OSIRIS-REx on its way back to Earth

OSIRIS-REx today fired its engines and successfully put itself on course for returning its samples from the asteroid Bennu to Earth on September 24, 2023.

The May 10 departure date was precisely timed based on the alignment of Bennu with Earth. The goal of the return maneuver is to get the spacecraft within about 6,000 miles (approximately 10,000 kilometers) of Earth in September 2023. Although OSIRIS-REx still has plenty of fuel remaining, the team is trying to preserve as much as possible for a potential extended mission to another asteroid after returning the sample capsule to Earth. The team will investigate the feasibility of such a mission this summer.

The spacecraft’s course will be determined mainly by the Sun’s gravity, but engineers will need to occasionally make small course adjustments via engine burns.

The science team has already proposed one option, sending the spacecraft on a rendezvous with the potentially dangerous asteroid Apophis shortly after its 2029 close-fly of Earth. It could be that there are other targets as interesting that they need to choose from.

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 completes last close-fly of Bennu

OSIRIS-REx today successfully completed its last close-fly of Bennu before it will fire its engines on May 10th and begin its journey back to Earth to return its samples.

During the flyby, OSIRIS-REx imaged Bennu for 5.9 hours, covering more than a full rotation of the asteroid. It flew within 2.1 miles’ (3.5 kilometers) distance to the surface of Bennu – the closest it’s been since the TAG sample collection event.

It will take until at least April 13 for OSIRIS-REx to downlink all of the data and new pictures of Bennu’s surface recorded during the flyby. It shares the Deep Space Network antennas with other missions like Mars Perseverance, and typically gets 4–6 hours of downlink time per day. “We collected about 4,000 megabytes of data during the flyby,” said Mike Moreau, deputy project manager of OSIRIS-REx at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Bennu is approximately 185 million miles from Earth right now, which means we can only achieve a downlink data-rate of 412 kilobits per second, so it will take several days to download all of the flyby data.”

While they will get images of the asteroid’s entire surface, the region scientists are most interested in is the Nightingale sample return site where the spacecraft grabbed its samples. To best understand the asteroid they need to have before and after shots, and this last fly-by gave them the latter.

OSIRIS-REx begins last approach of Bennu

Approaching Bennu for the last time
Click for full image.

The science team for OSIRIS-REx released today the first picture taken as the spacecraft begins its last approach of the asteroid Bennu. To the right is that picture, cropped and expanded to post here. From the caption:

This image shows a top-down view of asteroid Bennu, with a portion of the asteroid’s equatorial ridge and northern hemisphere illuminated. It was taken by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on March 4, from a distance of about 186 miles (300 km). The spacecraft’s cameras are pointed directly at Bennu’s north pole. Two large equatorial craters are visible on the asteroid’s edge (center and center left).

If all goes well, the probe will fly past the asteroid on April 7th, obtaining high resolution images of its entire surface as it rotates below, including very close-up images of the Nightingale sample grab location during its closest approach.

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 to make one last observation of Bennu before heading back to Earth

The OSIRIS-REx science team has figured out a way to make one last observation of Bennu and the Nightingale sample return site before heading back to Earth on May 10th.

This activity was not part of the original mission schedule, but the team is studying the feasibility of a final observation run of the asteroid to potentially learn how the spacecraft’s contact with Bennu’s surface altered the sample site. If feasible, the flyby will take place in early April and will observe the sample site, named Nightingale, from a distance of approximately 2 miles (3.2 kilometers). Bennu’s surface was considerably disturbed after the Touch-and-Go (TAG) sample collection event, with the collector head sinking 1.6 feet (48.8 centimeters) into the asteroid’s surface. The spacecraft’s thrusters also disturbed a substantial amount of surface material during the back-away burn.

The mission is planning a single flyby, mimicking one of the observation sequences conducted during the mission’s Detailed Survey phase in 2019. OSIRIS-REx would image Bennu for a full rotation to obtain high-resolution images of the asteroid’s northern and southern hemispheres and equatorial region. The team would then compare these new images with the previous high-resolution imagery of Bennu obtained during 2019.

Getting at look at Nightingale post-sample-grab is critical to better understanding the nature of the asteroid. Knowing how much changed from that contact will tell scientists a lot about the density, interior, and surface of this rubble-pile asteroid.

