Tag: asteroids

Today’s OSIRIS-REx sample grab from Bennu

10:28 am

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

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

6:32 pm

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.

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More results about Bennu from OSIRIS-REx

10:21 am

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.

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OSIRIS-REx team confirms date for sample grab from Bennu

12:16 pm

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.

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

9:21 am

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.

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Rocks from Vesta on Bennu?

8:51 am

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.

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Japan chooses Hayabusa-2’s next asteroid target

8:47 am

The new colonial movement: It appears that Japan has chosen the next asteroid that its probe Hayabusa-2 will visit in 2031, after it releases its samples to Earth in December from Ryugu.

Japan’s Hayabusa2 space explorer will aim to probe the asteroid “1998KY26” located between the orbits of Earth and Mars in 2031 after completing its current mission of collecting samples from another asteroid, the country’s science minister said Tuesday.

It is hoped Hayabusa2 will approach the ball-shaped asteroid, which has a diameter of around 30 meters and rotates about every 10 minutes, in July 2031, Education, Culture, Sports, Science and Technology Minister Koichi Hagiuda said.

It will not obtain samples from this second asteroid, only observe it close up by camera.

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Bennu tosses particles from its surface routinely

9:26 am

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.

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

6:16 pm

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.

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

3:53 pm

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.

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Tiny asteroid sets record for closest fly-by of Earth

9:10 am

Astronomers using the robotic Zwicky Transient Facility (ZTF) at the Palomar Observatory in California on August 16 spotted a tiny asteroid just after it had zipped past the Earth at a distance of only 1,830 miles, the closest any asteroid has ever been seen to do so without hitting the ground.

Asteroid 2020 QG is about 10 to 20 feet (3 to 6 meters) across, or roughly the size of an SUV, so it was not big enough to do any damage even if it had been pointed at Earth; instead, it would have burned up in our planet’s atmosphere.

“The asteroid flew close enough to Earth that Earth’s gravity significantly changed its orbit,” says ZTF co-investigator Tom Prince, the Ira S. Bowen Professor of Physics at Caltech and a senior research scientist at JPL, which Caltech manages for NASA. Asteroids of this size that fly roughly as close to Earth as 2020 QG do occur about once a year or less, but many of them are never detected.

The ability to spot these things is continuing to improve, though it does not appear they have yet obtained enough information to predict 2020 QG’s full orbit, or when or if it will return.

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Giant impact covered almost half of Gandymede’s surface

9:31 am

Artist's illustration of Ganydmede
Click for full illustration.

The uncertainty of science: Computer modeling and a review of images taken by Voyager 1 and 2 and the Galileo orbiter of Jupiter’s moon Ganymede now suggest the existence of a giant impact so large that it covers almost half the moon’s surface.

The artist’s illustration of Ganymede on the right, based on our presently incomplete set of global images, shows this impact area as the circular dark region.

Many furrows, or trough formations, have been observed on the surface of Ganymede, one of the Jovian moons. This research group comprehensively reanalyzed image data of Ganymede obtained by NASA’s Voyager 1, Voyager 2, and Galileo spacecrafts. The results revealed that almost all of these furrows appear to be arranged in concentric rings centered around a single point, indicating that this global multiring structure may be the remains of a giant crater. The radial extent of the multiring structures measured along Ganymede’s surface is 7800 km. For comparison, the mean circumference of Ganymede is only 16,530 km. If correct, this is the largest crater yet identified in the Solar System. The previous record holder with a 1900 km radius is on Calisto, another Jovian moon.

The conclusion reached here is very uncertain, since we really do not have a high resolution global map of Ganymede. All three spacecraft were only able to send back a scattering of high resolution images. The global map is based on Earth observations and images from the Hubble Space Telescope.

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OSIRIS-REx completes closest sample grab rehearsal

8:45 am

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:
» Read more

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Cryovolcanism on Ceres still ongoing?

10:02 am

3D simulation of Occator Crater on Ceres
Click for full 3D simulation image.
Click here for animated movie.

According to a new detailed analysis of data from the Dawn mission, scientists are now postulating that cryovolcanism in Occator Crater on Ceres began immediately after impact about 22 million years ago and has continued in fits and starts since.

Occator Crater was formed about 22 million years ago by a large impact. As in many other impact craters on Earth and on other planets, a central peak was formed, which collapsed again after some time. About 7.5 million years ago, brine rose to the surface within the remnants of the central peak. The water evaporated and certain salts, so-called carbonates, remained. They are responsible for the prominent bright deposits we see today, called Cerealia Facula, in the center of Occator Crater. Due to the loss of material in the interior, the inner part of the crater subsided. A round depression with a diameter of about 15 kilometers formed.

