Why You Should Care About Asteroid 101955 Bennu

Selection of radar images of asteroid 101955 Bennu, formerly known as 1999 RQ36, made with the Goldstone radio dish in Goldstone, Calif. in 2010. It doesn’t look like much but we’ll soon know it intimately. Credit: NASA

In September 2016 NASA will launch a very special mission. The target: near-Earth asteroid 101955 Bennu. The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security– Regolith Explorer (OSIRIS-REx) mission is to return the first pristine samples of carbon-rich material from the surface of a primitive asteroid.

“Primitive” in regard to asteroids means original material formed at the beginning of the solar system that’s been little altered by heat or pressure. Look around you and you won’t find these sorts of rocks on Earth. Most have been reworked and recycled through water and wind erosion and the great engine of plate tectonics.

While there are plenty of primitive asteroids out there, Bennu stands out as the most easily accessible one by space probe.

Contest winner Michael Puzio

Third grader Michael Puzio from North Carolina named it after an Egyptian mythological bird associated with the sun, creation and rebirth. A fitting name given Bennu’s origin at the time the clock first starting ticking in our planetary system.

Puzio was one of more than 8,000 students from around the world who entered the “Name That Asteroid” contest sponsored by the University of Arizona, the Planetary Society and LINEAR Project.

Illustration showing OSIRIS-REx in orbit around asteroid Bennu. Credit: NASA

With a girth of about 1,600 feet (0.5 km) Bennu’s a larger than average Earth-approaching asteroid. It orbits around the sun every 436 days (1.2 years) and passes close to the Earth every six years.

Why should you care about this hunk of rock? During those close approaches, Bennu comes within 278,867 miles (448,794 km) of our blue orb. Calculation of its future orbits indicate that Bennu has one of the highest impact probabilities in the next few centuries of any known asteroid.

The OSIRIS-REx spacecraft will extend a sampling device to gather a little over 2 ounces (60 g) of Bennu’s soil called regolith. Credit: NASA

Currently there’s a 1 in 1800 chance of impact with Earth on September 24, 2182. Another study by mathematician Andrea Milani reveals eight potential hit opportunities between 2169 and 2199. Little Bennu could turn into an angry bird.

Not only do scientists need to study the asteroid up close to gain a deeper understanding of the origin and evolution of asteroids and planets, but also to precisely determine its orbit and composition. In particular, we need to know how solar heating affects Bennu’s orbit. Something as simple as baking in the afternoon sun can have dramatic consequences over an asteroid’s lifetime.

Heat radiating from the hot “afternoon” side of an asteroid acts like a gentle rocket thrust that gradually changes an asteroid’s orbital speed. It’s called the Yarkovsky Effect. Because Bennu rotates from east to west instead of west to east like the Earth, the thrusting action causes its orbit to shrink inward toward the sun. Credit: NASA

Heat absorbed by Bennu during the day radiates back into space, pushing back against the asteroid like a rocket thruster. All those tiny pushes add up over the centuries, causing Bennu to slow down and its orbit to shrink. Whether a shrinking orbit will ultimately reduce or increase the probability of an impact, no one knows. That’s why getting up close will help nail down Bennu’s future amblings.

OSIRIS-REx is set to launch in September 2016 and reach the asteroid in October 2018. Once in orbit, the probe will map and study the surface from a distance of between 3 and 0.4 miles (5 and 0.7 km) for 505 days.

OSIRIS-REx will release the sample canister for re-entry back into Earth’s atmosphere and landing by parachute in Utah in September 2023. Credit: NASA/Goddard/University of Arizona

During this time the probe will extend the SAM or Sample Acquisition Mechanism down to the surface and collect at least 2.1 ounces (60 grams) of pristine material. The sample will be returned to Earth in a capsule that will float down by parachute over the Utah desert in September 2023.

Fragments of the Sutter’s Mill carbonaceous chondrite that fell in Sutter’s Mill, Calif. on April 22, 2012. Credit: NASA

Based on studies from Earth, Bennu most resembles the carbon and organic chemistry-rich carbonaceous chondrite meteorites. The Sutter’s Mill meteorite that fell in California last April is similar to what we expect for Bennu – dark, crumbly and rich in ancient clays and organics.

That chip-off-an-asteroid we could handle – it broke up harmlessly in the atmosphere. If Bennu came to Earth and remained intact, well, that would be another thing altogether.

19 Responses

  1. Bob Akers

    Astro Bob,
    I, another AstroBob, find your blog absolutely amazing. Keep up the great work. I just ran across the site by accident, but will be revisiting when time permits (are you retired??)- I’ve got it bookmark now!

    1. astrobob

      Hi Bob,
      Well, nice to meet you. Thanks for the kind words about the blog. There are times I wish I were retired but no, I’m still holding down a job until the aliens finally abduct me.

  2. Lynn

    Hi Bob, I know this blog is from a while ago but I was just looking up some of your old blogs and was wondering if this is similar to your other blog you done in 2011 and it’s just the name that’s changed as it was 1999 RQ36, thanks Bob

        1. astrobob

          Hi Rocky,
          One in 800 chance of hitting Earth in 2182. Predictions often change as the asteroid’s orbit is further refined. Usually, the chance of hitting Earth is even less likely to happen the more we study the asteroid’s orbit.

    1. astrobob

      The probability of asteroid 101955 Bennu striking Earth is extremely small. I suppose it could be struck by another asteroid and have its orbit changed. If that were to happen – and it’s not predicted or foreseen by anyone at the moment – Bennu could just as likely be knocked on a path that would pose even less threat to Earth than it does now. So yes, it’s possible but that doesn’t guarantee that Earth would be in any danger.

  3. Steve

    I see in your recent reply to another reader you indicate 1 in 800 while above it was 1 in 1800; has the probability changed recently based on new calculations or was it a typo?


    1. astrobob

      Hi Steve,
      Whoops! I was right the first time. I missed the “1” in my reply. Thanks for pointing that out. Yes, there’s presently a 1 in 1800 chance of an impact in 2182.

      1. Steve

        Thanks Bob.

        Nice blog – read most of it last night. Heavy cloud cover in East Central Wisconsin, so no sunset eclipse for us unfortunately.


  4. jorge

    hi i read SOMEWHERE that the bennu would be coming near us in this month of september do u have any other updates as far of the impact time frame? and is there a web site i could see it?
    thnak you and keep up the great work

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