This last flyby will also give them the chance to assess the spacecraft’s equipment following the touch-and-go sample grab. They want to know if everything still works as designed in order to plan any post-Bennu missions, including the possibility that OSIRIS-REx will rendezvous with the asteroid Apophis in ’29, shortly after the asteroid makes its next close flyby of Earth.

Another movie of OSIRIS-REx’s sample-grab-and-go at Bennu

The OSIRIS-REx science team has released another movie showing the sample-grab-and-go at Bennu, this time from a different camera.

The movie, made up of 189 images taken over three hours by the spacecraft’s navigation camera NavCam-2, can be seen at the link.

In the middle of the sequence, the spacecraft slews, or rotates, so that NavCam 2 looks away from Bennu, toward space. OSIRIS-REx then performs a final slew to point the camera (and the sampling arm) toward the surface again.

As the spacecraft nears site Nightingale, the sampling arm’s shadow comes into view in the lower part of the frame. Shortly after, the sampling head impacts site Nightingale (just outside the camera’s field of view to the upper right) and fires a nitrogen gas bottle, which mobilizes a substantial amount of the sample site’s material. Several seconds later, the spacecraft performs a back-away burn and the sampling arm’s shadow is visible against the disturbed surface material.

The team continues to investigate what caused the extremely dark areas visible in the upper and middle parts of the frame. The upper area could be the edge of the depression created by the sampling arm, a strong shadow cast by material lofted from the surface, or some combination of the two. Similarly, the middle dark region that first appears in the lower left of the image could be a depression caused by one of the spacecraft thrusters as it fired, a shadow caused by lofted material, or a combination of both.

It strikes me that getting post impact images of Nightingale is essential, if at all possible.

OSIRIS-REx completes storage of Bennu sample

OSIRIS-REx has now completed placing its sample from the asteroid Bennu in the Sample Return Capsule that will bring it back to Earth.

On the afternoon of Oct. 28, following the backout check, the mission team sent commands to disconnect the two mechanical parts on the TAGSAM arm that connect the sampler head to the arm. The spacecraft first cut the tube that carried the nitrogen gas that stirred up the sample through the TAGSAM head during sample collection, and then separated the collector head from the TAGSAM arm itself.

That evening, the spacecraft completed the final step of the sample stowage process –closing the SRC. To secure the capsule, the spacecraft closed the lid and then fastened two internal latches. As of late Oct. 28, the sample of Bennu is safely stored and ready for its journey to Earth.

Because they decided it was better to store the sample immediately and not risk losing it, they were unable to do the spin test that would told them how much sample they obtained. For this reason we will not know the amount until the sample capsule is opened here on Earth, after its return on September 24, ’23.

UPDATE: You can watch if short movie of the Sample Return Capsule closing here.

OSIRIS-REx engineers successfully place sample collector in return capsule

Stowing OSIRIS-REx's sample from Bennu
Click for full image.

OSIRIS-REx engineers have successfully placed the sample collector head holding the material captured from the asteroid Bennu in the return capsule that will bring it back to Earth.

Yesterday, NASA’s OSIRIS-REx mission successfully placed the spacecraft’s sample collector head into its Sample Return Capsule (SRC). The first image shows the collector head hovering over the SRC after the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) arm moved it into the proper position for capture. The second image shows the collector head secured onto the capture ring in the SRC. Both images were captured by the StowCam camera.

Today, after the head was seated into the SRC’s capture ring, the spacecraft performed a “backout check,” which commanded the TAGSAM arm to back out of the capsule. This maneuver is designed to tug on the collector head and ensure that the latches – which keep the collector head in place – are well secured. Following the test, the mission team received telemetry confirming that the head is properly secured in the SRC.

The next step will be to seal the capsule for return to Earth. However, based on the two images above, the sample is now relatively secure, as the opening where material could escape is now held face down in the capsule.

The spacecraft will head back to Earth in March ’21, with the sample capsule landing on Earth on September 24, 2023. I do not know whether it will be possible in the next six months to get new images of the Nightingale touch-and-go site, but have emailed Erin Morton, head of the communications for the science team, and asked. I will update this post when I hear back from her.

OSIRIS-REx sample grab so successful they are losing material

The samples from Bennu
Click for full two frame gif movie.

In a briefing today and press release, the OSIRIS-REx science team announced that they estimate that they have gathered a lot of material from the asteroid Bennu, at least 100s of grams, about twice the minimum of what they hoped to get.