In the following millions of years, activity concentrated mainly on the eastern part of the crater floor. Through cracks and furrows, brine also rose to the surface there and produced further bright deposits, the Vinalia Faculae. About 2 million years ago the center of the crater woke up again: brine rose to the surface and within the central depression a dome of bright material was formed. “This process continued up to a million years ago and maybe even until today,” Dr. Nico Schmedemann from the University of Münster summarizes.

This hypothesis is further supported by second paper that proposes there remains a reservoir of salty underground liquid water in the tiny planet’s interior. Both add weight to the idea that any object in space that is large enough for gravity to force it into a spherical shape is going to behave like a planet, with a complex and active geology.

The first paper has a lot of uncertainty, however, centering entirely on its dependence on crater counts to determine age. While providing a rough age estimate, the method depends on many assumptions, is indirect, and could easily be entirely wrong.

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Hayabusa-2’s future asteroid targets

10:02 am

Link here. There are two prime candidate asteroid targets, both near Earth astroids.

The possible secondary targets include the oblong asteroid 2001 AV43 or the asteroid 1998 KY 26. They’re each about the size of a large house and both orbit the Sun in roughly 500 days. The proposed plan would see Hayabusa 2 arriving at 2001 AV43 in the late 2029 time frame, or reaching 1998 KY 26 in July 2031. Both asteroids have a low enough relative speed relative to the spacecraft to put them within (eventual) reach after Hayabusa 2’s December flyby.

Interestingly, 2001 AV43 will fly 313,000 km from Earth (0.8 times the Earth-Moon distance) on November 11, 2029.

The two asteroids were selected from an initial field of 354 candidates, which was winnowed down based on accessibility and scientific interest. Both are fast rotators, as evidenced by their light curves, each spinning on its respective axis once every 10 minutes. This represents the shortest “day” of any known object in the solar system, suggesting that these asteroids are in fact solid objects and not simply loosely aggregated “rubble piles.” A visit to one of these asteroids would mark the first time a space mission has seen such an enigmatic fast rotator up close.

The asteroid 1998 KY26 is also a possible carbonaceous (C-type) asteroid, and Hayabusa 2’s exploration of such a space rock would be another first.

Going to 1998 KY26 would also require a distant pass of another asteroid. Going to 2001 AV43 would require a fly-by of Venus, which could provide more data on that planet. Based on this information, my guess is that they will opt for 1998 KY26.

The decision must likely be made before Hayabusa-2 drops off its Ryugu samples to Earth on December 6, 2020.

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OSIRIS-REx preps for final rehearsal of sample grab

9:40 am

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.

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Astronomers find freshly fallen meteorites based on tracking their fall

11:36 am

Australian astronomers have found two meteorites on the ground after spotting them in the sky before they fell, with one found only

The first had been spotted in the sky only a few weeks earlier, while the second had been spotted back in November 2019. They had had to postpone the search for the second until the restrictions for the Wuhan flu were lifted.

The discovery of the first was amusing:

Astronomer Dr Hadrien Devillepoix and planetary geologist Dr Anthony Lagain originally went on a reconnaissance mission to assess the latest fall site near Madura, taking drone imagery of the area. Dr Devillepoix said that as they were walking back to their car along the old telegraph track near Madura Cave, they spotted what appeared to be a real meteorite on the ground just in front of them.

“I thought Anthony was playing a prank on me, that he planted one of the fake meteorites we were using for the drone training session. But after a closer inspection, it was evident that the fist-sized, 1.1 kilogram rock we just found was indeed the meteorite we were after,” Dr Devillepoix said. Dr Devillepoix explained that although the rock was very close to the predicted fall position, the team was not expecting to find it that quickly in this bushy terrain.

Based on its track as it fell, the astronomers think it might be from the Aten family of asteroids, which orbit the Sun between Venus and Earth. Such asteroids are hard to find because of the glare of the Sun, and are thus not as well studied. This makes this find even more significant.

Finds like this, which are beginning to happen more and more, are important because, first, the meteorite doesn’t spend much time in the Earth environment, and second, they can precisely identify where the asteroid came from. Both facts allow scientists a much better understanding of the asteroids themselves.

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A gravel pile floating in space that might hit the Earth

2:10 pm

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.
» Read more

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OSIRIS-REx spots sun-caused erosion on Bennu

10:08 am

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.

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