On Friday, Oct. 11, the OSIRIS-REx team should have been preparing to point their spacecraft cameras precisely over the asteroid Bennu to capture high-resolution images of a region known as Osprey. It is one of four sites scientists are considering from which the spacecraft can safely collect a sample in late 2020.

NASA's Goddard Space Flight Center, By Lonnie Shekhtman

© NASA | Engineers pull off daring rescue of OSIRIS-REx asteroid mission

But early that morning, the team learned that a telecommunications facility near Madrid had suffered an unexpected network outage. Part of NASA's Deep Space Network (DSN) of global communications facilities, the Spanish complex is home to giant radio antennas. One of these was scheduled to ping OSIRIS-REx for a critical data download.
The data download would have kicked off a 24-hour marathon process known as a "late update" to predict the spacecraft's trajectory in time for a flyover of Osprey. Among the litany of complex tasks the navigation team needed to do that day was to download images of Bennu. The team uses these images to identify landmarks on the asteroid in order to update the spacecraft's position and velocity.
But the DSN outage threatened to throw the mission off track.
The OSIRIS-REx team identified Osprey as one of the most promising sites on Bennu's rugged surface, based on its relatively smooth terrain and lack of large, potentially hazardous boulders. Osprey is set inside an approximately 66-foot- (20-meter-) wide crater near Bennu's equator. 

On Oct. 12, engineers were planning to collect critical images of the surface in order to assess Osprey's population of rocks that might be small enough to be ingested into OSIRIS-REx's sample collection head when the spacecraft ultimately touches Bennu next year. This assessment was the key piece of information the team needed to choose the top sample collection site from the final four.
The Osprey flyover was the second of the four sites to be surveyed during the reconnaissance campaign. It would bring the spacecraft a little more than half a mile, or 1 kilometer, from Bennu's surface. The missed opportunity to download the images of Bennu on Oct. 11 meant that there would not be enough time to follow the usual 24-hour process to update the spacecraft position at the time of the critical observations. This update is required for the spacecraft's cameras to be pointed correctly at Osprey on Oct. 12.
Missing the Osprey observations would have set off a domino effect of delays, said Kenneth Getzandanner, OSIRIS-REx flight dynamics manager based at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We knew that if we didn't get the Osprey data, we wouldn't be able to make the decision about our top sites for sample collection.

Flying a spacecraft within a kilometer of a small body like Bennu requires ultimate precision. Since engineers can't see their spacecraft in space, they often rely on DSN antennas to collect signals that allow them to determine its speed and location. But tracking through the DSN is not precise enough for a spacecraft that's both far from Earth (more than 155 million miles, 250 million kilometers) and needs to get very close to a planetary body, as was the case with OSIRIS-REx and Osprey.
For such close encounters—the closest that any spacecraft has orbited its celestial object of study—OSIRIS-REx engineers relied on images of Bennu's surface taken by the spacecraft's cameras in a technique known as optical navigation. Unique landmarks in the images, such as boulders and craters, help reveal where the spacecraft is located in relation to the asteroid. Together with sophisticated mathematical models that take into account forces such as the slight pull of Bennu's gravity or the slight push of radiation from the Sun, these images allow engineers to predict where the spacecraft is headed, and ultimately where it'll have to point its cameras when a region of interest is being observed. But the predictions aren't perfect. With each burn of the engine, for instance, the spacecraft can boost itself farther or closer than anticipated.
"Most missions aren't that sensitive to small changes in position, but this one is because we're so close to the asteroid that small changes in position result in big changes in where you want to be pointed, particularly when you want to be pointed at a really small patch of the asteroid such as Osprey," said Burns.
Having pulled off dozens of detailed observations under these constraints earlier in the mission, the OSIRIS-REx engineers, like highly trained athletes with fine-tuned motor skills, were able to complete the compressed procedure. On Oct. 12, they sent the updated positions to the spacecraft and waited for the resulting images of Osprey. As the images materialized, crisp and clear and perfectly centered on Osprey, it was evident that the race had paid off.
"It's a testament to the preparation and skill of the team that we were able to accomplish this in less than four hours. It speaks to the fact that we have a stellar team as we head into the most critical and challenging phase of this mission: the sample collection campaign," Burns said.
NASA will announce the primary sample site, as well as a backup, on Dec. 12. Two final reconnaissance flyovers at even lower altitudes beginning in January will allow the OSIRIS-REx team to collect final, detailed images of these sites. 

This article was originally published at NASA's Goddard Space Flight Center.
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