In fact, images of the TAGSAM sample grab equipment suggest that there are some larger rocks lodged in its opening (preventing the flap from closing), and that the small movements they have done to photograph it has caused some of the captured material to escape. The image to the right shows this. You can see floating specks and their shadows (the horizontal streaks) that have escaped. At about 9 o’clock you can see a curve in the contact between a lighter material and blackness to its outside, bending towards the center of the TAGSAM. At other exposures they can clearly see a rock there, distorting the shape and thus preventing the flap from closing properly.

Because of this, they are foregoing the spin maneuver that would have weighed the sample, as well as one engine burst that would have slowed the spacecraft’s movement away from Bennu.

This means they will not know the exact amount captured until the sample gets back to Earth. This is a gamble, but they are confident that they have gotten a lot of material. According to Dante Lauretta, the principal investigator, the sample grab-and-go “got very down” into Bennu, as much as 19 inches. He is also confident that they grabbed more than a 100 grams.

They are therefore going to as quickly as possible store the samples in the Sample Return Capsule for return to Earth, beginning on October 27. They need to do a complex series of steps to make this happen, which is why it cannot happen until then.

One more detail: In their simulations prior to the touch-and-go, they had a range of estimates of how deep the spacecraft would penetrate. According to Lauretta, OSIRIS-REx plunged into Bennu at the softest part of that range, telling them that the asteroid is probably much more loosely packed than expected.

Because they are not doing that last engine burst means that they are moving away from Bennu for good. They will not return to the asteroid. Whether they will be able to get post sample grab images of Nightingale is unknown.

Images taken during OSIRIS-REx sample grab on Bennu

Below is an embed of a short eight second video of OSIRIS-REx’s sample grab yesterday from the surface of Bennu, created from 82 images, and covering at high speed the five minutes of approach, contact, and retreat. If you set the speed rate at 0.25, you can get a better view of the whole sequence of events.

From the science team’s press release,

The spacecraft’s sampling arm – called the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) – is visible in the lower part of the frame. The round head at the end of TAGSAM is the only part of OSIRIS-REx that contacted the surface during the sample collection event. In the middle of the image sequence, the sampling head positions itself to contact the asteroid’s surface head-on. Shortly after, the sampling head impacts site Nightingale and penetrates Bennu’s regolith. Upon initial contact, the TAGSAM head appears to crush some of the porous rocks underneath it. One second later, the spacecraft fires a nitrogen gas bottle, which mobilizes a substantial amount of the sample site’s material. Preliminary data show the spacecraft spent approximately 5 of the 6 seconds of contact collecting surface material, and the majority of sample collection occurred within the first 3 seconds.

The TAGSAM is designed to catch the agitated surface material, and the mission team will assess the amount of material collected through various spacecraft activities. After touchdown, the spacecraft fired its thrusters to back away from Bennu. As expected, this maneuver also disturbed the Nightingale site, and loose debris is visible near the end of the image sequence. Preliminary telemetry shows the spacecraft remains in good health. The spacecraft was traveling at 0.2 mph (10 cm/sec) when it contacted sample site Nightingale and then backed away at 0.9 mph (40 cm/sec). [emphasis mine]

At the moment it appears they don’t yet know how much sample they have gotten, but they are very optimistic that they have gotten enough, based on the performance above. On October 24th, when they have gotten far enough away from the asteroid, they give the spacecraft a spin to measure its present mass and compare that to a spin done prior to the sample grab. The difference will tell them how much sample they have captured.

They will also be looking at images of TAGSAM over the next few days, which will also indicate what’s been captured.

Sample grab appears to be a success at Bennu

OSIRIS-REx has apparently successfully touched the surface of Bennu, grabbed a sample, and backed away without damage.

The link takes you to my embed of NASA’s live stream, which is mostly pr garbage. However, it is providing live updates from the mission control team, as it happens. Most of time, the NASA people running their pr effort even have the sense to shut up when such updates come it.

Right now we do not know how much of a sample was obtained. It will take some analysis of data and images to find out. They will know by the time of tomorrow’s press conference at 5 pm (Eastern).

Today’s OSIRIS-REx sample grab from Bennu

Nightingale landing site on Bennu
The Nightingale landing site on Bennu, with
OSIRIS-REx superimposed. Click for full image.

Spaceflight Now today published a nicely detailed article summarizing the entire OSIRIS-REx mission to the asteroid Bennu, in anticipation of today’s attempt to grab a sample from that asteroid’s surface.

