December 3, 2018 – OSIRIS-REx has arrived at asteroid Bennu. On December 3, the spacecraft completed its two-year, multimillion-mile cruise phase to come within 12 miles (20 km) of Bennu. The arrival marks the start of proximity operations, the up-close science phase of the mission.
Now that OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) is at the asteroid, it will begin the official preliminary survey phase on December 31. That survey is part of a series of tasks that will prepare OSIRIS-REx to do something no other U.S. spacecraft has done: collect and return a pristine asteroid regolith (soil) sample to Earth.
OSIRIS-REx’s proximity operations (prox-ops) will also take the Lockheed Martin team “flying” the spacecraft into new territory. No one has ever operated a spacecraft so close to a small celestial body and in a microgravity environment.
“The risks of being that close to an asteroid are driving the main challenges on this mission,” said Olivia Billett, Science Phase Lead for the OSIRIS-REx mission. “In January, we will get into the smallest orbit that any mission has ever been in, and we will get even closer next summer. Fundamentally, our team always runs the risk of a mission-ending impact because of how close we are operating the spacecraft to Bennu.”
To keep OSIRIS-REx safe during prox-ops, Lockheed Martin Space engineers designed and built OSIRIS-REx with an onboard system not usually needed for spacecraft operating in larger orbits. The systems can warn the spacecraft of danger ahead and prompt it to take autonomous corrective actions through back-away burns.
“It’s trained so that it knows how to get away from the asteroid,” said Mark Fisher, the Spacecraft Engineer who leads OSIRIS-REx’s engineering.
The spacecraft will also perform the same evasive maneuvers if it loses ground contact with the spacecraft operations team in Lockheed Martin Space’s Mission Support Area for too long.
“If it’s just sitting there and it says, ‘Hey, it’s been too many days and I haven’t heard. I’m going to assume the worst and I’m going to get away from the asteroid,’” Fisher said.
In addition, because Bennu is so small — just 1,612 feet (492 meters) in diameter, about the size of the Empire State Building — the gravitational forces that a spacecraft might encounter around a much larger planet don’t exist.
“The non-gravitational forces are much more of a factor in our navigation than they normally are,” Billett said. “It’s a very different paradigm.”
For instance, moving a few hundred meters has a minimal effect on a planetary orbiter, but for an asteroid orbiter it means that the sun’s angles on the spacecraft can change significantly and affect power and thermal profiles ultimately adjusting the orbit of OSIRIS-REx.
Those dynamic changes require constant management by the ground team. “To fly the trajectory that we’re going to fly over the next two years, we have to do maneuvers once or twice a week throughout the entire mission because our orbit can degrade so quickly,” Billett said. “We’re in effect continually flying the spacecraft along its path to make sure it’s going where we need to map the entire asteroid and meet our science objectives.”
For the rest of the science mission following the December 3 arrival, the Lockheed Martin team will move between surveys and orbit. The orbits and surveys will gather data and images needed to map Bennu’s surface features and understand its spectral, thermal and geologic properties. Scientists will use that information to arrive at a dozen potential sample sites. Then, after several reconnaissance flyovers that will bring OSIRIS-REx as close as 200 meters (650 feet) from Bennu’s surface, they will choose the final site.
“The phases build on each other,” Billett said. “It’s not just a question of coming up, taking a bunch of pictures at the site and diving in because the acquisition of the sample has to be precise.”
The challenges are worth the effort—for OSIRIS-REx and future missions. “It’s trailblazing — from the sample acquisition to the design of the mission,” Fisher said.
“We’re seeing our planning bear out,” Billett said. “All of that can feed forward into any future missions that want to go to an asteroid. We are learning what the risks are, the areas of vulnerability and strength, and about establishing proximity operations as a paradigm.”
A Mission Of Firsts
The seven-year mission of the Lockheed Martin-built OSIRIS-REx spacecraft is full of firsts. It will be the first NASA explorer to carry asteroid regolith (soil) samples back to Earth and the first opportunity for scientists to study a pristine asteroid sample that could provide clues to the origins of life.
The mechanism that will collect that sample is unprecedented as well. TAGSAM, or Touch-And-Go Sample Acquisition Mechanism, is a Lockheed Martin invention. Instead of landing on tiny Bennu, OSIRIS-REx will rely on TAGSAM’s arm to reach out and touch Bennu’s surface and gather regolith into its sampler head.
The tasks represent a lot of work for the team.
“Normally with an orbiter we do orbit insertion just once, and then once in orbit, we do trim maneuvers no more than once a month,” Billett said. “Some missions are stable enough for once a year. With OSIRIS-REx, it’s a much, much faster cadence than most missions have had to deal with.”
The team will be sending up to 50 command packages each week, which translates to approximately 30,000 commands executing on board the spacecraft every week. Those commands need to be designed and implemented on a fast-turnaround schedule.
“Normally when you are building a maneuver it takes about three weeks to go through a design, work with a navigation team, test it, review it,” Billett said. “We’re taking that and compressing it down to a few hours.”
The team spent months preparing for prox-ops. “One aspect was training the right people and the other was providing them with the right tools,” Fisher said. “We went through our entire toolset that we would normally use for a planetary mission (to determine) where is this not going to meet our requirements, where is this not going to be not fast enough? We came up with a new set of tools and new ways of using our (existing) tools to make this cadence happen.”
Then the team tested those tools—and themselves. “We did several elaborate tests really pretending that we’re at the asteroid,” Fisher said. “We did wall clock simulations with the entire team, passing files back and forth, getting data down, having to react to that data.”
They even simulated the time of year — down to celebrating Thanksgiving in May.
“We had the honey-baked ham and the mustard and the green beans,” Fisher said. “That was a signal to ourselves that we’re very serious about this.”