Bits of pebbles and dust from the asteriod Bennu that were collected during the long journey of the OSIRIS-REx spacecraft should be landing in the Utah desert later this month.
The delivery will be a first for NASA — its first sample return from an asteroid and one of a very small handful of space objects ever brought to Earth by humans from anywhere but the moon.
The roughly two ounces (60 grams) of regolith collected from the surface of Bennu — a 4.5 billion year old remnant of the early solar system — are expected to give new insights into how our solar system planets were formed and about the mix of organic compounds present when life began on Earth.
The landing will be the finale to a quite remarkable 4.7 million mile journey to, around and onto a tiny ball of dirt, gravel and pebbles, and then back to Earth. The spacecraft studied the asteroid from close orbit for almost two years before making its hazardous touch-and-go attempt to scoop up some regolith.
Though successful, that contact was a lot more fraught than expected. The asteroid is held together by only very week gravitational forces, the scientists found, and it nearly swallowed OSIRIS-REx as a swamp would, kicking up a wall of debris into space that threatened the spacecraft’s safety.
Now comes the final challenge of the return capsule drop-off. Once on Earth, the samples will go to NASA’s Johnson Space Center for curating, examining and ultimately distributing to scientists for their long-awaited chance to learn up close about a celestial body untouched by the teeming biosphere of Earth.
The returning asteroid sample from Bennu is not the first of its kind to be flown to Earth — that honor goes to the Hayabusa and Hayabusa2 spacecraft sent by the Japan Aerospace Exploration Agency. They returned with bits of dust and soil from two other asteroids, Ryugu (2020) and Itokawa (2010.)
Like Bennu, Ryugu is a carbonaceous asteroid, with a material makeup that includes substantial carbon. These are the type of asteroid most common in the solar system and of the most interest to space scientists since they generally contain the organic (i.e, containing carbon and hydrogen) building blocks of life. Itokawa is a stony asteroid.
So why did NASA send a spacecraft on its round trip, via an Earth gravity assist, to a “rubble pile” of rock and dirt in the general near-Earth region of the solar system.
I asked astrobiologist Jason Dworkin of the Goddard Spaceflight Center, the longtime project scientist for OSIRIS-REx, and he described why Bennu was such a desirable scientific target:
“As a fragment of the early solar system, Bennu represents unaltered, uncontaminated material representative of the sort that was available on the early Earth (or elsewhere) via meteoritic infall.”
“Unlike meteorites today, Bennu has not been exposed to the biosphere and the sampling event demonstrated that Bennu is fragile and thus meteorites from of this type of object may be absent in our collections.”
“Furthermore, we observed carbonate veins on the surface of Bennu which likely originated from hydrothermal systems on Bennu’s parent body. These, and other domains, could serve as an abiotic model of the abiotic chemistry that happened on the early Earth (or elsewhere).”
In other words, Bennu and asteroids like it, could provide innumerable chemical clues about how life began.
The Touch-And-Go Sample Acquisition Mechanism of OSIRIS-REx descending to the surface of Bennu and collecting dirt and dust. (NASA)
This building of a plausible case for the emergence of life on Earth being tied to the delivery of necessary molecules via asteroids-that-become-meteorites has been going on for some time, as has the effort to understand how similar pre-biotic compounds could arise autonomously on Earth.
Carbonaceous meteorites such as the iconic Murchison, the highly unusual Ivuna that formed outside the solar system. and the ancient and widely studied Allende have taught scientists that molecules necessary for life, as well as water molecules, do reside on asteroids and have been delivered to Earth via meteorites for eons. The suite of these molecules that have been identified in meteorites is impressive.
But sample returns offer something more — the surety that the molecules area result of Earthly contamination. These are entirely clean samples, and they also carry information about the asteroid material that can otherwise be substantially altered during their travels through space and their scorching passages through Earth’s atmosphere.
Some results from the examination of the asteroid Ryugu, brought to Earth in 2020 in Hayabusa2 mission, illustrate the great benefits. In a March 2023 paper in Nature Communications, the JAXA team (which includes Dworkin and other NASA scientists) reported the presence of uracil in the sample, a molecule never found in an incoming meteorite.
Uracil is one of five fundamental units of the genetic code and is found in all RNA molecules, which contain the instructions for how to build and operate living organisms.
Also found in the Ryugu sample was nicotinic acid, also known as Vitamin B3 or niacin, which is an important cofactor for metabolism in living organisms.