If you want to understand what is happening today, this article does a nice job of outlining everything.

I have embedded the live stream of the sample grab below the fold. It begins at 5 pm (Eastern) today. Be warned that it will show very little of the actual event, as the spacecraft will not be sending much data back to Earth today, during these operations. All we will really find out is if the grab happened, or was aborted to avoid risks, or occurred but the spacecraft was impacted by flying material during the grab. (Let us hope that this last option does not occur.)

The first images and data will not arrive until tomorrow, to be released during a press conference scheduled for 5 pm (Eastern).
» Read more

NASA announces broadcast schedule for OSIRIS-REx’s sample grab at Bennu

NASA today announced the broadcast schedule that will be available to the public of OSIRIS-REx’s sample grab at the asterod Bennu on October 20, 2020.

Much of the schedule is NASA’s public relations blather, filled with some good information intermixed with a lot of lobbying for the government agency. Much of it will also be the equivalent of watching paint dry, as nothing will be happening quickly.

However, if you wish to watch the important part, tune into NASA TV from 5 to 6:30 p.m on October 20th.

Hosted by Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, and Michelle Thaller, science communicator at Goddard, the broadcast will cover milestones in the last 90 minutes leading up to TAG and spacecraft back-away. It will include perspectives from team members and science leaders about the mission’s challenges and accomplishments.

This will be on the public feed. If you find yourself choking on the NASA hype, you can then switch over to the media feed, which will be “A clean feed of the Mission Support Area during TAG [touch-and-go].”

Regardless, the actual attempt will be heart-stopping, because there is a real chance flying rocks from the asteroid’s surface will hit and damage the spacecraft.

More results about Bennu from OSIRIS-REx

Scientists using the OSIRIS-REx spacecraft have now published a special collection of papers outlining some of their discoveries made during that spacecraft’s observations of the asteroid Bennu from February to October 2019.

These papers just make official much of what was revealed during a conference I attended in November 2019. To sum up the papers:

  • Bennu has a lot of carbonates across its surface.
  • Some of that material came from another object that had to have had water.
  • The asteroid’s boulders come in two types, dark-porous and bright-solid, with the latter likely from that water-bearing other asteroid
  • Bennu’s surface is fresh, only recently exposed to space, including the sample site Nightingale.
  • Bennu’s interior has large voids, and its equatorial region is less dense.

The discovery of carbonates, produced from the interaction of water and carbon dioxide, is a big deal. As Dante Lauretta, OSIRIS-REx principal investigator, explained at that November conference, “To me this is one of the most exciting results from the conference.”

These findings have allowed scientists to theorize that Bennu’s parent asteroid likely had an extensive hydrothermal system, where water interacted with and altered the rock on Bennu’s parent body. Although the parent body was destroyed long ago, we’re seeing evidence of what that watery asteroid once looked like here – in its remaining fragments that make up Bennu. Some of these carbonate veins in Bennu’s boulders measure up to a few feet long and several inches thick, validating that an asteroid-scale hydrothermal system of water was present on Bennu’s parent body.

The material could not have been created on Bennu itself, which means it formed on a different object that was large enough and existed long enough to create the veins in these boulders. That material was then flung back into space to settle onto Bennu’s surface.

The freshness of Bennu’s surface is also a big deal, as it means that etither the asteroid is not that old, or that its surface somehow gets plowed over periodically. It also means that when OSIRIS-REx grabs samples at the Nightingale site on October 20th, they will be grabbing material that has not been altered much by the harsh environment of space.

Finally, the data about Bennu’s interior and density is maybe the neatest discovery. As the press release notes,

The reconstructed gravity field shows that the interior of Bennu is not uniform. Instead, there are pockets of higher and lower density material inside the asteroid. It’s as if there is a void at its center, within which you could fit a couple of football fields. In addition, the bulge at Bennu’s equator is under-dense, suggesting that Bennu’s rotation is lofting this material.

Bennu’s very weak gravity makes it a very alien and hard-to-comprehend place. It appears that the gravel in this floating gravel pile is barely held together, some interlocking in a way that leaves many open gaps, with other pieces pulled outward by the spin of the asteroid.

In reading these results, my first impression was an overwhelming sense of time and its inconceivable vastness. Much of Bennu’s most primitive material comes from the early solar system, about six billion years ago. Other material is newer, but required many many millions of years to get created elsewhere, and then somehow end up in space to be captured by this asteroid.

