In less than a couple weeks from now, on Oct. 20, 2020, NASA’s OSIRIS-REx will become the space agency’s first mission to obtain samples from an asteroid – Bennu – that will then be returned to Earth. The touchdown itself will be a brief, but exciting milestone in asteroid exploration.
OSIRIS-REx has been busy exploring Bennu from orbit, and has made some very significant discoveries about the asteroid, including ones that may help scientists better understand how life began on Earth.
The findings provide scientists with valuable clues as to what the rock and dust samples might reveal when they are eventually returned to Earth and studied in laboratories.
The first paper reveals a very significant discovery, that Bennu is covered with carbon-bearing, organic matter. This doesn’t automatically mean there was once life on Bennu, or the original larger asteroidal body that it came from (which broke apart billions of years ago), but it does show that the kinds of organic compounds that life is made from are present. OSIRIS-REx already found that the landing site for the sample-taking, called Nightingale, contains these compounds as well.
“The abundance of carbon-bearing material is a major scientific triumph for the mission. We are now optimistic that we will collect and return a sample with organic material – a central goal of the OSIRIS-REx mission,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson.
This carbon material can also give scientists insights into how much water Bennu used to have and still has now.
“Our recent studies show that organics and minerals associated with the presence of water are scattered broadly around Bennu’s surface, so any sample returned to Earth should contain these compounds and minerals,” said SwRI’s Dr. Vicky Hamilton, a coauthor on all three papers. “We will compare the sample’s relative abundances of organics, carbonates, silicates and other minerals to those in meteorites to help determine the scenarios that best explain Bennu’s surface composition.”
Some of the boulders on Bennu have been found to contain mineral veins composed of carbonate. On Earth, such carbonate minerals often precipitate from hydrothermal systems that contain both water and carbon dioxide. Some of the boulders with veins are close to the sampling site, meaning that, hopefully, the samples returned to Earth will contain some carbonate too.
“Boulders strewn about near the Nightingale site have bright carbonate veins,” Hamilton said. “Bennu shares this compositional trait with aqueously altered meteorites. This correlation suggests that at least some carbonaceous asteroids were altered by percolating water in the early Solar System.”
Having the samples in hand will tell scientists a lot more about Bennu’s geology, of course, but based on other data collected by OSIRIS-REx so far, researchers think that Bennu’s parent asteroid or planetary body likely had an extensive hydrothermal system. The fact that some of the veins are a few feet long and several inches thick is evidence for this.
The regolith at Nightingale is also geologically quite young and pristine. This means that it hasn’t been exposed for as long a period of time as other locations on the asteroid to the harsh space weathering environment with strong radiation (which hits Bennu’s surface since Bennu has no atmosphere).
“Bennu’s diverse surface includes abundant primitive material potentially from different depths in its parent body plus a small proportion of foreign materials from another asteroid family littered about its surface,” said SwRI’s Dr. Kevin Walsh, a coauthor of one of the papers. “In addition, both the primary and back-up sample sites, Nightingale and Osprey, are situated within small spectrally reddish craters that are thought to be more pristine, having experienced less space weathering than most of Bennu’s bluish surface.”
These findings will help scientists better understand how cosmic rays and the solar wind affect the surface of asteroids. In false-color imaging, the longer that surface material has been exposed to this kind of space weathering, the bluer it looks. The freshest material on Bennu still looks reddish, but the average on Bennu is a less intense blue color.
Another paper highlights the different kinds of boulders on Bennu; some are dark and rough, while others are brighter and smoother. The dark boulders are weaker and more porous, while the bright ones are stronger and less porous. It is also the brighter boulders that contain the mineral veins.
Both types of boulders are weaker than had been expected, however. That’s actually a good thing, because it means that similar types of meteorites would probably not survive the plunge through Earth’s atmosphere. That, in turn, means that the composition of the samples returned to Earth will likely be significantly different from any meteorites that have ever been collected.
One of the other papers deals with Bennu’s gravity. Scientists have been able to map the asteroid’s gravity field by analyzing its effects on the trajectories of both the spacecraft and small particles that get ejected from Bennu’s surface.
