Posted on by Evan Gough

This is the Closest OSIRIS-REx has Gotten to Bennu. Just 680 Meters Above the Asteroid

NASA’s OSIRIS-REx spacecraft has been at asteroid Bennu since Dec. 3rd, 2018. On that day, it went from travelling to the asteroid to travelling around it. Since then it’s been surveying and mapping Bennu.

Now OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) has reached a new milestone. On June 12th, the spacecraft executed another maneuver, and in the process broke its own record for the closest orbit of a planetary body by a spacecraft.

This signalled the beginning of a new phase for the OSIRIS-REx mission called Orbital B. In Orbital B, the spacecraft is orbiting asteroid Bennu at an altitude of 680 meters (2,231 feet). (This beats its own previous record of 1.3 kilometers (0.8 miles) above the surface.)

An illustration of OSIRIS-REx as it executed a maneuver into Orbital B, which took it down to an altitude of only 680 meters (2,231 ft.) Image Credit: University of Arizona.

OSIRIS-REx will remain in Orbital B until the second week of August. Following that, it will raise its orbit to 1.3 km (0.8 miles) above the surface. During the first two weeks of Orbital B, the spacecraft will investigate particles being ejected into space. It first spotted those particles when it arrived at Bennu, and scientists want to investigate by taking frequent images of the asteroid’s horizon.

Scientists were surprised to see these particles being ejected from Bennu. During the first part of Orbital B, the spacecraft will investigate the phenomenon. Image Credit: NASA/Goddard/University of Arizona/Lockheed Martin
Scientists were surprised to see these particles being ejected from Bennu. During the first part of Orbital B, the spacecraft will investigate the phenomenon. Image Credit: NASA/Goddard/University of Arizona/Lockheed Martin

For Orbital B’s remaining five weeks, the spacecraft will investigate Bennu with its science instruments. These include:

  • OSIRIS-REx Laser Altimeter (OLA) which will produce a full map of Bennu’s terrain.
  • PolyCam, which will create a high-resolution, global image mosaic of the asteroid.
  • OSIRIS-REx Thermal Emission Spectrometer (OTES) which will create an infrared global map.
  • REgolith X-ray Imaging Specvtrometer (REXIS) which will produce a global X-ray map.

These instruments will work together to help scientists select the best spot to collect a sample from Bennu.

Beyond choosing the sites with the best samples, mission personnel have to assess them for likely success. The team operating the spacecraft will identify four possible sites for sample collection. Following the Orbital B phase is the Reconnaissance Phase of the mission. During that phase, OSIRIS-REx will make a series of low-altitude observations of the final two sample site candidates. At an altitude of only 225 m (738 ft) above the surface, the spacecraft will be able to identify objects as small as 2 cm (0.8 inches.)

This image of asteroid Bennu was taken at an altitude of only 690 m (0.4 miles) from the asteroid’s surface. It was captured by NavCam 1, one of three cameras in the spacecraft’s TAGCAMS (the Touch-and-Go Camera System) suite. The asteroid’s largest resident boulder is on the bottom edge. At this resolution, details as small as 0.5 m (1.6 ft) can be resolved in the center of the image. Image Credit: NASA/Goddard/University of Arizona/Lockheed Martin

One of the obstacles to a successful sample-collection is the unexpected rockiness of Bennu’s surface. The sample collection portion of the mission is called Touch-And-Go (TAG) and the rocky surface means that the plans for TAG need to be adjusted.

OSIRIS-REx found that Bennu's surface is unexpectedly rocky. The large light-colored rock in the center is about 7.4 meters (24 ft.) wide. That's about half the width of a basketball court. This image was captured by the spacecraft's PolyCam camera. Image Credit: NASA/Goddard/University of Arizona
OSIRIS-REx found that Bennu’s surface is unexpectedly rocky. The large light-colored rock in the center is about 7.4 meters (24 ft.) wide. That’s about half the width of a basketball court. This image was captured by the spacecraft’s PolyCam camera. Image Credit: NASA/Goddard/University of Arizona

In order for a successful TAG, the team needs to find a landing site that is clear of large rocks and boulders. The site also needs to be level. If it’s too tilted, the sampling arm may not be able to do its job. According to NASA, the unexpected rockiness is adding an additional challenge.

An artist's illustration of OSIRIS-REx's TAGSAM collecting a sample from Bennu's surface. Image Credit: By NASA/Goddard Space Flight Center - http://www.nasa.gov/image-feature/osiris-rex-grabs-a-sample (image link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=52203236
An artist’s illustration of OSIRIS-REx’s TAGSAM collecting a sample from Bennu’s surface. Image Credit: By NASA/Goddard Space Flight Center – http://www.nasa.gov/image-feature/osiris-rex-grabs-a-sample (image link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=52203236

If anyone’s up to the challenge, it’s probably NASA. Rich Burns is the project manager of OSIRIS-REx at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. In a press release he said, “Bennu has issued us a challenge to deal with its rugged terrain, and we are confident that OSIRIS-REx is up to the task.”

OSIRIS-REx is a seven year mission to study asteroid Bennu. Asteroids like Bennu are primitive bodies from the early days of the Solar System’s formation 4.5 billion years ago. Retrieving a sample from this ancient piece of rock will help scientists piece together the history of our Solar System, including dear old Earth. 

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