A million years is a very long time. A billion years is a thousand times longer. To conceive such time frames and all that can happen during that time is practically impossible. Bennu has shown us just a hint of how much can happen, some of which we would never have imagined otherwise.

OSIRIS-REx team confirms date for sample grab from Bennu

Nightingale landing site on Bennu
Click for full image.

The OSIRIS-REx science team has confirmed October 20, 2020 as the date the spacecraft will attempt a sample grab from the asteroid Bennu.

OSIRIS-REx is charged with collecting at least 2 oz. (60 grams) of Bennu’s rocky material to deliver back to Earth – the largest sample return from space since the Apollo program – and the mission developed two methods to verify that this sample collection occurred. On Oct. 22, OSIRIS-REx’s SamCam camera will capture images of the TAGSAM head to see whether it contains Bennu’s surface material. The spacecraft will also perform a spin maneuver on Oct. 24 to determine the mass of collected material. If these measures show successful collection, the decision will be made to place the sample in the Sample Return Capsule (SRC) for return to Earth. If sufficient sample has not been collected from [the primary landing site] Nightingale, the spacecraft has onboard nitrogen charges for two more attempts. A TAG attempt at the back-up Osprey site would be made no earlier than January 2021.

The press release at the link provides a lot of technical and interesting details about the sample-grab-and-go attempt, expected to put the spacecraft in contact with the asteroid’s surface for no more than sixteen seconds.

The maneuver itself is quite risky, as the available smooth landing area, as shown in the image above, is only half the size the equipment was designed for, and surrounded by large boulders.

Ryugu, like Bennu, appears to have rocks from other asteroids

Japanese scientists today announced that 21 rocks identified by Hayabusa-2 on the asteroid Ryugu have a composition that suggests they were formed on another asteroid.

Although Ryugu’s surface is uniformly dark [because it is a C-type asteroid], the scientists behind the new research found numerous boulders scattered across the asteroid that were 1.5 or more times brighter than their surroundings — that is, they reflected at least 50% more light than most of the rest of Ryugu. This contrast made the researchers suspect these boulders may have come from outside the asteroid.

By analyzing the spectrum of light reflected off 21 of these boulders, the scientists deduced they were made of minerals known as anhydrous silicates. Prior studies have suggested that such water-poor, silicon-rich rocks make up silicaceous or S-type asteroids, the most common kind of asteroid found in the inner main asteroid belt. The brightness of these boulders also matches the brightness of S-type asteroids.

This result compliments the result yesterday from scientists studying Bennu with OSIRIS-REx, and was in fact released at the same time. Both asteroids apparently contain material from other asteroids, suggesting that asteroids in their initial formation (as rubble piles) are routinely a mixture of material from many asteroids, thrown out during impacts and then recaptured.

Rocks from Vesta on Bennu?

Scientists reviewing data of Bennu from OSIRIS-REx have found six very bright boulders that have a make-up similar to that found on Vesta, which suggests they initially came from that asteroid.

The unusual boulders on Bennu first caught the team’s eye in images from the OSIRIS-REx Camera Suite. They appeared extremely bright, with some almost ten times brighter than their surroundings. They analyzed the light from the boulders using the OSIRIS-REx Visible and Infrared Spectrometer instrument to get clues to their composition. … The signature from the boulders was characteristic of the mineral pyroxene, similar to what is seen on Vesta and the vestoids, smaller asteroids that are fragments blasted from Vesta when it sustained significant asteroid impacts.

Of course it’s possible that the boulders actually formed on Bennu’s parent asteroid, but the team thinks this is unlikely based on how pyroxene typically forms. The mineral typically forms when rocky material melts at high-temperature. However, most of Bennu is composed of rocks containing water-bearing minerals, so it (and its parent) couldn’t have experienced very high temperatures. Next, the team considered localized heating, perhaps from an impact. An impact needed to melt enough material to create large pyroxene boulders would be so significant that it would have destroyed Bennu’s parent-body. So, the team ruled out these scenarios, and instead considered other pyroxene-rich asteroids that might have implanted this material to Bennu or its parent.

The make-up of Vesta matches. While these rocks might have been flung from Vesta during an impact there, eventually to settle on the surface of Bennu, Vesta is not the only possibility. We do not have a good census of the asteroids in the solar system. Others whose make-up is not yet determined could be a source, as well as an asteroid that no longer exists, destroyed by a collision long ago.

