New OSIRIS-REx photos of Bennu sample site after sample grab

Nightingale before and after sample grab
Click here and here for original images.

The OSIRIS-REx science team today released the photos the spacecraft obtained in its last fly-by of the asteroid Bennu on April 7th of the Nightngale sample site, showing how it changed after the sample grab in October 2020.

The two photos to the right compare that site, with the top image taken before the sample grab and the bottom image taken on April 7th.

Comparing the two images reveals obvious signs of surface disturbance. At the sample collection point, there appears to be a depression, with several large boulders evident at the bottom, suggesting that they were exposed by sampling. There is a noticeable increase in the amount of highly reflective material near the TAG [touch-and-go] point against the generally dark background of the surface, and many rocks were moved around.

Where thrusters fired against the surface, substantial mass movement is apparent. Multiple sub-meter boulders were mobilized by the plumes into a campfire ring–like shape — similar to rings of boulders seen around small craters pocking the surface.

Jason Dworkin, the mission’s project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, noticed that one boulder measuring 4 feet (1.25 meters) across on the edge of the sampling site seemed to appear only in the post-TAG image. “The rock probably weighs around a ton, with a mass somewhere between a cow and a car.”

Dante Lauretta, of the University of Arizona and the mission’s principal investigator, later pointed out that this boulder is likely one of those present in the pre-TAG image, but much nearer the sampling location, and estimates it was thrown a distance of 40 feet (about 12 meters) by the sample collection event.

The sample grab arm penetrated the surface by more than a foot, and that event is marked by that dark depression at the center of the site.

OSIRIS-REX will now prepare for its May 10th engine firing that will send it on its route back to Earth to return that sample in September 24, 2023. After that the spacecraft could be sent on another mission, this time to the potentially dangerous asteroid Apophis.

OSIRIS-REx has begun its return to Bennu

On January 14th the OSIRIS-REx team fired the spacecraft’s engines to halt its drift away from the asteroid Bennu and begin its return for one last reconnaissance before heading to Earth with its samples.

OSIRIS-REx executed the first maneuver on Jan. 14, which acted as a braking burn and put the spacecraft on a trajectory to rendezvous with the asteroid one last time. Since October’s sample collection event, the spacecraft has been slowly drifting away from the asteroid, and ended up approximately 1,635 miles (2,200 km) from Bennu. After the braking burn, the spacecraft is now slowly approaching the asteroid and will perform a second approach maneuver on Mar. 6, when it is approximately 155 miles (250 km) from Bennu. OSIRIS-REx will then execute three subsequent maneuvers, which are required to place the spacecraft on a precise trajectory for the final flyby on Apr. 7.

OSIRIS-REx is scheduled to depart Bennu on May 10 and begin its two-year journey back to Earth. The spacecraft will deliver the samples of Bennu to the Utah Test and Training Range on Sep. 24, 2023.

While they will gather images of the whole asteroid, their number one goal will be to get high resolution photos of the sample-grab site Nightingale to see how it was changed by that sample grab. The spacecraft pushed into the asteroid’s rubble pile about 1.6 feet, and that act certainly disturbed both the interior and surface. By comparing the before and after pictures scientists can garner a lot of information about the asteroid’s make-up, density, and structure. It will also teach future engineers what to expect when next they try to touch another rubble-pile asteroid.

OSIRIS-REx’s landing spot on the asteroid Bennu

Bennu, annotated
Click for full resolution unlabeled image.

The OSIRIS-REx science team today released another image of the asteroid Bennu, this time showing the planned Nightingale touch-and-go sample grab landing site.

The image to the right, reduced, cropped, and annotated by me, is that image. From the caption:

The crater where sample site Nightingale is located can be seen near the top, center of the image – it is a small region containing dark, fine-grained material. Bennu’s prime meridian boulder, Simurgh Saxum , is also visible in the lower left of the image, near the asteroid’s limb. Directly east of Simurgh is Roc Saxum . The field of view is 0.3 miles (0.5 km). For reference, Simurgh is 125 ft (38 m) across, which is about the size of a commercial airliner.

Nightingale is only about 50 feet across, which is about a third the size of the kind of smooth areas they had designed their grab-and-go equipment around. This global image illustrates the difficulties they face with that sample grab. Though there appear to be larger areas in this photo that seem smooth, they really are not. The asteroid has no dust, and the sample grab equipment is designed to suck up particles smaller than 0.8 inches in diameter. Most of the surface is covered with pebbles and gravel larger than this.

Thus they needed to find a spot where the bulk of the material is “fine-grained.” Nightingale fits that bill, though it has a small footprint and also has larger particles that pose a risk to the sample grab because they could damage the spacecraft, or clog the sample grab equipment.

Either way, for the spacecraft to autonomously guide itself accurately down to this small spot, surrounded as it is by much larger boulders, will be challenging, and is why they have done one dress rehearsal already, getting as close as 213 feet, and will do a second in June, getting down to 82 feet.

OSIRIS-REx’s sample grab location on Bennu

Nightingale site on Bennu
Click for full image.

On April 14th engineers for the probe OSIRIS-REx will do the first of two dress rehearsals of their planned touch-and-go sample grab from the asteroid Bennu, presently planned for August 25.

The image to the right was taken on March 3, 2020 from about 1,000 feet away during the spacecraft’s third reconnaissance phase, and is centered on that touch-and-go site, dubbed Nightingale by the science team. It illustrates why that sample grab carries risks that were unexpected. As they point out on the image’s release page, “the rock in the [upper right] of the image is 2 ft (70 cm) long, which is about the length of a small ice chest.” Moreover, across the entire touchdown site are numerous other rocks ranging in size from fists to laptops.

When they designed the mission, they had assumed there would be places on Bennu’s surface made up mostly of dust. areas where such dust would have gathered into ponds, as seen in other asteroids. The expectation also assumed these areas would be larger than any of the smooth areas found on Bennu. As they have noted:
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