Roughly 80 million miles from Earth, a spacecraft is stretching.
The probe slowly extended its 11-foot robotic arm this week, something it hasn’t done since before its 2016 launch. This motion is a precursor to making history as this week’s robotic arm test is a preamble to the mission’s ultimate goal—when the spacecraft will descend onto an asteroid’s surface to collect a sample.
In December this intrepid probe, called OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer), or O-REx (as the engineers call it), will arrive at Bennu, an asteroid the size of the Empire State Building. When it does, Bennu will become the smallest celestial body ever orbited by a spacecraft.
“It’s on the project list to claim that Guinness Book of World Records, when the time comes,” says Sandy Freund, Lockheed Martin’s operations manager for O-REx.
A Tricky Landing
Astronomers discovered asteroid 101955 Bennu in 1990. It’s classified as a B-type asteroid, with about a diameter of 500 meters and 100 stories tall in length.
That’s pretty small by cosmic standards, but dangerously large considering the slim chance this asteroid could strike Earth one day. Every six years, Bennu swings close to Earth—within 65,000 miles, which is closer than the moon. That fact puts a sense of urgency on knowing the composition of near-Earth asteroids like Bennu.
To get O-REx onto an asteroid as it hurdles through space requires some careful flying, serious engineering, and a bit of guesswork. Bennu is too small to have an atmosphere to slow down the spacecraft and no useful gravity will keep it on the surface.
But scientists actually know very little about what this craft is going to be landing on, whether an unforgiving rocky surface or a more soft, regolith-filled landing zone—so O-REx will only connect with the surface for a scant few seconds. This touch-and-go tactic required the creation of a collection head called the Touch-and-Go Sample Arm Mechanism (TAGSAM).
“We don’t really know the surface properties of an asteroid,” says Lockheed’s Beau Bierhaus. “The asteroid is going to be what it’s going to be.”
A solid and unmoving surface would require machinery that reacts against it, similar to a bed of gravel, but that same system could sink into the surface of an asteroid covered in a soft bed of regolith. But Bennu is so unknown, the engineers didn’t know what to expect.
“Designing a landing system to accommodate all of that range of scenarios is very challenging,” says Bierhaus. “One of the key design decisions that we converged on relatively early is that we want to get the sampling done quickly, because the longer you’re in contact with the surface of the asteroid, the more your design is subject to the uncertainties of the properties of the surface.”
A Dangerous Asteroid Rendezvous
Our picture of Bennu is becoming clearer as O-REx approaches its target.
“We’ve had data taken from Spitzer Space Telescope, and we even got a radar shape model, so we’ve known some bulk or some gross aspects of this asteroid for a while,” Bierhuas says. “But we haven’t really known what it is really like in detail. And just a few weeks ago, after 10 years of wondering, we’re now finally seeing what Bennu is actually like. So yeah, the sense of excitement with everybody is pretty exceptional.”
Sandy Freund joined O-REx the end of 2013, first as an TAGSAM engineer, then working on the integration of the payload for launch, and now as the head of Lockheed’s missions operations team. The focus on O-REx is getting more and more intense, she says, because the mission “is in the heart of the science.”
The spacecraft will officially arrive at Bennu on December 3 and start a series of flybys to examine the asteroid in unprecedented detail, helping researchers understand how asteroids move around the solar system. One question, which scientists hope to answer, is how sunlight can change conditions on an asteroid’s surface that can also influence its trajectory. The craft will also map the chemistry and mineralogy of this carbon-rich asteroid, a point of interest for anyone interested in a future career in space mining.
In the months to come, O-REx will begin the first steps of its ambitious sample return mission. Next year, the ground team will start a preprogrammed automated sequence that will bring the spacecraft to within a few feet of the surface.
“We can’t be in the loop just due to the [seven-minute] lag time that we have,” Freund says. “So we will send the commands from the ground to essentially start TAG, and then from that point forward it’ll be autonomous.”
A Drive-By Asteroid Sampling
Following this week’s testing, O-REx’s robotic arm will be extended into the position needed for touching the surface. The circular collection device is at the tip of the arm, and the round face is the only part of the spacecraft that will touch Bennu, resting on the surface. The contact pushes springs that trigger a blast of nitrogen gas that stirs up pieces of the regolith. These are collected, and then O-REx moves away from the surface. The whole process will take five seconds, but the engineers have enough fuel to make several attempts.
“We can try many times,” says Freund. “The limiting factor is the number of bottles [of nitrogen] that we carry.” And they have only three.
The use of nitrogen speaks to the mission’s demand for pristine samples. The mission, among other things, is also hunting for organic matter on asteroids, which includes molecules such as carbon and hydrogen. Nitrogen doesn’t react to much of anything, which is why its great for preserving documents like the Declaration of Independence and safely purging fuel from rocket engines.
“We will know a few hours after the TAG event that the sequence executed as expected,” Freund says. “But really, the measurement of success is how much sample we gathered, and that is a multi-day process for us.”
This extreme and ambitious mission has many milestones to go, but even the team of engineers who built it says the science is more compelling than the machinery.
“OSIRIS-REx, by virtue of returning material from Bennu, is getting those early ingredients that led to the formation of Earth,” Bierhaus says. “We get to solve these really neat engineering challenges, but fundamentally they really are going after these really big questions.”