Regardless, these rocks confirm that in the process of formation in the early days of the solar system, asteroids of all types exchanged material.

Bennu tosses particles from its surface routinely

Objects ejected from Bennu
Tracked particles after August 28, 2019 ejection event.

During OSIRIS-REx’s more than twenty months flying close to the Bennu, scientists have found that the asteroid routinely kicks particles from its surface into space, with these events linked to the asteroid’s day-night cycle.

Since arrival the scientists have seen and tracked more than 300 ejection events, with the almost seven hundred objects detected ranging from about an eighth to a half inch in size. Most moved about eight inches per second, comparable to “a beetle scurrying across the ground.”

The image to the right, cropped, reduced, and brightened to post here, comes from the introductory paper of a suite of papers on the subject, published today.

The timing of the events however reveals the most.

As Bennu completes one rotation every 4.3 hours, boulders on its surface are exposed to a constant thermo-cycling as they heat during the day and cool during the night. Over time, the rocks crack and break down, and eventually particles may be thrown from the surface. The fact that particle ejections were observed with greater frequency during late afternoon, when the rocks heat up, suggests thermal cracking is a major driver. The timing of the events is also consistent with the timing of meteoroid impacts, indicating that these small impacts could be throwing material from the surface. Either, or both, of these processes could be driving the particle ejections, and because of the asteroid’s microgravity environment, it doesn’t take much energy to launch an object from Bennu’s surface.

The link includes a cool movie showing the ejections events and the tracked paths of the ejected particles.

What OSIRIS-REx will grab from the asteroid Bennu in October

Closest view of Nightingale taken by OSIRIS-REx

On August 11th the spacecraft OSIRIS-REx did a sample grab-and-go rehearsal that put the spacecraft as close as 135 feet from the asteroid Bennu. During the rehearsal the spacecraft’s mapping camera (MapCam) snapped 22 images of the approach, showing the landing site, dubbed Nightingale, at the highest resolution yet.

From those images the science team created a movie. To the right is the closest image from that movie, lightened slightly and reduced to post here. It gives us the best view of the Nightingale landing site we will have prior to the October sample grab.

In essence, we are looking at the material that OSIRIS-REx will grab, though which particular rocks will be grabbed from this gravel pile are of course unknown. The spacecraft’s equipment is designed to capture pebbles smaller than 0.8 inches across. There are a good number of such rocks here, interspersed with a lot of larger rocks, including the several more than a foot across.

As I have noted previously, this landing site is about half the diameter of the landing sites the spacecraft was designed to touch down on. The rehearsal however gives us strong hope that OSIRIS-REx will be able to hit the bullseye. See this second movie, which shows the approach from two different cameras, with a wider context image provided to show how the spacecraft successfully hones in on its target.

Movie of OSIRIS-REx’s last rehearsal before sample grab

Closest point to Nighingale landing site during OSIRIS-REx's last rehearsal
Click image for full movie.

The OSIRIS-REx science team has released a movie made by the spacecraft’s navigation camera during its August 11th final rehearsal prior to the planned sample grab-and-go now set for October.

The image to the right is a capture of one image when the spacecraft was closest to the asteroid, about 131 feet above the surface. The target landing site, dubbed Nightingale, is the somewhat smooth area near the top half of the frame.

These images were captured over a three-hour period – the imaging sequence begins approximately one hour after the orbit departure maneuver and ends approximately two minutes after the back-away burn. In the middle of the sequence, the spacecraft slews, or rotates, so that NavCam 2 looks away from Bennu, toward space. Shortly after, it performs a final slew to point the camera (and the sampling arm) toward the surface again. Near the end of the sequence, site Nightingale comes into view at the top of the frame. The large, tall boulder situated on the crater’s rim (upper left) is 43 feet (13 meters) on its longest axis. The sequence was created using nearly 300 images taken by the spacecraft’s NavCam 2 camera.

Nightingale might be their best choice, but it remains about half the size they had originally wanted for their grab-and-go site, with far too many objects larger than planned. They designed the grab-and-go equipment to catch objects smaller than 0.8 inches. Little at this location, or on the entire surface of Bennu, is that small. The asteroid is truly a pile of gravel, with no dust.

