OSIRIS-REx finds possible pieces of Vesta on Bennu

In an interplanetary faux pas, it appears some pieces of asteroid Vesta ended up on asteroid Bennu, according to observations from NASA's OSIRIS-REx spacecraft. The new result sheds light on the intricate orbital dance of asteroids and on the violent origin of Bennu, which is a "rubble pile" asteroid that coalesced from the fragments of a massive collision. "We found six boulders ranging in size from 5 to 14 feet (about 1.5 to 4.3 meters) scattered across Bennu's southern hemisphere and near the equator," said Daniella DellaGiustina of the Lunar and Planetary Laboratory, University of Arizona, Tucson. "These boulders are much brighter than the rest of Bennu and match material from Vesta." "Our leading hypothesis is that Bennu inherited this material from its parent asteroid after a vestoid (a fragment from Vesta) struck the parent," said Hannah Kaplan of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Then, when the parent asteroid was catastrophically disrupted, a portion of its debris accumulated under its own gravity into Bennu, including some of the pyroxene from Vesta."The unusual boulders on Bennu first caught the team's eye in images from the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) Camera Suite (OCAMS). 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 (OVIRS) instrument to get clues to their composition.

A spectrometer separates light into its component colors. Since elements and compounds have distinct, signature patterns of bright and dark across a range of colors, they can be identified using a spectrometer. 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.

Observations reveal it's not unusual for an asteroid to have material from another asteroid splashed across its surface. Examples include dark material on crater walls seen by the Dawn spacecraft at Vesta, a black boulder seen by the Hayabusa spacecraft on Itokawa, and very recently, material from S-type asteroids observed by Hayabusa2 at Ryugu. This indicates many asteroids are participating in a complex orbital dance that sometimes results in cosmic mashups.

As asteroids move through the solar system, their orbits can be altered in many ways, including the pull of gravity from planets and other objects, meteoroid impacts, and even the slight pressure from sunlight. The new result helps pin down the complex journey Bennu and other asteroids have traced through the solar system.

Based on its orbit, several studies indicate Bennu was delivered from the inner region of the Main Asteroid Belt via a well-known gravitational pathway that can take objects from the inner Main Belt to near-Earth orbits.

There are two inner Main Belt asteroid families (Polana and Eulalia) that look like Bennu: dark and rich in carbon, making them likely candidates for Bennu's parent. Likewise, the formation of the vestoids is tied to the formation of the Veneneia and Rheasilvia impact basins on Vesta, at roughly about two billion years ago and approximately one billion years ago, respectively.

"Future studies of asteroid families, as well as the origin of Bennu, must reconcile the presence of Vesta-like material as well as the apparent lack of other asteroid types. We look forward to the returned sample, which hopefully contains pieces of these intriguing rock types," said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson. "This constraint is even more compelling given the finding of S-type material on asteroid Ryugu. This difference shows the value in studying multiple asteroids across the solar system."

The spacecraft is going to make its first attempt to sample Bennu in October and return it to Earth in 2023 for detailed analysis. The mission team closely examined four potential sample sites on Bennu to determine their safety and science value before making a final selection in December 2019. DellaGiustina and Kaplan's team thinks they might find smaller pieces of Vesta in images from these close-up studies.

DellaGiustina and Kaplan are primary authors of a paper on this research appearing in Nature Astronomy September 21.

New small satellite mission to rendezvous with binary asteroids

The University of Colorado Boulder and Lockheed Martin will soon lead a new space mission to capture the first-ever closeup look at a mysterious class of solar system objects: binary asteroids. These bodies are pairs of asteroids that orbit around each other in space, much like the Earth and Moon. In a project review on Sept. 3, NASA gave the official go-ahead to the Janus mission, named after the two-faced Roman god. The mission will study these asteroid couplets in never-before-seen detail. Known as Key Decision Point-C (KDP-C), this review and approval from NASA allows for the project to begin implementation, and baselines +the project's official schedule and budget. It will be a moment for twos: In 2022, the Janus team will launch two identical spacecraft that will travel millions of miles to individually fly close to two pairs of binary asteroids. Their observations could open up a new window into how these diverse bodies evolve and even burst apart over time, said Daniel Scheeres, the principle investigator for Janus. "Binary asteroids are one class of objects for which we don't have high-resolution scientific data," said Scheeres, distinguished professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences at CU Boulder. "Everything we have on them is based on ground observations, which don't give you as much detail as being up close." The mission, which will cost less than $55 million under NASA's SIMPLEx program, may also help to usher in a new era of space exploration, said Lockheed Martin's Janus Project Manager Josh Wood. He explained that Janus' twin spacecraft are designed to be small and nimble, each one about the size of a carry-on suitcase.

"We see an advantage to be able to shrink our spacecraft," said Wood. "With technology advancements, we can now explore our solar system and address important science questions with smaller spacecraft."

Janus is led by the University of Colorado Boulder, where Scheeres is based, which will also undertake the scientific analysis of images and data for the mission. Lockheed Martin will manage, build and operate the spacecraft.

The mission will rendezvous with two binary pairs-named 1996 FG3 and 1991 VH-each showcasing a different kind of orbital pattern. The pair called 1991 VH, for example, has a "moon" that whips around a much bigger "primary" asteroid following a hard-to-predict pattern.

The team will use a suite of cameras to track the dynamical motion in unprecedented detail. Among other goals, Scheeres and his colleagues hope to learn more about how binary asteroids move-both around each other and through space.

"Once we see them up close up, there will be a lot of questions we can answer, but these will raise new questions as well," Scheeres said. "We think Janus will motivate additional missions to binary asteroids."

Wood added that the mission's twin spacecraft, each of which weigh just about 80 pounds, will travel farther than any small satellite to date.

After blasting off in 2022, they'll first complete an orbit around the sun, before heading back toward Earth and sling-shotting their way far into space and beyond the orbit of Mars.

"I think it's a great test for what is achievable from the aerospace community," Wood said. "And the Colorado-centric development for this mission, combining the space talent of both CU Boulder and Lockheed Martin, is a testament to the skills available in the state."