≡ Menu

OSIRIS-REx: Orbital Operations at Bennu

Sometimes one mission crowds out another in the news cycle, which is what has happened recently with OSIRIS-REx. The study of asteroid Bennu, significant in so many ways, continues with the welcome news that OSIRIS-REx is now in orbit, making Bennu the smallest object ever to be orbited by a spacecraft. That milestone was achieved at 1943 UTC on December 31, which in addition to the upcoming New Year’s celebration was also deep into the countdown for New Horizons’ epic flyby of MU69, the Kuiper Belt object widely known as Ultima Thule.

Image credit: Heather Roper/University of Arizona.

I suppose the classic case of mission eclipse was the Voyager flyby of Uranus, which occurred on January 24, 1986. I was flying commercial students in a weekend course four days later in Frederick, MD and anxious to hear everything I could about the flyby, its images and their analysis, but mid-morning between flights I learned about the Challenger explosion, and the news for days, weeks, was filled with little else. Now, of course, we can study the striking images of Uranus’ rings and the tortured moon Miranda, putting them in the great context of Voyager exploration, but for a time the story was mute.

OSIRIS-REx has a long period of mapping and sampling ahead of it, with the sample site selection gearing up, and we’ll have plenty to say about it in coming weeks. Ponder that the spacecraft orbits Bennu from a distance just 1.75 kilometers from its center, a tighter value even than Rosetta’s, when it orbited 7 kilometers from the center of comet 67P/Churyumov-Gerasimenko.

From OSIRIS-REx flight dynamics system manager Mike Moreau (NASA GSFC):

“Our orbit design is highly dependent on Bennu’s physical properties, such as its mass and gravity field, which we didn’t know before we arrived. Up until now, we had to account for a wide variety of possible scenarios in our computer simulations to make sure we could safely navigate the spacecraft so close to Bennu. As the team learned more about the asteroid, we incorporated new information to hone in on the final orbit design.”

Using 3-D models of Bennu’s terrain created from OSIRIS-REx’s recent global imaging and mapping campaign, the mission team intensifies its navigation survey, and will analyze changes in the spacecraft’s orbit to study the minute gravitational pull of the object, which should tighten up existing models of not just the gravity field, but Bennu’s thermal properties and spin rate. By the summer of 2020, controllers will be ready for the spacecraft to touch the surface for sampling operations, with the sample scheduled for return to earth in September of 2023.


{ 9 comments… add one }
  • Bruce D. Mayfield January 2, 2019, 14:28

    Those pics of Bennu we marveled at before where from about 24 km. Now the probe is only about 1.5km from its surface! It’s orbital period is about 62 hrs. The orbital speed is is therefore 2Pi*1.75km/62 hrs., which if I’ve crunched correctly amounts a speed relative to Bennu’s center of less than 5cm/second ! (compare to NH’s wiz by of UT at over 14km/s)

    The upshot: Expect absolutely eyepoping closeups of Bennu!

    • Robin Datta January 2, 2019, 17:53

      Closeups? Eyepopping? Maybe a stray plastic water bottle or shopping bag – maybe with indecipherable writing?

  • ljk January 3, 2019, 11:11

    Emily Lakdawalla • December 31, 2018

    News brief: OSIRIS-REx arrives in orbit at Bennu

    Today at 19:43 UTC, OSIRIS-REx entered orbit at asteroid Bennu. In so doing, it accomplished both the tightest orbit (at an altitude under 2 kilometers) and the orbit of the smallest object ever. Read more at the OSIRIS-REx website.

    UPDATE: At a New Horizons panel discussion during the public event leading up to their flyby of Kuiper belt object 2014 MU69, OSIRIS-REx team member Olivier Barnouin shared a wealth of early OSIRIS-REx science results. I collected them all in a Twitter thread beginning here.


  • ljk January 11, 2019, 11:25

    Steam-Powered Asteroid Hoppers Developed through UCF Collaboration

    By using steam rather than fuel, the microwave-size spacecraft prototype can theoretically explore celestial objects “forever.”

    By Zenaida Gonzalez Kotala

    January 10, 2019


    • Bruce D. Mayfield January 12, 2019, 14:16

      Thanks for posting that link ljk. What a useful exploratory tool that may become! Nice collaboration effort too. With gov and academic involvement in addition to the private sector we can hope that discoveries might be published instead of just becoming business secrets.

  • ljk January 30, 2019, 17:18

    Jason Davis • January 30, 2019

    OSIRIS-REx stable in Bennu orbit, team refines sample collection plans

    NASA’s OSIRIS-REx spacecraft is performing well as mission controllers monitor its extremely close orbit around asteroid Bennu. Meanwhile, mission principal investigator Dante Lauretta tells The Planetary Society his team has yet to find a sample collection site on the rocky asteroid that matches their pre-arrival expectations, meaning their plans will probably have to change.


    To quote:

    There are no science operations planned during Orbital A; this phase of the mission is designed to give the navigation team experience flying the spacecraft. And fly it they have! The orbit is so stable, OSIRIS-REx hasn’t yet had to tidy up its trajectory with any thruster firings.

    Lauretta said there was skepticism back in OSIRIS-REx’s design phase over whether the spacecraft could actually orbit a world so small. Bennu is just 500 meters across. Its gravity is so weak, it’s the same order of magnitude as the force from solar radiation pressure — the same force that pushes a solar sail like LightSail 2. To stay in orbit, OSIRIS-REx must orient itself so that sunlight pushes on it evenly as the spacecraft circles Bennu. It does this by circling the asteroid perpendicular to incoming solar photons, constantly flying over the terminator, where day transitions to night on the asteroid. That’s why those Orbital A pictures of Bennu (and all other photos that OSIRIS-REx will take during Orbital A) show Bennu lit from the side.

  • ljk March 12, 2019, 9:56

    Emily Lakdawalla • February 28, 2019

    Fun With a New Data Set: The OSIRIS-REx Earth Flyby

    The OSIRIS-REx team recently issued their first data release to the Planetary Data System. This release doesn’t include any closeup pictures of asteroid Bennu, but it does include all the pictures they took during their September 2017 Earth flyby. Looking at familiar targets like Earth and the Moon is helpful for getting into a new data set. This is true for amateurs and professionals both — read Vicky Hamilton’s article for The Planetary Report on how the Earth flyby helped the science team.


Leave a Comment