OSIRIS-REx completes closest sample grab rehearsal

OSIRIS-REx yesterday successfully completed its closest sample grab rehearsal, bringing the spacecraft about 131 feet from the surface of the asteroid Bennu.

The approximately four-hour “Matchpoint” rehearsal took the spacecraft through the first three of the sampling sequence’s four maneuvers: the orbit departure burn, the “Checkpoint” burn and the Matchpoint burn. Checkpoint is the point where the spacecraft autonomously checks its position and velocity before adjusting its trajectory down toward the event’s third maneuver. Matchpoint is the moment when the spacecraft matches Bennu’s rotation in order to fly in tandem with the asteroid surface, directly above the sample site, before touching down on the targeted spot.

Four hours after departing its 0.6-mile (1-km) safe-home orbit, OSIRIS-REx performed the Checkpoint maneuver at an approximate altitude of 410 feet (125 meters) above Bennu’s surface. From there, the spacecraft continued to descend for another eight minutes to perform the Matchpoint burn. After descending on this new trajectory for another three minutes, the spacecraft reached an altitude of approximately 131 ft (40 m) – the closest the spacecraft has ever been to Bennu – and then performed a back-away burn to complete the rehearsal.

During the rehearsal, the spacecraft successfully deployed its sampling arm, the Touch-And-Go Sample Acquisition Mechanism (TAGSAM), from its folded, parked position out to the sample collection configuration. Additionally, some of the spacecraft’s instruments collected science and navigation images and made spectrometry observations of the sample site, as will occur during the sample collection event. These images and science data were downlinked to Earth after the event’s conclusion.

The actual sample-grab-and-go attempt is scheduled for October 20th.

The short video below the fold shows spacecraft as it makes its closest approach. From the youtube page:
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OSIRIS-REx preps for final rehearsal of sample grab

The OSIRIS-REx science and engineering team is getting ready for its August 11th final rehearsal of the sample grab-and-go at the asteroid Bennu that it plans to do in October.

If the rehearsal goes right, the spacecraft will descend to within 131 feet of the surface of Bennu as it deploys its equipment as if it would continue down to the surface. It will also fly in formation above the Nightingale sample site when it does this, taking the highest resolution images yet of the surface of the asteroid.

It will then back off, returning to its home orbit farther from Bennu. Engineers will then review what happened, and use that data to prepare for the actual sample grab-and-go, set for October 20, 2020.

A gravel pile floating in space that might hit the Earth

flat rock near Osprey
Click for full image.

Come October the probe OSIRIS-REx will attempt a quick touchdown on the asteroid Bennu to grab some tiny particles, all smaller than 0.8 inches across.

Bennu is what scientists have described as a “rubble-pile” asteroid. They use that name because it is simply a conglomeration of a lot of rocks, pebbles, boulders, and stones, all jagged and of all sizes. The overall gravity has never been strong enough to squeeze them together, at least as far as we can see, and so they are piled up loosely across the asteroid’s surface wherever we look.

I think a better name for this asteroid would a floating gravel pile, since the material on it, as clearly shown in the image to the right (reduced and rotated to post here), more resembles the tailings one finds at a mine or quarry. This photo was taken by OSIRIS-REx on May 26, 2020 during its first dress rehearsal over its back-up touch-and-go sample grab site, Osprey. As the release caption notes,

The field of view is 12 ft (3.8 m). For reference, the bright rock [near] the tip of the boulder is 1 ft (0.3 m) across, which is about the size of a loaf of bread.

I have rotated the image 90 degrees so that east is up, because the full mosaic of the entire Osprey landing site, shown below, is oriented that way, and by rotating it to match it is easier to locate this image within it.
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OSIRIS-REx spots sun-caused erosion on Bennu

Rock on Bennu showing exfoliation
Click for full figure.

An analysis of images taken by OSIRIS-REx of the asteroid Bennu has allowed scientists to identify places where the changing temperatures from day to night has caused the surfaces of rocks to flake away, a process geologists label exfoliation.

The image on the right, cropped and reduced to post here, is from figure 1 in the paper. The yellow arrow points to a typical example of exfoliation, which is a process you can see on many rocks here on Earth.

