A NASA spacecraft zooming toward Earth is on track to fling a capsule the size of an automobile tire onto a Utah bombing range Sunday morning. Inside will be fragments of an asteroid that may contain clues about the origin of life, and that is even more notable for being the most dangerous space rock yet detected.
The mission is OSIRIS-REx. That is NASA’s acronym for what is more fully known as the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer. Even more complicated than the name is the aerospace feat that OSIRIS-REx has attempted. This is NASA’s first attempt to scoop a sample of an asteroid, and it has turned out to be surprisingly tricky.
The asteroid is Bennu. It has an orbit around the sun that crosses Earth’s orbit. Although it’s not on a trajectory to hit our planet in anyone’s lifetime, it might someday.
With a diameter of about 1,640 feet, which is slightly more than the height of the Empire State Building including the antenna, Bennu is far smaller than the mountainous rock that struck Earth 66 million years ago and ended the reign of the dinosaurs. But that’s plenty big enough to get the attention of people whose job description includes the term “planetary defense.”
Gravitational interactions of the asteroid, Earth and the moon make Bennu’s orbital path hard to predict for future centuries. But according to a calculation published in 2021, Bennu has a 1 in 2,700 chance of hitting Earth in September 2182; that estimate will be refined after a close pass in 2135.
“Bennu is the most potentially hazardous asteroid in the solar system,” said the mission’s top scientist, Dante Lauretta, a planetary scientist at the University of Arizona.
NASA chose to sample Bennu in part because it’s a potentially hazardous asteroid. The successful DART mission of last year – when NASA slammed a spacecraft into a small asteroid and altered its motion – offered a demonstration of how a dangerous rock might be knocked off course. Knowing the nature of dangerous asteroids, and how they might respond to something plowing into them, is one justification for sending OSIRIS-REx on this long and difficult voyage.
But the fundamental goal is science, not planetary preservation. This type of asteroid – very old – contains molecules that date to the formative period of the solar system some 4 billion years ago. Scientists think Bennu will provide insight into how Earth, which was bombarded by space rocks early in its history, became fertile ground for the appearance of life.
“I’m really interested in how the Earth got its oceans, and maybe more importantly, the chemical building blocks for life,” said Danny Glavin, a senior scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Fragments of asteroids do reach Earth in the form of meteorites, but they get contaminated. That was one of the issues with ALH84001, the headline-grabbing Mars rock that NASA scientists in 1996 claimed was laden with evidence of ancient microfossils. Rarely in the annals of science has there been a bigger claim with bigger blowback. Among the chief objections to the Mars rock as evidence of extraterrestrial life was that it was thoroughly contaminated with material from Earth as it lay on the ice for thousands of years in Antarctica.
Glavin and his colleagues hope OSIRIS-REx will deliver pristine samples of Bennu that will offer clearer clues to the origin of life on Earth, among the greatest unknowns in science.
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Weak gravity and a missing beach
The mission, which is led by NASA’s Goddard, has managed to overcome a number of serious technical challenges since the spacecraft was launched in 2016. One of the most difficult problems was getting the spacecraft into orbit around Bennu.
Gravity is remarkably subtle as fundamental forces of the universe go, and the gravitational attraction of this asteroid was almost immeasurably feeble. It wasn’t much different from the force applied by solar radiation hitting the spacecraft’s solar panels. That made maneuvering into a stable orbit a more difficult task than anticipated. The engineers eventually managed to keep gravity and solar radiation in balance as the spacecraft orbited the rock.
The next great challenge was the sampling event. The plan all along had been to land on a smooth part of the rock with fine grains of material, an area that engineers when planning the mission referred to as “the beach.”
Except … there was no beach.
Bennu up close turned out to be vastly rougher, rockier and nastier than anticipated. It was what astronomers call a rubble pile.
“I was frequently saying, ‘Where is this beach?’” said project manager Rich Burns, a NASA aerospace engineer. “We had many moments where we questioned whether it would work or not. It was challenging in ways that were never envisioned.”
The engineers managed to pick out a place for the sampling maneuver. But again, Bennu produced a surprise.
