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The Second Moons of Earth

Posted by samzenpus on from the that's-no-moon dept.

Hugh Pickens writes "Despite a large body of work on satellite capture by the gas giants, mainly Jupiter and Saturn, there has been little published about the Earth's natural satellites other than the moon. Now Scientific American reports that although the moon has been with us for billions of years, Earth has also had countless other satellite companions and probably has one right now. These 'second moons' are boulders from the large population of near-Earth asteroids that get snagged by our gravity, orbit the Earth for a few months, then escape and move on. Known as 'Temporarily-Captured Orbiters' (TCOs), the irregular natural satellites are hard to see but astronomers spotted one such transient satellite in 2006. Dubbed 2006 RH120, the asteroid was a few meters in diameter, was captured by Earth for about a year and made four Earth orbits before being ejected after its June 2007 perigee back to interplanetary space. But TCOs are not just of academic interest. 'Once TCOs can be reliably and frequently identified early enough in a capture event they create an opportunity for a low-cost low-delta-v meteoroid return mission. The scientific potential of being able to first remotely characterize a meteoroid and then visit and bring it back to Earth would be unprecedented (PDF).'"

14 of 92 comments (clear)

  1. metroid capture by Anonymous Coward · · Score: 4, Funny

    not the best idea... (i think they're dangerous)

  2. Better ideas by vlm · · Score: 5, Interesting

    But TCOs are not just of academic interest. 'Once TCOs can be reliably and frequently identified early enough in a capture event they create an opportunity for a low-cost low-delta-v meteoroid return mission.

    Boring. I'd put a whole freaking base on it while its in earth orbit, then see where it goes. If not a manned base, at least a robot research station. Should be pretty interesting to see where it ends up. At least a radio beacon?

    --
    "Science flies us to the moon. Religion flies us into buildings." - Victor Stenger
    1. Re:Better ideas by Grishnakh · · Score: 3, Informative

      I'd put a whole freaking base on it while its in earth orbit, then see where it goes. If not a manned base, at least a robot research station.

      Did you miss the part in the description that said the last such meteoroid was only a few meters across? Are you going to send Lilliputians up there to build a base on it?

      Should be pretty interesting to see where it ends up. At least a radio beacon?

      I'm sure they can calculate exactly where it'll go once they know enough about its position and velocity. Even if it makes it out of the Solar System, it's going to be rather slow; if you want to send a probe out of the solar system, it's probably a lot faster to just build one and send it out there with rockets. To be captured by Earth's gravity, these things can't be going very fast.

    2. Re:Better ideas by Nyeerrmm · · Score: 5, Interesting

      Actually, trajectories of small bodies like that are quite interesting. Two things stand out to me (I did some of my graduate work looking at missions to small-ish asteroids like Apophis which is ~300 meters, so bigger than this but smaller than large asteroids).

      1. If this is loosely captured by Earth with multi-month orbits it is on the edges of the Earth's sphere of influence where the Earth and the Sun's gravity really interplay in weird ways and small uncertainties in its current state could turn into huge uncertainties later.

      2. For a very small asteroid, the surface-area-to-mass ratio is very high, meaning effects of solar pressure and the Yarkovsky effect will cause it to behave very differently. The ability to track an asteroid like this could greatly inform models of these effects.

      If you could find many of these and have a spacecraft able to rendezvous and deposit a tracker on new ones as we find them, it could greatly benefit studies of near-Earth objects. Of course, a mission to do that sounds extremely challenging (but very interesting to work on).

    3. Re:Better ideas by Anonymous Coward · · Score: 3, Insightful

      What about the n-body problem?

    4. Re:Better ideas by Nyeerrmm · · Score: 3, Interesting

      Understanding the effects for a small asteroid could inform our understanding of how larger asteroids would behave as well, thus serving to help us better predict contintent-killers like Apophis.

      Of course, I'd much rather bring them in closer and mine them, but that would be more difficult, so tracking would probably happen first (and be good practice for eventual capture missions).

      As far as allocation of resources go, that really depends. I'd have to see detailed studies on what a mission like this would cost.

    5. Re:Better ideas by dotancohen · · Score: 5, Interesting

      I'm sure they can calculate exactly where it'll go once they know enough about its position and velocity.

      No, all we know is that it is in a possibly-unstable orbit. If the orbit were simply enough that we could calculate it long term, then it would be stable enough to not be reejected. In fact, landing on the object in order to enjoy it's boost out of Earth orbit would require matching its orbit exactly, which would put the lander on an escape orbit itself. No chuck of iron necessary.

      If you didn't notice from the summary, one item made four orbits in a year, that is one orbit every three months. By Kepler's laws, that means that the object's distance from the Earth is twice the moon's distance. You would already be at escape velocity there, seeing as it just butts up against the Earth's Hill sphere. And all that is assuming a circular orbit, which is very unlikely. More likely, apogee is outside the Hill sphere and the only reason that the object stays in "orbit" is when the apogee is opposite the sun. As soon as the orbit rotates a bit, the object is lost.

      --
      It is dangerous to be right when the government is wrong.
  3. At the risk of being declared a space nut by Dexter+Herbivore · · Score: 4, Insightful

    I really hope this does lead to capture of a metal rich meteoroid, it may cause havoc on the world metal markets but in the long run cheap minerals have to be good for the world economy.

    1. Re:At the risk of being declared a space nut by daem0n1x · · Score: 3, Funny

      I think Heavy Metal is not allowed by the regime, because it's satanic and pollutes youth's minds.

  4. And at least one .. by n5vb · · Score: 5, Interesting

    .. is man-made .. :)

  5. Temporarily captured? by Hatta · · Score: 3

    What causes these things to be only temporarily captured? If they escape orbit by their own momentum, they were never really in orbit in the first place. So either this is a misnomer, or there's some external force causing these objects to leave orbit. What is it? Tidal effects from the moon?

    --
    Give me Classic Slashdot or give me death!
    1. Re:Temporarily captured? by Trepidity · · Score: 4, Informative

      If I understand the paper correctly, there are configurations where the zero-velocity surfaces of the sun and a planet coincide at some points, so an object orbiting the sun can transfer to a planetary orbit at one of the intersections without any other energy input, and then transfer back out again at the same or another intersection, again without any other energy input. Figure 1 on page 8 of the PDF has an illustration of some cases.

      --
      10 PRINT CHR$(205.5+RND(1)); : GOTO 10
    2. Re:Temporarily captured? by Anonymous Coward · · Score: 3, Funny

      That's exactly it. There's a very nice animated gif here of the old Saturn V third stage captured out of the blue a few years ago (mentioned by n5vb earlier in the thread). You can see that every time it gets near the moon, it gets either slowed down or sped up depending on if it comes before or after. On the final and sixth orbit, it comes just behind the moon which slingshots it away completely.

  6. Oblig xkcd by 16384 · · Score: 5, Funny

    http://xkcd.com/307/