At 8.55 a.m. local time on 24 September, a small and precious cargo is due to touch down in Utah’s West Desert, ending a journey of more than two years and two billion kilometres. Released 100,000 kilometres from Earth by NASA’s OSIRIS-REx spacecraft, the sample capsule contains roughly 250 grams of material transported from the near-Earth asteroid 101955 Bennu — the largest ever asteroid sample to be brought back to Earth.

As principal US scientist for the Mars Sample Return (MSR) programme, a collaboration between NASA and the European Space Agency aiming to retrieve samples from Mars in the 2030s, I know how technologically difficult this is. I am also conscious that sample-return missions have too few cheerleaders, even among planetary scientists. As the complexity of MSR has become clearer, funding has come under threat. In July, the US Senate proposed to slash 2024 funding to NASA for MSR, and to cancel it altogether if the project cannot stay within budget.

As we strive to simplify MSR, we should also build awareness of what these missions can do. Extraterrestrial samples have taught us incredible, fundamental things about ourselves, Earth and the Solar System. It’s because of the lunar samples, collected during NASA’s Apollo missions between 1969 and 1972, that we know that the Moon was probably formed when a Mars-sized planet smashed into Earth 4.5 billion years ago.

Over the past two decades, our understanding of how the Solar System and planets formed has also been improved by such efforts. First, by NASA’s Genesis mission, which spent two years collecting samples of the solar wind, then delivered them to Earth in 2004; and then through tiny pieces of the asteroids 25143 Itokawa (about one milligram) and 162173 Ryugu (around 5 grams) retrieved by the Japan Aerospace Exploration Agency’s Hayabusa and Hayabusa2 missions in 2010 and 2020. The NASA Stardust mission collected about 1 mg of particles from the tail of comet Wild-2 in 2006, which have been used to understand the make-up of comets and the dynamics of the nebula from which the Solar System formed. Chemical, isotopic and mineral analyses of these samples also underpin our understanding of how the rocky planets formed in the early Solar System — and how one planet happened to be in just the right place to nurse life.

For OSIRIS-Rex, a successful touchdown will be only the start of a painstaking scientific journey that could help us to understand whether the water and organic molecules involved in the origin of life came from carbon-rich asteroids similar to Bennu crashing into the early Earth.

OSIRIS-REx will provide an enduring, global resource. Samples can be preserved for decades, possibly centuries, and be analysed with methods that haven’t yet been invented. Over the next two years, the OSIRIS-REx science team will catalogue the samples and conduct scientific analyses on up to 25% of them by mass. Six months after return, any scientist can propose a study. NASA will grant access to the samples through a peer-review process, as it does for other extraterrestrial materials under its purview, including the Apollo lunar samples.

I want the same for materials from Mars. I’ve dreamt of bringing back samples from there ever since I was a graduate student studying meteorites in a laboratory.

Around 3.5 billion to 4 billion years ago, Mars was probably warmer and wetter than it is today — and had a thicker atmosphere — much like Earth when life emerged here. On Earth, the rocks of that era have been destroyed by plate tectonics and weathering. But more than half of Mars’ pristine surface dates to that time. To fully understand whether life ever existed on Mars — and life’s possible origins on rocky planets in general — we must bring back a variety of samples from its ancient terrains and study them at the spatial resolution and with the analytical precision that are possible only in Earth-based labs.

Yes, the projected cost of MSR is higher than that of other robotic sample-return missions, but it is worth the challenge because of what we might gain, including knowledge crucial for sending humans to Mars and ensuring their safe return. Innovations from NASA missions underpin technologies that improve people’s lives, including laptops, computed tomography, magnetic resonance imaging and air purifiers.

The first step of MSR is already under way, with NASA’s Perseverance rover documenting and collecting samples in Jezero Crater. The next phase, set to start in 2028 or later, would be to retrieve these carefully selected samples and bring them back to Earth, which won’t happen before 2033.

Once those samples arrive, many thousands of scientists around the world, including geologists and geochemists, biologists and astrobiologists, atmospheric and climate scientists, theorists and modellers, will be able to analyse them and interpret the resulting data for decades to come. Those samples could answer some of humanity’s biggest existential questions: How do rocky planets such as Mars and Earth form and evolve? How and why did the climate on Mars change so drastically? How, when and where did life emerge? Are we alone in the Universe?

I encourage any scientists who are interested in these questions to sign up to be an MSR Affiliate ( The imminent return of the Bennu samples by OSIRIS-REx reminds me of what an exhilarating time this is and the profound possibilities of these precious materials.