May 16 2013

NASA picks Near Earth Object for return sample mission

NASA has announced the prime candidate for an ambitious unmanned mission that will visit a Near Earth Asteroid and return a sample of it to earth. The winner is 101955 Bennu, a member of the Apollo group of asteroids roughly half a klick in diameter:

NASA Homepage — Bennu could hold clues to the origin of the solar system. OSIRIS-REx will map the asteroid’s global properties, measure non-gravitational forces and provide observations that can be compared with data obtained by telescope observations from Earth. OSIRIS-REx will collect a minimum of 2 ounces (60 grams) of surface material.

“The entire OSIRIS-REx team has worked very hard to get to this point,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson. “We have a long way to go before we arrive at Bennu, but I have every confidence when we do, we will have built a supremely capable system to return a sample of this primitive asteroid.”

This thing has been cooked for eons, so probably not much left in the way of volatiles, the kind of stuff you’d find on mint condition comet nuclei. The advantage in going for an Apollo ‘roid over a traditional member of the asteroid belt or a distant comet is it doesn’t take anywhere near the delta V most interplanetary objects demand. That’s a big deal when heading out and matching orbits, it’s a humongous deal when coming back with the sample. Starting out the vehicle has lots of stages, lots of fuel, but much of that is used up punching through our atmosphere, getting off the earth and reaching the object. What’s usually left over is just a dot at the top of that big stack, not much room for fuel left. Low delta V and low surface gravity are two critical considerations in a return mission.


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  1. 1

    Thanks for mentioning Bennu!
    A correction:
    It has been the target for the OSIRIS-REx mission for quite some time now, since well before the mission was given final approval by NASA in 2011. The new thing is that Bennu now has its name, as opposed to its catalog designation 1999 RQ36.
    Bennu does have some volatiles in its composition, just not as ice – water and carbon compounds relatively weakly chemically bonded to the silicates are stable at temperatures <200 C or ao.

  2. 2
    steve oberski

    I’m wondering what sort of “non-gravitational forces” 101955 Bennu might possess ?

    Centrifugal force, electrostatic force ?

  3. 3

    Interesting question! Wikipedia knows all: they’re interested in the Yarkovsky Effect.

  4. 4
    Stephen "DarkSyde" Andrew

    Wow, thanks Timber, learn something new every day. That’s one more tiny chaotic input into a very complex n-body system system which would make precise long term predictions, the kind critically important to monitoring NEO behavoir, even more difficult.

  5. 5

    I read an article a few years ago in which an astronomer had plotted rates of revolution against mass for a large number of asteroids. He said that if they were single big rocks, the upper limit to rotation rates would be the tensile strength of a rock. But if they were piles of dirt, the upper limit would be the one at which such a pile would fly apart and no longer be a pile. The research showed that there are no fast-spinning asteroids. They’re all piles of rocks and dirt. This means that it would be very hard to attach a rocket to one and thus adjust its orbit.

    Whodathunk that painting an asteroid could be far more effective?

  6. 6

    Huh, thought the title was referring to Paul Fidalgo. I was interested what samples they might have been trying to collect.

    I suppose this is cooler.

  7. 7

    @Stephen @4:
    Yes, the Yarkosky radiation-pressure forces do make predicting asteroid trajectories quite hard. Fortunately, with good enough data we can measure the Yarko acceleration directly. We have a fairly good measurement for Bennu, thanks to several epochs of radar ranging (I say ‘we’, although I personally have only had a little involvement in that project – the group at Arecibo did all the work). For other objects, we can model Yarko fairly well, if we know the object’s size, shape, and spin state.
    @timberwoof @5:
    Not quite. Almost all near-Earth asteroids are indeed rubble piles, but they aren’t entirely lacking in cohesion. Objects smaller than ~100-200 m (so, quite a bit smaller than Bennu) have enough internal cohesion that they can hold themselves together even when spinning significantly faster than the strengthless-spin limit. Larger objects, like Bennu, don’t spin faster than once every 2 hours, because the rubble piles don’t have enough cohesion to handle the stresses from spinning much above the strengthless limit.
    There are three preferred methods to change an asteroid’s trajectory. 1. Hit it with another object going as fast as you can. This is good for sudden changes to trajectory, but isn’t well-controlled. 2. Put a spacecraft near the object and hover, angling the rocket exhaust such that it misses the rock. Its gravity pulls on the spacecraft, the spacecraft’s gravity pulls on it, and the whole thing moves slowly in the direction you want to go. This is called a gravity tractor, and is good for fine control and very gradual changes to trajectory. 3. For small asteroids, put the asteroid inside a vectran bag and pull it tight. Then light up the rockets (some variation of solar-powered electric propulsion). This is more rapid than the gravity tractor, but only works for small objects – such as the notional target of the proposed Asteroid Retrieval Mission.
    Painting an object with a grenade inside a sack of powdered aluminum is possible, and would slowly change its trajectory, but the gravity tractor has far better control.

  8. 8
    StevoR : Free West Papua, free Tibet, let the Chagossians return!

    Wonderful news – hope we see this mission happen and all goes nominally.

    @5. timberwoof :

    The research showed that there are no fast-spinning asteroids.

    We-ell, there are a few – see :


    Excerpt :

    It is the second-fastest spinning asteroid ever seen, astronomers say. The space rock, called asteroid 2010 WA, flew within 24,000 miles (38,000 kilometers) of Earth Tuesday night (Nov. 16). ..(snip) .. “We measured the rotation rate of the asteroid at about 31 seconds,” astronomer Eileen Ryan, the observatory’s director, told SPACE.com in an e-mail. “This makes it the second fastest rotating asteroid discovered to date.” The fastest spinning asteroid currently known is an asteroid called 2010 JL88, which spins once every 24.5 seconds and was also discovered using Magdalena Ridge Observatory’s telescope, Ryan said.

    In addition to :


    The pattern revealed that the asteroid revolves once every 42.7 seconds, making it a “superfast rotator”. The International Astronomical Union has since confirmed the discovery. The rotation rate of 2008 HJ smashes the record set by the previous holder, 2000 DO8, discovered eight years ago. That asteroid rotates around half as fast, once every 78 seconds.

    (Hmm .. yeah I know these seem to conflict. Records change / findings revised?)

    But, generally, yeah, see :


    Although a recent article in the April 2013 issue of ‘Australian Sky &Telescope’ magazine – by Michael Shepard titled Why do Asteroids come in pairs?” – suggests that one reason many asteroids come in pairs or /& have moonlets is due to them rotating rapidly and spinning up with the YORP effect and then fissioning into two or more components.

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