Getting Hyped for OSIRIS-REx’s Return

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A recent poll from the Pew Research Center shows that the space topic that Americans are most concerned about is asteroids: finding them, studying them, learning how to deflect them. So let’s talk about an asteroid mission with a major milestone coming up that I am very excited about. Plus the story involves a giant space rock trying to eat a spacecraft, and that’s always fun.

Allow me to introduce the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer, aka OSIRIS-REx (which, frankly, is one of the less ridiculous acronyms NASA has used). This mission left Earth on September 8, 2016 and began a two year cruise to the asteroid Bennu with the ultimate goal of collecting a sample from the asteroid and returning it to Earth. 

What’s So Cool About Bennu?

Bennu is a small asteroid roughly a third of a mile across. It’s what we call a rubble pile asteroid—that is, instead of being a big chunk of solid rock, it’s a whole lot of smaller stuff held together by gravity. Basically a somewhat cohesive pile of pebbles. Its orbit carries it as far out as Mars and as far in as just inside Earth’s orbit. This orbit made Bennu 

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 an attractive target for two reasons. One: it was an easy asteroid for us to get a spacecraft to because it’s close. And two: there is a non-zero chance that Bennu will, someday, hit Earth. The chances are very, very, very small, but they’re not zero. So it’s not a bad object for us to get familiar with.

Bennu also carries carbon-rich material from the very early solar system, which is something that’s hard to find elsewhere and something that astronomers would love to get their hands on. One of the reasons asteroids make great sites for sample return missions is that they’re pristine fossils from the solar system’s early days. They haven’t changed much in the last 4.5 billion years, so information about our system’s infancy which has been erased from other worlds can still be found in asteroids. They’re like time capsules. Giant, dirty, hard-to-reach time capsules.

OSIRIS-REx reached Bennu and entered orbit in December 2018. It then spent nearly two additional years circling the asteroid, mapping and taking sensor readings to pick the best spot to take a sample from. On October 20, 2020 the spacecraft descended to the surface to grab that precious asteroid dust. And that’s where things went slightly sideways.

Bennu Hungers

Here’s what was supposed to happen: as OSIRIS-REx made a gentle 0.2 mph descent, the 11 foot long arm with the collecting head on the end would delicately touch down on the surface and rest there for about five seconds. During those seconds gas jets would blow onto the surface to swirl bits of it up into the collection head. Then the spacecraft would fire its thrusters to push back from the asteroid, hopefully with up to 60 grams of material from Bennu safely stored in its collection head.

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What actually happened is that the asteroid tried to swallow the spacecraft. Bennu turns out to be an even looser rubble pile than we used to think, and when the collection arm touched the surface, the surface turned out not to be solid at all. The arm kept going, plunging into the ground with nothing to stop it. The assumption is that it would have just kept on going deeper if that five-second timer hadn’t gone off, prompting the spacecraft to fire its thrusters and push away. Of course, the thrusters firing caused a new problem.

When the thrusters went off, they hit that loose surface material like a hurricane. They not only reversed OSIRIS-REx’s motion, they also blasted six metric tons of material off the surface of Bennu, blowing a 30 foot crater into the asteroid. The spacecraft got pummeled with debris (which is never a thing you want for your space babies), and a number of particles landed directly on the camera for the spectrometer.

Downside: the camera could no longer see anything but those particles. Upside: the spectrometer had an incredibly detailed view of those particles which were, after all, some of the very bits of Bennu the spectrometer had been sent to study. And it turns out Bennu is full of exactly the ancient, carbon-rich materials we were hoping it was. And there was a whole lot of that material in the sampler just waiting to come back to Earth!

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Only…there was another problem. OSIRIS-REx had an embarrassment of riches. Its plunge into the asteroid had, hilariously, resulted in the sample container grabbing too much asteroid. The sample container was so full it couldn’t close. 

 NASA ultimately had to figure out how to shake loose the excess particles so that they could get the lid closed.

There was originally a plan to do a second touchdown, in case OSIRIS-REx hadn’t managed to collect 60 grams of material on the first one. After the sample collector was finally closed, NASA estimated it had at least 250 grams of material trapped inside of it. In case you were wondering, the total amount of asteroid material that has been returned to Earth by previous asteroid sampling missions, most especially Hayabusa and Hayabusa2, totals less than 8 grams. The second touchdown was canceled.

Homeward Bound

Following its unexpectedly eventful sampling, OSIRIS-REx spent another several months in orbit. It left Bennu behind on May 10, 2021 to begin the long flight back to Earth. On September 24, almost exactly seven years after it launched, OSIRIS-REx will swing back by Earth and drop its sample container into the atmosphere for a landing in the desert of Utah. NASA has been practicing the recovery all summer.

Hundreds of grams of pristine matter from the early days of the solar system will make their way to labs around the world, where they can be studied in a level of detail impossible for a spacecraft to obtain. And we’ll be able to learn more than ever about an asteroid that could, someday, have its sights set firmly on Earth.

OSIRIS-APEX

Once it’s dropped the sample container, OSIRIS-REx will still be out there, making its way through the dark. So what can a poor spacecraft do to top seven years of flight, fighting off a ravenous asteroid, and delivering an unprecedented amount of highly coveted research material back to its home planet?

Why, change its name and go visit an asteroid that used to be the stuff of nightmares, of course!

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Apophis is another small asteroid whose orbit brings it close to Earth. We once thought it might screw up our collective day in either 2029 or 2036, when its orbit brings it veeeeeery close to Earth. There was, at one point, a 2.7% possibility that on that Apophis would impact. If it had, it wouldn’t have been an extinction-level event by any means—Apophis isn’t big enough. But it would have made thousands of square miles of Earth a very lethal place to be in.

2.7% is small but it’s most definitely not zero, and for a while there asteroid trackers had bad dreams about this rock. The good news is that further observations of its path have confirmed that Earth is safe from Apophis for at least the next hundred years. The other good news is that in April 2029 Apophis will be less than 20,000 miles from Earth. And we’ll just happen to have a still-healthy asteroid-studying spacecraft hanging around that needs a new target.

When the spacecraft drops off its sample container in September, the OSIRIS-REx mission will officially end and the spacecraft will be redubbed OSIRIS-APEX (the APEX is for APophis EXplorer. Yes, that’s not quite how acronyms work, but space agencies don’t care). The newly-named spacecraft will enter orbit around Apophis on April 21, 2029 and spend 18 months circling it. OSIRIS-APEX will no longer have a sample container, but it will perform the same descent-and-thruster-fire maneuver to try and disturb Apophis’s surface to expose the layers beneath it for study. Here’s hoping this asteroid doesn’t try to eat the spacecraft.

2029 feels like a long way off, but Apophis’s visit will be here before we know it. In the meantime, we Earthlings will just have to console ourselves with the small mound of sweet, sweet primordial solar system dust that will be heading our way when OSIRIS-REx comes home on September 24. Mark your calendars, it’s going to be a big day!