Credit: NASA/Goddard/University of Arizona
It’s almost TAG time! On October 20, 2020 NASA’s OSIRIS-REx spacecraft will slowly descend from its orbit 2,500 feet (770 meters) above Bennu to briefly touch the asteroid’s pebbly surface with its TAGSAM instrument, quickly sucking in and filter-capturing a small amount of material which will be returned to Earth for scientific study in 2023. This will be NASA’s first-ever collection of pristine asteroid material and needless to say, it’s a pretty exciting!
Launched Sept. 8, 2016 (I was there!) OSIRIS-REx has been in orbit around the main asteroid Bennu since December 2018. During that time it’s learned so much about this small but fascinating member of our solar system, including mapping its surprisingly rocky surface in exquisite detail. Recently posted by NASA’s Goddard Space Flight Center, here are a few things to know about Bennu in advance of the spacecraft’s touch-and-go (TAG) maneuver, and enjoy a fly-around tour as well:
Bennu is classified as a B-type asteroid, which means it contains a lot of carbon in and along with its various minerals. Bennu’s carbon content creates a surface on the asteroid that reflects about four percent of the light that hits it — and that’s not a lot. For contrast, the solar system’s brightest planet, Venus, reflects around 65 percent of incoming sunlight, and Earth reflects about 30 percent. Bennu is a carbonaceous asteroid that hasn’t undergone drastic, composition-altering change, meaning that on and below its deeper-than-pitch-black surface are chemicals and rocks from the birth of the solar system.
Bennu has been (mostly) undisturbed for billions of years. Not only is it conveniently close and carbonaceous, it is also so primitive that scientists calculated it formed in the first 10 million years of our solar system’s history — over 4.5 billion years ago. Thanks to the Yarkovsky effect — the slight push created when the asteroid absorbs sunlight and re-emits that energy as heat — and gravitational tugs from other celestial bodies, it has drifted closer and closer to Earth from its likely birthplace: the Main Asteroid Belt between Mars and Jupiter.
Is Bennu space trash or scientific treasure? While “rubble pile” sounds like an insult, it’s actually a real astronomy classification. Rubble-pile asteroids like Bennu are celestial bodies made from lots of pieces of rocky debris that gravity compressed together. This kind of detritus is produced when an impact shatters a much larger body (for Bennu, it was a parent asteroid around 60 miles [about 100 km] wide). Bennu, for contrast, is about as tall as the Empire State Building. It likely took just a few weeks for these shards of space wreckage to coalesce into the rubble-pile that is Bennu. Bennu is full of holes inside, with 20 to 40 percent of its volume being empty space. The asteroid is actually in danger of flying apart, if it starts to rotate much faster or interacts too closely with a planetary body.
Fact: OSIRIS-REx was designed to be navigated within an area on Bennu of nearly 2,000 square yards (meters), roughly the size of a parking lot with 100 spaces. Now, it must maneuver to a safe spot on Bennu’s rocky surface within a constraint of less than 100 square yards, an area of about five parking spaces!
Gravity isn’t the only factor involved with Bennu’s destiny. The side of Bennu facing the Sun gets warmed by sunlight, but a day on Bennu lasts just 4 hours and 17.8 minutes, so the part of the surface that faces the Sun shifts constantly. As Bennu continues to rotate, it expels this heat, which gives the asteroid a tiny push towards the Sun by about 0.18 miles (approximately 0.29 kilometers) per year, changing its orbit.
See these and more things to learn about Bennu from NASA here, and learn more about OSIRIS-REx’s TAG sample maneuver here. And follow along on October 20 on the OSIRIS-REx Twitter feed too!
Lights in the Dark 
now do Apophis. 🙂
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