The missions to Ryugu and Itokawa (where the first Hayabusa only collected tiny bits of the asteroid) were revelatory, but the Bennu mission could be even more so.
Itokawa is thought to have formed from a collision between asteroids about 1.5 billion years ago. Ryugu’s surface is thought to be even younger, about 9 million years old. Bennu, on the other hand, is thought to have formed at the beginning of the solar system’s history, traveling through space over 4.5 billion years without undergoing any major alterations of its composition.
The best evidence suggests that within 10 million years of our solar system’s formation, Bennu’s present-day chemistry and mineralogy were already established.
By that time, the original carbonaceous material underwent some geologic heating and chemical transformation inside a much larger planetoid, one capable of producing the requisite pressure, heat and hydration to form into more complex minerals.
Bennu likely was broken off from a much larger carbon-rich asteroid about 700 million to 2 billion years ago after an impact of some kind. It likely formed in the main asteroid belt between Mars and Jupiter and drifted much closer to Earth since then.
Bennu has wandered into near-Earth space because of gravitational interactions with the giant planets and other space dynamics. The asteroid orbits on average about 105 million miles from the Sun, which is not much further out than Earth’s average orbital distance of 93 million miles.
The OSIRIS-REx probe has set two space navigating records: Bennu is the smallest body a spacecraft has ever orbited (the space rock is not quite as round as the Empire State Building is tall), and OSIRIS-REx performed the closest orbit ever of a small body, at just 1 mile from the surface. Asteroids smaller than 650 feet in diameter spin too rapidly for a spacecraft to safely land on them.
During the sampling operation, OSIRIS-REx managed to collect those precious two ounces of regolith from Bennu’s surface. The spacecraft had the capability to attempt two more collections, but the amount of material scooped up on the first try was more than enough and the team did not want to risk harming the spacecraft with additional high-drama, touch-and-go maneuvers.
After the payload lands, it will be transported to a temporary clean room on a military base in Utah for initial processing and disassembly in preparation for its journey by aircraft to the Johnson Space Center in Houston. That NASA facility is where many of the 842 pounds of moon rocks and pebbles from the Apollo era are studied, stored, and prepared for researchers elsewhere with expertise in relevant space and planetary science.
It will take some time, but samples protected against any conceivable contact with our atmosphere will be distributed to NASA researchers, JAXA researchers and then many others at institutes and universities here and abroad.
A long list with scores of Bennu hypotheses has been created by the OSIRIS-REx team and they will be tested by NASA and the many working groups tasked with testing them and trying to confirm or reject them.. These hypotheses relate to Bennu’s mineralogical, chemical and molecular makeup, its pre-solar and formation histories, its relationship with known meteorites, its inventory of prebiotic organics and much more.
And then, of course, there will be surprises — like the presence of the nucleobase uracil in the Ryugu sample.
Already Bennu has produced a number of those substantial surprises, including the presence of hydrated minerals on its surface that indicate the long-ago presence of water on the asteroid.
And then there’s the unexpectedly pliable nature of the asteroid surface when the spacecraft touched it. As NASA reported, the particles making up Bennu’s exterior were so loosely packed and lightly bound to each other that if a person were to step onto Bennu they would feel very little resistance, as if stepping into a pit of plastic balls.
Because of the powerful forces of biology, geology and constant weathering on Earth, anything as ancient as Bennu would have been modified many times over on our planet. That’s why scientist such as Dworkin are so keen to examine the Bennu samples; they provide an open window into a world that disappeared here long ago.
Dworkin has been working on the OSIRIS-REx project for quite a long time — since 2004, in fact. That was when he became co-principal investigator for an early OSIRIS-REx proposal that was not accepted, and the same happened in 2007.
He was deputy project scientist for the ultimately successful 2011 proposal and became project scientist soon after. The mission launched in 2017 and so there’s been a rather long wait for results even after the mission was up and running.
Dworkin is a sample scientist, and he said that consequently everything he has been working on related to asteroid sample return for the last 19 years is to enable and maximize the science generated from the mission. He will be conducting Bennu sample research for the next two years, he said, and the scientific community for decades. Indeed, a portion of the sample will be stored away for future advances in technology and know-how.
There are still things that could go wrong before he and others see any Bennu samples. So when asked his emotions as the landing approaches he said, not surprisingly, that he was “both excited and nervous.”
Many Worlds 
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