Bennu sample. In contrast, access to the Moon regolith retrieved in 2020 by the Chinese Chang’e-5 mission has been limited. However, on 2 August 2023 the CNSA announced an application process through which it would provide aliquots of its Moon sample to a broad array of international researchers [
4].
That sample was collected from the Moon’s Oceanus Procellarum by the Chang’e-5 lunar sample-return mission, which launched on 23 November 2020 and returned the following month with a cargo of
of regolith [
5]. It was the first lunar sample return since the Soviet Union’s Luna 24 mission, which collected
of regolith in 1976 [
13].
The first window for applications from international researchers opened from 6 November to 22 December 2023, but the timing was even tighter for interested NASA-funded scientists. Since 2011, US legislation known as the Wolf Amendment has effectively banned NASA from bilateral cooperation with Chinese organizations [
14]. However, in a rare exception announced in an internal e-mail sent to researchers on 29 November 2023, NASA said it had "certified its intent to Congress to allow NASA-funded researchers to apply to the China National Space Administration for access to lunar samples returned to Earth on the Chang’e-5 mission" [
15].
Clive Neal, professor of planetary geology at the University of Notre Dame (Notre Dame, IN, USA), is excited about the prospects for the broader research the Chinese offer could foster. As a member of an international collaboration not involving any NASA funding, Neal participated in early research on lunar basalts returned by Chang’e 5. "These samples, taken from a different area than the Apollo-era samples, have revolutionized the way we interpret the age of planetary surfaces and quantitative textural analysis-in many of the samples we have not seen anything like it before," Neal said. "Chang’e 5 was a remarkable achievement and those samples, with the CNSA’s curation, will be a gift that keeps on giving, with discoveries made with them for decades."
In an interview published 22 December 2023 on the China Daily website, a CNSA spokesperson, Hongliang Xu, welcomed researchers, including US scientists, who had applied and were intending to apply for access to the Chang’e-5 sample [
16]. He also urged "farsighted people in the US" to consider abolishing the Wolf Amendment, with a view to the United States resuming international cooperation with China in space science and exploration.
Certainly, the Wolf Amendment has cast a long shadow. According to Neal, many NASA scientists are wary to accept China’s offer and apply for sample access, despite the exception NASA has made. "In the past, I have known scientists to have funding frozen just for the giving the perception of bilateral collaboration," he said, adding that the US political landscape could quickly change in this presidential election year.
The political landscape has also been unsettled around one of NASA’s flagship projects, the Mars Sample Return (MSR) mission [
17]. In 2023, the Planetary Science Decadal Survey, put together by the US National Academies of Sciences, Engineering, and Medicine identified completion of the MSR mission as NASA’s "highest scientific priority" for the following decade [
18]. But the mission is now under immense financial pressure.
In September 2023, an independent review board (IRB) judged that MSR had "unrealistic budget and schedule expectations from the beginning" [
19]. Initially estimated to cost NASA 5.3 billion USD, MSR was targeted for launch readiness by 2028. However, the IRB found that the budget required would likely rise to between 8 billion and 9.6 billion USD, with launch readiness pushed back to 2030 at the very earliest [
19].
The revised costs are in part a result of the sheer magnitude and complexity of a collaborative mission in which many components must work with miniscule margins for error (
Fig. 4) [
20]. In an interview with The Planetary Society’s podcast, Planetary Radio [
21], Orlando Figueroa, former NASA director for Mars exploration and the chairperson of the IRB that evaluated the MSR mission, likened the situation to NASA’s development of the James Webb Space Telescope [
22]. "When you look at James Webb, you know that the community at large was alarmed by the cost growth, but the astrophysics community never doubted the impact of such a machine," Figueroa said. "Mars sample-return is of that ilk. The Mars community as a whole must remain united behind the Mars Exploration Program and Mars sample-return within it."
In US President Biden’s annual budget request for 2024, the administration requested 949 million USD for the MSR mission [
23]. However, in response, the 2024 Senate appropriations bill stated: "If NASA is unable to provide the Committee with a MSR life-cycle cost profile within the
$5,300,000,000 budget profile, NASA is directed to either provide options to de-scope or rework MSR or face mission cancellation" [24]. The bill proposed funding for MSR of "not less than $300,000,000." On 3 March 2024, however, the US House and Senate Appropriations Committees took this ultimatum off the table in the final Fiscal Year 2024 appropriations bills [
25], which instead directed NASA to report on a recommended path forward for MSR, within a balanced science portfolio[
26].
NASA released its response to the IRB report on 15 April 2024. Its proposals included "a revised MSR mission design with improved mission resiliency and risk posture, reduced overall complexity... descoped content..." [
27]. In an accompanying statement, NASA Administrator Bill Nelson said, "We need to look outside the box to find a way ahead that is both affordable and returns samples in a reasonable timeframe" [
28].
While the future of MSR remains unclear at present, there is more space sample retrieval on the near horizon, including from Mars. CNSA’s Chang’e-6 mission, which blasted off in May 2024, aims to be the first mission to collect and return regolith from the dark side of the Moon [
29], and the Chang’e-6 mission successfully returned its sample to Earth in June 2024. Also in 2025, the CNSA is expected to launch Tianwen-2, a sample-return mission designed to collect
of regolith from a comet known as Kamo’oalewa [
30]. In 2026, the Japan Aerospace Exploration Agency will launch its Martian Moons Exploration mission, to travel to Mars and survey its two moons, Phobos and Deimos, before collecting a sample from Phobos and bringing it back to Earth [
31]. Finally, considered less complex than MSR, Tianwen-3, CNSA’s Mars sample-return mission is currently expected to launch in
[
32].
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