By analyzing the asteroid’s gravity field, scientists determined that Bennu’s interior is patchy. Some regions are much less dense than others, and the center is very low density, almost like there’s a big hole inside the center of Bennu. The bulge around Bennu’s equator is also less dense, and scientists think this means that material inside the asteroid is being “lofted” into the equatorial region.
Touch-And-Go (TAG) sample collection
But before those precious samples can be sent back to Earth for analysis, they have to be collected. That will be OSIRIS-REx’s mission on October 20, when the first Touch-And-Go (TAG) sample collection will be attempted. If it works, it will be the first-ever sample collection from an asteroid by any NASA mission.
Nightingale, the touchdown site for the sample-taking, is a rocky area 52 feet (16 meters) in diameter in the northern hemisphere of the asteroid. OSIRIS-REx will descend to the surface for only a few seconds, but that’s all that is needed to collect the samples. There is also a backup site called Osprey.
The entire sample collection event, including descent, sampling and ascent will last about 4.5 hours. The spacecraft will leave its orbit around Bennu when it is still 2,500 feet (770 meters) above the surface.
To obtain the rock and dust samples, OSIRIS-REx will use its robotic sampling arm, called the Touch-And-Go Sample Acquisition Mechanism (TAGSAM). The spacecraft will need to do this autonomously, since it takes about 18.5 minutes for signals to travel from Bennu back to Earth. To do this, it will use its Natural Feature Tracking (NFT) navigation system.
Navigation images will be taken 90 minutes after OSIRIS-REx leaves orbit, which are compared to an image catalog onboard the spacecraft. This helps the spacecraft maneuver to the right location on Bennu.
There is also a hazard map in the navigation system, to help OSIRIS-REx avoid large boulders.
If all goes as planned, OSIRIS-REx will collect at least 2 oz. (60 grams) of material from Bennu’s surface, to be stored for later delivery to Earth. How will the scientists know for sure that it worked? The spacecraft’s SamCam camera will capture images of the TAGSAM head to see whether it actually contains the material two days later, on October 22. The spacecraft will also perform a spin maneuver on October 24, which will let scientists know just how much material was collected.
If everything worked, the material will be stored in the Sample Return Capsule (SRC). But what happens if it didn’t work and there is too little or no material? In that case, up to two more attempts to collect the samples will be made.
OSISIRIS-REx is scheduled to leave Bennu sometime in 2021 and will deliver the samples to Earth on September 24, 2023.
Bennu, which resides in the inner Solar System, was discovered on September 11, 1999, and is named after the Bennu, the ancient Egyptian mythological bird associated with the Sun, creation, and rebirth. It has a mean diameter of 490 meters (1,610 feet).
Bennu is classed as a near-Earth asteroid that has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199.
Bennu also has some similarities to another near-Earth asteroid called Ryugu. Both asteroids are thought to have originated from larger objects in primitive asteroid families within the main asteroid belt between Mars and Jupiter. The Japanese Hayabusa2 spacecraft rendezvoused with Ryugu in 2018. It studied the asteroid for a year and half before collecting its own samples. Those samples are on their way back to Earth now, and will arrive on December 6, 2020.
It will be interesting to see how the composition of those samples compares to those from Bennu. This is important, as it will help scientists better understand how asteroids like these formed and what the original parent bodies were like a few billion years ago.
In another surprise, OSIRIS-REx also recently found some fragments of asteroid Vesta on the surface of Bennu. Six boulders were discovered that are brighter than the rest of Bennu and match material known to exist on Vesta. Vesta resides in the main asteroid belt and is the second-largest asteroid, after the dwarf planet Ceres.
“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 in-depth studies of asteroids like Bennu, Ryugu and Vesta will not only help scientists better understand their origins, but also that of the Solar System overall. And if the discoveries on Bennu are any indication, they may even reveal clues about the origin of life itself.
OSIRIS-REx was launched on Sept. 8, 2016 aboard a ULA Atlas V 411 rocket, from Cape Canaveral Air Force Station in Florida.
More information about OSIRIS-REx is available on the mission website.