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Sampling an Ancient Rubble Pile

Sampling an Ancient Rubble Pile

Oct 21, 2020
Tags: Comets, Meteors, and Asteroids , Astrobiology , Español
asteroid Bennu
Asteroid Bennu

By Seth Shostak, Senior Astronomer

This week, a robotic spacecraft bearing the unwieldy name OSIRIS-REX silently sidled up  to a small asteroid, one of millions that ply the dark spaces of the solar system.  The spacecraft was there to collect a small sample of material – dusty rock that could shed light on two of science’s most intriguing puzzles: How was the solar system formed, and did that process bequeath its planets any of the necessary molecules for life?

If you could go back in time and visit our small part of the galaxy 4.6 billion years ago, you wouldn’t find the Earth, the Sun, or any of the other familiar components of the solar system.  They didn’t exist.  Instead, you would encounter a strange soup: a scramble of dark grains afloat in a cloud of gas.  But if you had the patience to wait a few million years, you would see these materials self-assemble into planets, moons, comets, asteroids, and the Sun.  You’d witness the slow, quiet birth of the solar system.

At least, that is what we think you’d see. But planetary scientists still aren’t sure of the details. And we can’t go back billions of years to sharpen our understanding.  But what we can do is take a leaf from the paleontologists’ book. They can’t return to the Mesozoic to study the evolution of  the dinosaurs, so they examine fossils, remnants from the past that can tell the story. 

Spacecraft over Bennu

The OSIRIS-REX mission is searching for geologic remnants; material that was present during the birth of our solar system – 50 times older than dinosaur bones.  And it’s doing so in a first-rate fossil field: an asteroid named Bennu.    

Go to a NASA web site, and you’ll find plenty of stunning photos of Bennu.  It’s rhomboid in shape, and carpeted by loose rubble.  It’s darker than an asphalt highway and small enough to fit in the Oakland Coliseum.

But the most interesting thing about Bennu is its age.  The pebbles and rocks that litter its surface are the same materials that built the planets, and they’ve remained largely unchanged for billions of years.  They’re dusty leftovers that we expect to be as informative as dinosaur bones.

However, as the advertising copy says, “there’s more.”  In addition to being the raw materials of planets, the rubble littering Bennu might also contain compounds that were necessary to spawn life.  That would be incredibly interesting, for it would show that the rain of asteroids that collided with the young Earth was critical to the emergence of biology.

The OSIRIS-REX mission has been long.  The spacecraft reached Bennu’s vicinity two years after its 2016 launch, carefully matching its speed to that of the asteroid. It has assiduously made photographs so that mission scientists could choose the best places to grab a sample. In a tour de force of space engineering, it used small thrusters to precisely pace the asteroid’s trajectory and rotation.

On Tuesday, October 20, the sampling arm, whose business end resembles a Honda air filter, was extended down to Bennu’s surface.  Exhaling a brief blast of nitrogen gas, it stirred up loose rock, packing some of it into its collector head.  The entire sampling process took but a few seconds, after which the spacecraft backed away.

Two years of travel and the delicate adjustment of the spacecraft’s trajectory were all in the service of those few seconds.  How much material was gathered is, as yet, unknown.  The minimum science requirement is two ounces – enough to fill a perfume bottle.  If the sample is less, OSIRIS-REX has enough nitrogen to attempt two more grabs.

Next Spring, OSIRIS-REX will bid Bennu farewell and start its return trip to Earth.  The gritty specimen in its bowels cost four years and a trillion dollars. Eventually that material will be robotically packed into a capsule and dropped onto the Utah desert for retrieval.

At that point, scientists will begin their work.  And the mission has provoked considerable excitement.

“We will undoubtedly learn something about the sort of prebiotic material that asteroids brought to Earth billions of years ago,” says John Marshall, a planetary scientist at the SETI Institute and a member of the OSIRIS-REX team.  “This may prove that very slow chemistry in space – long before our planet was even born – could be a necessary part of biological evolution.”

If that turns out to be true, humankind will have learned something essential about its own existence.

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