Rocks expand when sunlight heats them during the day and contract as they cool down at night, causing stress that forms cracks that grow slowly over time. Scientists have thought for a while that thermal fracturing could be an important weathering process on airless objects like asteroids because many experience extreme temperature differences between day and night, compounding the stress. For example, daytime highs on Bennu can reach almost 127 degrees Celsius or about 260 degrees Fahrenheit, and nighttime lows plummet to about minus 73 degrees Celsius or nearly minus 100 degrees Fahrenheit. However, many of the telltale features of thermal fracturing are small, and before OSIRIS-REx got close to Bennu, the high-resolution imagery required to confirm thermal fracturing on asteroids didn’t exist.

The mission team found features consistent with thermal fracturing using the spacecraft’s OSIRIS-REx Camera Suite (OCAMS), which can see features on Bennu smaller than one centimeter (almost 0.4 inches). It found evidence of exfoliation, where thermal fracturing likely caused small, thin layers (1 – 10 centimeters) to flake off of boulder surfaces. The spacecraft also produced images of cracks running through boulders in a north-south direction, along the line of stress that would be produced by thermal fracturing on Bennu.

The typical erosion processes that can cause exfoliation (weather, gravity) are not possible on tiny Bennu, so the solution appears to rest with sunlight and sunlight alone.

This is not really a surprising result, but it is the first time it has been documented by data.

Bennu’s forbidding gravelly surface

Gravelly Osprey landing site on Bennu
Click for a higher resolution version.

On May 26 the OSIRIS-REx science team completed their first rehearsal and close approach to their back-up sample-grab-and-go site on Bennu, dubbed Osprey, getting as close as 820 feet. The image to the right, cropped and reduced to post here, shows that sample site within the white box. According to the image caption, the “long, light-colored boulder to the left of the dark patch, named Strix Saxum, is 17 ft (5.2 m) in length.” Note also that they have rotated the image so that east is at the top in order to make it more easily viewed.

This particular spot in this crater is actually a revision from their first choice from early in 2019, which originally was to the right and below the dark patch in the center of the crater. After six months of study, they decided instead on the present target area above the dark patch, because it seemed safer with the most sampleable material.

So how safe is this new location? Let’s take a closer look.
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OSIRIS-REx rehearsal and landing rescheduled

The OSIRIS-REx science team today announced that, in order to give them more preparation time needed because of the coronavirus protocols, they have rescheduled their second rehearsal of the spacecraft’s touch-and-go sample grab from the asteroid Bennu from June to August, and delayed the actual touch-and-go sample grab from August to October.

The mission had originally planned to perform the first Touch-and-Go (TAG) sample collection event on Aug. 25 after completing a second rehearsal in June. This rehearsal, now scheduled for Aug. 11, will bring the spacecraft through the first three maneuvers of the sample collection sequence to an approximate altitude of 131 ft (40 m) over the surface of Bennu. The first sample collection attempt is now scheduled for Oct. 20, during which the spacecraft will descend to Bennu’s surface and collect material from sample site Nightingale.

Previously they had said that the rehearsal would get as close as 82 feet. Nothing has changed. That distance was the closest they expected the spacecraft to get. The new number, 131 feet, is in the middle of possible ranges. As explained to me by Erin Morton, head of communications for OSIRIS-REx in the Principal Investigator’s Office, “I originally chose the lowest altitude in that range to include in our public outreach materials, but later realized that it made more sense to use the mid-point altitude number, since that’s the average of the high and low possibilities.”

Though they have the ability to do two more sample grabs if the first in October is unsuccessful, they won’t bother if it succeeds. They must leave Bennu regardless in mid-2021 to return the sample to Earth on September 24, 2023.

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.

Bennu’s equatorial craters

Bennu's craters
Click for full image.

The OSIRIS-REx science team today released a neat image of Bennu, highlighting the string of impact craters along the rubble-pile asteroid’s equatorial ridge. The image to the right, cropped and reduced to post here, shows that image. From the release:

Bennu’s darkest boulder, Gargoyle Saxum , is visible on the equator, near the left limb. On the asteroid’s southern hemisphere, Bennu’s largest boulder, Benben Saxum , casts a long shadow over the surface. The field of view is 0.4 miles (0.7 km). For reference, the largest crater in the center of the image is 257 ft (78 m) wide, which is almost the size of a football field.

The photo was taken from a distance of six miles on April 28. The craters illustrate well the rubble pile/sandbox nature of this asteroid. They all look like what you’d expect if the impact was able to easily drive itself deep into the a pile of sand and loose rocks. The resulting crater thus has a very indistinct rim and a sloping floor down to a central point.

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