When the OSIRIS-REx sample return spacecraft briefly touched down on the water-rich asteroid Bennu last week, NASA hoped to collect at least two ounces (60 grams) of rock and dust samples. The probe's sample collection head made contact with the surface at 6:08 p.m. EDT on Oct. 20, firing a puff of nitrogen gas to stir up dust and pebbles for the collector to capture.
Everything appeared to go off without a hitch — at first. Later, scientists learned that so much material had filled the cannister that bits and pieces had wedged open the mylar cap designed to prevent the precious specimens from escaping. Now they can't shut the lid.
In the photo you can see bits and pieces of rock floating away from the partially open seal. Before any more of it is lost to space NASA is acting quickly to stow and secure the material in the Sample Return Capsule. Despite anxiety at losing a portion of the sample, the good news is that OSIRIS-REx captured such a beefy amount that there should be plenty to go around when the capsule returns to Earth in September 2023.
Bennu is a small, dark asteroid just a third of a mile (0.5 km) across that orbits between Mars and Jupiter in the main asteroid belt. If you could examine a rock from its surface it would look almost exactly like a hunk of charcoal. NASA picked its first sample return mission because Bennu is a carbon-and-water-rich asteroid related to carbonaceous chondrite (car-bon-AY-shuss KON-dryts) meteorites found on Earth.
Volcanic rocks have been subjected to extreme heat that alters their structure and composition, but carbonaceous rocks like those comprising Bennu have been heated very little. Scientists call them pristine — little-altered since the formation of the solar system 4.6 billion years ago.
Carbonaceous meteorites often contain complex carbon compounds including amino acids which are essential for life. Our bodies use amino acids to build the proteins that make up our bones, organs, skin, muscles and nails. Bennu and its ilk may have delivered some of these crucial compounds when the Earth coalesced from smaller bodies during its formation and later through bombardment. Can't you just feel that gritty asteroid dust in your veins?
Photos of Bennu's surface also reveal bright rocks. OSIRIS-REx found six boulders ranging in size from 5 to 14 feet (about 1.5 to 4.3 meters) scattered across its southern hemisphere and near the equator that are much brighter than their surroundings. Using the spacecraft's spectrometer, an instrument that measures a rock's composition by how it absorbs and reflects sunlight, scientists identified the mineral pyroxene. This is the same material discovered on the asteroid Vesta during the Dawn mission and also in certain meteorites.
It's very unlikely that pyroxene, which forms when rocks melt at high temperatures, would be forged on Vesta, which is composed mostly of water-rich minerals that never experienced a serious baking. Instead it appears that Bennu has inherited rocks blasted off Vesta when it was struck by smaller asteroids and meteoroids that gouged its many craters.
OSIRIS-REx stands for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer and is the first attempt by the United States to gather and return pieces of an asteroid. The probe will remain at Bennu until March 2021 and then begin the long journey home. On September 24, 2023 the spacecraft will drop off the capsule at Earth which will land by parachute in the Utah desert after plummeting through the atmosphere.
Returning a portion of an ancient asteroid to our planet will be the climax of a story that began with chaos and simplicity and led to beings forever curious about life's origin.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.