Meet Bennu, Who Is a Little Strange


OSIRIS-REx, the very ambitious effort to obtain a sample from an asteroid in space, is a success to this point. The manmade satellite is in orbit around the small asteroid 101955 Bennu. It’s a triumph of celestial navigation and technology.

There were serious critics of the OSIRIS-REx’s mission, who asked “What was the point, what will you find if you get there?” The answer, of course is that we didn’t know what it would find and that was why they were going to look. And, wow, have they found some surprises.

First, the surface of Bennu doesn’t look anything like what we expected.

This image shows a view across asteroid Bennu’s southern hemisphere and into space, and it demonstrates the number and distribution of boulders across Bennu’s surface. The image was obtained on Mar. 7 by the PolyCam camera on NASA’s OSIRIS-REx spacecraft from a distance of about 3 miles (5 km). The large, light-colored boulder just below the center of the image is about 24 feet (7.4 meters) wide, which is roughly half the width of a basketball court. Credit: NASA/Goddard/University of Arizona

This image shows a view across asteroid Bennu’s southern hemisphere and into space, and it demonstrates the number and distribution of boulders across Bennu’s surface. The image was obtained on Mar. 7 by the PolyCam camera on NASA’s OSIRIS-REx spacecraft from a distance of about 3 miles (5 km). The large, light-colored boulder just below the center of the image is about 24 feet (7.4 meters) wide, which is roughly half the width of a basketball court. Credit: NASA/Goddard/University of Arizona

Astronomers expected a dustball; what they have found is a kind of space tosheroon.1 Science expected asteroids to be evidence of the earliest origins of the solar system, the lumps, as it were that didn’t get incorporated into the sun or the planets. But not quite such large lumps, or such an extraordinary variety of lumps. Some of those rocks are 24 feet across, and obviously widely different. How were those rocks made? Where were those rocks made?

A second big surprise is that there are explosions on the surface of Bennu that eject streams of particles into space.

This view of asteroid Bennu ejecting particles from its surface on January 19 was created by combining two images taken by the NavCam 1 imager onboard NASA’s OSIRIS-REx spacecraft: a short exposure image (1.4 ms), which shows the asteroid clearly, and a long exposure image (5 sec), which shows the particles clearly. Other image processing techniques were also applied, such as cropping and adjusting the brightness and contrast of each layer. Credit: NASA/Goddard/University of Arizona/Lockheed Martin

This view of asteroid Bennu ejecting particles from its surface on January 19 was created by combining two images taken by the NavCam 1 imager onboard NASA’s OSIRIS-REx spacecraft: a short exposure image (1.4 ms), which shows the asteroid clearly, and a long exposure image (5 sec), which shows the particles clearly. Other image processing techniques were also applied, such as cropping and adjusting the brightness and contrast of each layer. Credit: NASA/Goddard/University of Arizona/Lockheed Martin

What kind of force is causing explosions strong enough to eject visible particles from the surface of the asteroid? Granted that the escape velocity from Bennu is very low, but still. The asteroid is assumed to be as old or older than the Earth, 4.6 billion years, plus or minus. Its orbit is stable; well, stable for a relatively small orbiting body approximately as close to the Sun as the Earth. The YORP effect pushes it around,2 but that will have to be the subject of another post. But the mechanism that causes these documented ejections of particles is a mystery. And if it has been going on for long, it’s a mystery why Bennu still exists. If a few grams are lost with each explosion, and explosions happen a couple of times a day, over 4.6 billion years the asteroid should be down to bread crumbs. If the explosions haven’t been going on that long, why did they start, and when?

The third mystery is that OSIRIS-REx has already detected magnetite among the ejected materials. So far as is known, magnetite only forms in the presence of water. There are also hydroxyl ions present, which is water trapped in mineral structures. Bennu is far too small to retain water in the raw vacuum of space. Where did the water come from?

Bennu was selected as the target for OSIRIS-REx because it was known to be a comparatively rare Type B carbonaceous asteroid. That’s pretty weird; there are only about 65 known Type B asteroids. But no one expected it to be this weird. Or this wonderful.

The goal of the OSIRIS-REx mission is to grab a sample of Bennu and bring it back to Earth.3 That would be a remarkable achievement. But OSIRIS-REx has already proven the value of pure science. WC can’t wait to see what turns up next.

 


  1. For a thorough discussion of tosheroons and their important, WC recommends Terry Pratchett’s novel Dodger. Set in early Victorian London, it’s completely different than most of Pratchett’s work. 
  2. YORP is the Yarkovsky–O’Keefe–Radzievskii–Paddack effect, or YORP effect for short, which changes the rotation state of a small astronomical body – that is, the body’s spin rate and the obliquity of its pole(s) – due to the scattering of solar radiation off its surface and the emission of its own thermal radiation. WC hasn’t  yet worked up the nerve to try and write a blog post on the YORP effect. 
  3. Bennu is also interesting because it has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199. Assuming we haven’t cooked the planet by then, that’s a concern. 
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One thought on “Meet Bennu, Who Is a Little Strange

  1. To be more than a little technical about footnote 3 – whether we (or anything else) have cooked the planet by 2199 will not affect whether this particular body impacts earth. Perhaps if we have it could trigger a global cooling event. #glasshalffull

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