On Oct. 20, 2020, the spacecraft slowly approached and extended a robotic arm. At the end of the arm was a sampling device resembling the air filter on a ‘57 Chevy. It touched the surface of the asteroid – and unexpectedly kept going, sinking about 20 inches into Bennu. The surface offered little resistance. Thrusters then fired and backed OSIRIS-REx away from Bennu.
“If the thrusters hadn’t activated, Bennu would have swallowed us up,” Glavin said.
“Bennu is behaving like a liquid droplet, but the particles are boulders and gravel and dust barely held together by gravity,” Lauretta said.
The operation was a success if not a thing of beauty. The lid of the sampler got jammed open by a rock during the process and some of the sample material spilled. But the team sealed what they had sampled, and concluded that they have plenty of stuff coming to Earth. Burns described the sample as 250 grams, plus or minus 101 grams.
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‘In the end game’
The spacecraft is now heading directly toward Earth (or, to look at it another way, Earth is heading directly toward the spacecraft as both orbit the sun). When the spacecraft is 67,000 miles from Earth – more than a quarter the distance between the planet and the moon – engineers will send a command for it to release the capsule containing the sample.
The process of releasing the capsule will spin it in a way that will stabilize its motion – akin to a quarterback throwing a tight spiral. The parent spacecraft will fire thrusters to change its trajectory and ensure that it does not plunge to Earth.
The capsule, meanwhile, will spend four hours approaching Earth – no navigation involved, just Newtonian physics at work – and then will enter the atmosphere at more than 27,000 mph. A heat shield will keep it from burning up. A drogue parachute will slow it down, and then the main parachute will deploy. Officials will track the falling capsule by radar and by airplane.
The target area is a military test range on the Bonneville Salt Flats. No one lives there. The mission leaders and their teams will be stationed off-site. As the capsule is heading toward the ground, recovery teams in four helicopters will race to the recovery area.
The first helicopter will dispatch military personnel to scout for unexploded ordnance. Then an employee of contractor Lockheed Martin, arriving in the second helicopter, will do a safety inspection on the capsule to make sure it is not emanating dangerous gases. Another helicopter will carry the NASA media team. Finally, the scientists will arrive and take samples of the area around the capsule for later examination in laboratories in case there are signs of contamination.
Recent rainstorms mean the desert is not as dry as the NASA team would like.
“We are prepared for it to be muddy,” said NASA scientist Nicole Lunning, who will curate the sample at NASA’s Johnson Space Center in Texas. “We are going to roll with what nature has given us.”
The capsule, bagged in three layers of plastic, will be hauled on a 100-foot line by helicopter to a hangar where a “clean room” awaits – a sterilized space that limits the odds of contamination. Then it will be flown to a clean facility at NASA Johnson for what will probably be many years of research.
There is no “Andromeda Strain” concern with this mission. Asteroids like Bennu – devoid of liquid water, bombarded by radiation and cruising through the cold vacuum of space – are sufficiently inhospitable objects that no one is worried that anything scary will crawl out of the capsule. The bigger issue is that Earth’s own microbes, or maybe just some Utah dust and dirt, will infiltrate the sample.
There are some unlikely scenarios that could cause the mission managers to decide against releasing the capsule, which would mean waiting another couple of years for the spacecraft and Earth to get into the right position again. But the go, no-go decision will probably be easy.
“This capsule’s coming back rain or shine,” deputy project manager Michael Moreau said. And while all this drama unfolds, OSIRIS-REx will be headed toward another potentially dangerous asteroid, Apophis, that’s predicted to come within 20,000 miles of Earth in 2029.
“I’m tossing and turning at night with excitement and anxiety,” Lauretta said. “Twenty years ago this seemed like magic, and now, it’s happening, and we’re in the end game.”
Lori Glaze, the head of planetary science for NASA, said Friday that a government shutdown might disrupt the long-awaited analysis of the Bennu sample, but it will remain “protected and safe.”
“The sample has waited for more than 4 billion years for humans to study it,” she said, “and if it take us a little bit longer, I think we’ll all be okay.”