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328Re: [find_orb] Congratulations, Bill !!!

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  • Bill Gray
    Oct 26, 2015
      Hi Grant,

      Thanks for the kind words. This has been a fun object to follow.
      (And has resulted in my having to make some improvements to Find_Orb.
      They're not quite ready to post yet, though.)

      Some bits of the story that might be of interest to those doing orbit
      computations... this becomes a bit of a long story! :

      When this object was picked up at Catalina on October 3, it was quickly
      clear that it was in a high earth orbit. I made my usual efforts to match
      it to known objects of that sort, such as the Chang'e 2 and 3 boosters and
      the DSCOVR booster:

      http://www.projectpluto.com/pluto/mpecs/che3.htm
      http://www.projectpluto.com/pluto/mpecs/che.htm
      http://www.projectpluto.com/pluto/mpecs/dscovr.htm

      WT1190F didn't match up with anything already known. The folks at Catalina
      had only gotten about 90 minutes of data, and it was still possible this was
      a rock passing by close and slow, rather than junk. But that would only be
      true if the data was much worse than Catalina usually gets. Junk seemed much
      more likely.

      The nominal orbit looked vaguely similar to one for an object last seen
      in 2013, then lost :

      http://www.projectpluto.com/pluto/mpecs/uda.htm

      So I started looking for a link, with no success. The gap between 2013
      and 2015 was such that the object might have had several lunar flybys in there,
      each adding a bit of chaos to the result. Project forward from the 2013 arc,
      and you could get the next close lunar flyby with decent accuracy. But the
      orbit after that would be just sufficiently uncertain that the _next_ flyby
      would be poorly determined, and the orbit solution for times after that would
      be almost pure garbage. The 2013 orbit was based on just two short arcs of
      observations nine months apart, so it was a little shaky to start with.

      However, in attempting the link, I noticed something interesting.
      When I ran the 2013 orbit forward, I got an impact on 2015 November 28.
      I had a very difficult time believing this, but I e-mailed Steve Chesley
      and Paul Chodas about it. My thought was that they have some better tools
      for tackling this sort of problem, and might be able to get the linkage.
      Once they had the linkage, I could essentially "look in the back of the
      book", see how they did it, and replicate it.

      Meanwhile, I was also hoping for more data. That eventually came in,
      from (K91) and (K92) in South Africa. With that, I noticed WT1190F was
      headed in for an impact. That was without taking solar radiation pressure
      into account (we didn't have nearly enough data to compute an area/mass
      ratio). But when I gave the object a guesstimated AMR, it _lowered_ the
      perigee and made an impact _more_ certain.

      The extra data did the trick, combined with a new orbit determination
      method. (New for me, anyway; it's a slight variation of the Metropolis
      algorithm, which has been around since the 1950s. The application to
      orbit determination may be new, for all I know.) The link with the 2013
      object was suddenly quite solid. (With the data we have now, it's less
      tricky to get the orbit for the entire arc.) Here's a solution that uses
      all the data I currently know about. It requires two non-gravitational
      parameters to get low residuals; the object is very much kicked around by
      sunlight.

      http://projectpluto.com/temp/mpec_all.htm

      Once we had a link, Eric Christensen and Alex Gibbs at CSS started to
      look for precoveries, and Marco Micheli started looking through the
      PanSTARRS archives. The CSS folks came up empty. Marco got a good bit of
      data, including in September of this year and some data in early 2013 and
      in December 2012. But that's probably all we'll get. I can find a lunar
      flyby in June 2012, which makes the orbit a little shaky before that
      point. And then there's one the previous month; go beyond _that_, and the
      orbit becomes almost garbage. It's a bit like looking at a tennis ball and
      saying where it was a couple of hits ago. Which makes it pretty much
      impossible for me to tell Eric and Alex and Marco, "You should try looking
      _here_ in your archives."

      All of which leaves me without much hope of putting an ID to this object.
      There is one faint possibility, which is that the object can be linked to
      an "unknown" from 2009 :

      http://www.projectpluto.com/pluto/mpecs/9u.htm

      Peter Birtwhistle suggested, long ago, that this object might be
      the same as the 2013 object. I wasn't able to get a link then, and I
      still haven't, and they may not actually be the same object. But the
      orbits are so chaotic that it's hard to say. I'm still investigating.

      I had hopes I might identify the object in the same manner as I identified
      the Chang'e boosters, the Chang'e 2 probe, LCROSS, DSCOVR, etc.: I ran
      the orbit backward and noticed that the object went past the moon at the
      same time a lunar mission arrived there. In this case, there aren't any
      missions matching up to the flybys. (Might be if I could go back beyond
      May 2012. But I can't.)

      There are several known objects in this sort of orbit, and I may be
      the only one really keeping track of them. Astronomers generally are
      interested in actual rocks, and the satellite folks are more interested in
      things in geostationary and lower orbits. I've pretty much had these
      high-flying bits of junk to myself. I expect interest in them may be
      a little greater in some circles now.

      Another odd thing about this object is that I've learned it stretches
      the limits of some orbit determination software. I've heard from one
      gent who usually deals with artificial satellites, and who has a much more
      complex model of Earth's gravity in his software than I do. He's modelling
      all the higher-order spherical harmonic effects that matter for low orbits.
      (Find_Orb includes the "J2" term -- the one caused by the earth being oblate --
      the "J3" term caused by it being a little bit pear-shaped, and the next
      J4 term, but nothing more.)

      However, he hadn't dealt with anything high enough for planetary
      perturbations to matter. (You can ignore such perturbations, mostly,
      for low-earth orbits, even for geosynchs.) So he wasn't able to get an
      orbit for this object.

      Another gent who usually deals with asteroids, comets, etc. couldn't
      get it, because the J2 term wasn't included in his software. You can go
      a long way before you find the earth's oblateness mattering for such
      objects. JPL's _Horizons_ system and OrbFit didn't include them until
      2011 MD flew past :

      https://groups.yahoo.com/neo/groups/MPML/conversations/messages/25656

      But it matters quite a bit if you attempt to fit all the WT1190F data;
      the mean residuals grow to about 80 arcseconds if you assume a round earth.
      Even J3 and J4 have a small, but noticeable, effect for WT1190F.

      -- Bill

      On 10/26/2015 10:19 AM, 'Blair, Grant' g.blair@... [find_orb] wrote:
      >
      >
      > Our very own Bill Gray has been mentioned several times in the context
      > of orbital calculations, in a recent article in NATURE.
      >
      > http://www.nature.com/news/incoming-space-junk-a-scientific-opportunity-1.18642
      >
      > Well done, Bill!
      >
      > Grant
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