A NASA spacecraft has started its return trip to Earth after collecting a sample from an asteroid last fall.
It was a big day when OSIRIS-REx briefly touched down on the asteroid Bennu back in October – and after collecting more sample than expected, it’s finally on its way home. Project Scientist Dr. Jason Dworkin says at 4:23 p.m. Monday, OSIRIS-REx fired its thrusters for a total of seven minutes, propelling away from Bennu at a speed of roughly 600 miles per hour.
“Taking it two orbits from the sun to intersect with Earth on September 24, 2023,” he adds.
Clearly, OSIRIS-REx still has a lengthy trip ahead of it (1.4 billion miles, to be exact). Dworkin says there’s no-straight path to Earth – both in a figurative and literal sense, as Bennu is currently on the opposite side of the sun. Rather, NASA navigators are steering OSIRIS-REx around the sun, toward where the Earth will be in two and half years.
So how will the return trip work?
"Everything is orbiting around the sun. The most fuel-efficient way [to travel] is to use the sun's orbit and follow that around, and change your distance from the sun just a little bit to encounter other objects you're going to," says Dworkin. "We have an amazing flight dynamics team, and they have a detailed model of the entire solar system — all the different forces that act on the spacecraft, from Bennu to Jupiter, to Earth, to, obviously, the sun. They can do weeks of calculations to make sure that we have the lowest-possible energy trajectory, and then do the burn, and there's a possibility for cleanup maneuvers if there's any error in that burn. This will be a series of burns as we get closer to Earth: we first aim toward 4,000 km from the earth, and then 400 [km] and then 40 [km]. Then, finally, it points at the earth, [and] deploys the sample return capsule four hours before Earth encounter. The spacecraft does a burn to divert away from the earth, and the sample canister lands in the Utah desert."
For those who maybe don't know or don't remember, why is NASA interested in this asteroid sample? What do you hope to learn from it?
"So Bennu is an ancient piece of rock. It's about 500 yards across, a very small asteroid. It's a remnant of the formation of the solar system — it's actually, we believe, older than the earth. And so, to understand how the earth was made, how life got started, how the solar system was forming, we need to look at leftover pieces. It's sort of like, if you're baking a pie, and all you have is the pie to look at — you don't really understand crust unless you can look at some remnants of the making of the pie on the cutting board. So this is sort of like leftovers on a cutting board, that we're examining close up in our laborities to understand how the pie got made that is our wonderful and delicious Earth."
Aside from the physical sample that it collected, what other data has OSIRIS-REx been collecting at Bennu?
"We did lots and lots of observations while at Bennu, all to make the sample more valuable, and to make it as safe as possible to collect that sample. So that included observing, for the first time, particles of bits of rock being ejected from the surface of Bennu. That was a total surprise. We discovered veins of carbonate rock on the surface of Bennu, organic material. The asteroid was much, much more rocky than we ever anticipated. That's based on a measurement called thermal inertia that was done from Earth. It's how fast and slowly the material warms up and cools down - so like sand has a very low thermal inertia, it warms up and cools down quickly, while pavement is much higher [and] takes much longer to warm up, and much longer to cool down. So anyway, for Bennu we measured thermal inertia properly, but our interpretation was totally wrong. It turns out that Bennu is very, very rocky — but those rocks are very compliant. Which is why, when the spacecraft went to collect a sample, the sampling collector arm sunk into the surface by a couple feet.
We discovered that we made a big mess. We blew a large crater in the surface of Bennu. A yard-long rock was thrown a dozen yards away. We discovered that the surface changed color a bit, it's now more speckled at that spot — that tells us something about what we might expect in our sample. And it gives the monitors who look at how asteroids get cratered some real data on what an impact looks like from a spacecraft. So there's a huge scientific boon of this last maneuver."
Very cool. For the return trip, what is the day-to-day going to look like for you on this project?
"Of course, we'll be monitoring the spacecraft, making sure that it continues to be healthy, and continues on its trajectory. But my main work is in the laboratory. We have a series of laboratory rehearsals with proxy samples that we'll be analyzing with all the laboratories around the world that will be looking at this material — to make sure that we are ready, and make sure that all of the laboratories are qualified. And of course, we'll be doing rehearsals in the Utah testing/training range for when the sample comes back, to make sure that we're ready and that we have every possibility covered in our sample recovery team."
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OSIRIS-REx first launched from Cape Canaveral in Florida in 2016. Dr. Jason Dworkin is a project scientist with NASA, which says it is considering a second trip for OSIRIS-REx to another asteroid, once the capsule with the Bennu sample returns to Earth.
