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2011MD

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  • Andrea Milani
    The passage of 2011 MD on 27 June 2011 was such a close approach that the orbit was significantly affected by the shape of the Earth; the effect was also
    Message 1 of 5 , Jul 5, 2011
      The passage of 2011 MD on 27 June 2011 was such a close approach that
      the orbit was significantly affected by the shape of the Earth; the
      effect was also amplified by the very low velocity (the asymptotic
      approach velocity to the Earth's sphere of influence was about 1.5
      km/s). These subtle effects do not change very much the orbit before
      the closests approach, but have dramatic effects on the post-encounter
      orbit. Moreover, this object has been very extensively observed
      immediately before the closest approach (more than 1000 observations)
      and also after reappearing on the other side of the Earth, 34 times in
      the first night and 35 times later.

      Our NEODyS system, which tries to automatically improve the NEA orbits
      by using all the new observations, has not been able to cope, in
      particular it has discarded all the post encounter observations, with
      residuals in some cases exceeding 20 arcsec. A simple order of
      magnitude computation shows that the perturbation due to the
      oblateness of the Earth (the C_20 term in the gravity potential
      expansion) was enough to generate the residuals of the post-encounter
      observations. The effects of lesser deviations of the shape of the
      Earth from a sphere, such as the ellipticity of the equator (C_22 and
      S_22 terms), are on the contrary negligible with respect to the
      observational accuracy.

      Thus we have implemented a model of the Earth gravity field including
      oblateness, which kicks in only when the distance from the geocenter
      is less than 0.001 AU, and recomputed the fit to the observations. Now
      the residuals of most of the observations after the encounter are
      compatible with the observational error. There is a signifcant number
      of observations still discarded, but about half of them should be due
      to timing errors of the order of 1 second, which give large residuals
      when the object is moving at 2.5 arcsec/s in RA and 4 arcsec/s in DEC,
      as it was the case for the last pre-encounter observations. These
      residuals are available, as usual, from the NEODyS service, at

      http://newton.dm.unipi.it/neodys2/index.php?pc=1.1.7.1&n=2011MD&ab=7

      We have also performed our usual impact monitoring, checking for
      possible collisions of 2011 MD with Earth after the encounter but
      before 2090, and found nothing.

      The NEODyS team

      Andrea Milani, Fabrizio Bernardi, Davide Farnocchia and Giovanni B.
      Valsecchi


      ================================================
      Andrea Milani Comparetti
      Dipartimento di Matematica
      Piazzale B. Pontecorvo 5
      56127 PISA ITALY

      tel. +39-050-2213254 fax +39-050-2213224
      cellular phone +39-349-4482751
      E-mail: milani@...
      WWW: http://adams.dm.unipi.it/~milani/
      ================================================
    • Gerhard Dangl
      Hello all, five 2011 MD video sequences consisting of more than 1000 single CCD images, captured in Nonndorf (C47). Integration time was 5s.
      Message 2 of 5 , Jul 5, 2011
        Hello all,

        five 2011 MD video sequences consisting of more than 1000 single CCD
        images, captured in Nonndorf (C47). Integration time was 5s.
        http://www.dangl.at/2011/2011_md/2011_md.htm#video

        Regards
        Gerhard
        www.dangl.at
      • Luca Buzzi
        Hi Andrea, I would like to ask what impact have the huge amount of observations made by code I27 to the orbital solution. I mean, do the residuals change if
        Message 3 of 5 , Jul 5, 2011
          Hi Andrea,
          I would like to ask what impact have the huge amount of observations made by
          code I27 to the orbital solution.
          I mean, do the residuals change if you remove all those observations? Do
          they create some sort of bias?
          Of course I really have nothing with I27 observations, I only ask what
          impact have a station that submit 1000 measures of the 1500 total.

          Best regards,
          Luca Buzzi
          # 204




          -----Messaggio originale-----
          Da: mpml@yahoogroups.com [mailto:mpml@yahoogroups.com]Per conto di Andrea
          Milani
          Inviato: martedi 5 luglio 2011 15.45
          A: Minor Planets Mailing List
          Oggetto: {MPML} 2011MD




          The passage of 2011 MD on 27 June 2011 was such a close approach that
          the orbit was significantly affected by the shape of the Earth; the
          effect was also amplified by the very low velocity (the asymptotic
          approach velocity to the Earth's sphere of influence was about 1.5
          km/s). These subtle effects do not change very much the orbit before
          the closests approach, but have dramatic effects on the post-encounter
          orbit. Moreover, this object has been very extensively observed
          immediately before the closest approach (more than 1000 observations)
          and also after reappearing on the other side of the Earth, 34 times in
          the first night and 35 times later.

          Our NEODyS system, which tries to automatically improve the NEA orbits
          by using all the new observations, has not been able to cope, in
          particular it has discarded all the post encounter observations, with
          residuals in some cases exceeding 20 arcsec. A simple order of
          magnitude computation shows that the perturbation due to the
          oblateness of the Earth (the C_20 term in the gravity potential
          expansion) was enough to generate the residuals of the post-encounter
          observations. The effects of lesser deviations of the shape of the
          Earth from a sphere, such as the ellipticity of the equator (C_22 and
          S_22 terms), are on the contrary negligible with respect to the
          observational accuracy.

          Thus we have implemented a model of the Earth gravity field including
          oblateness, which kicks in only when the distance from the geocenter
          is less than 0.001 AU, and recomputed the fit to the observations. Now
          the residuals of most of the observations after the encounter are
          compatible with the observational error. There is a signifcant number
          of observations still discarded, but about half of them should be due
          to timing errors of the order of 1 second, which give large residuals
          when the object is moving at 2.5 arcsec/s in RA and 4 arcsec/s in DEC,
          as it was the case for the last pre-encounter observations. These
          residuals are available, as usual, from the NEODyS service, at

          http://newton.dm.unipi.it/neodys2/index.php?pc=1.1.7.1&n=2011MD&ab=7

          We have also performed our usual impact monitoring, checking for
          possible collisions of 2011 MD with Earth after the encounter but
          before 2090, and found nothing.

          The NEODyS team

          Andrea Milani, Fabrizio Bernardi, Davide Farnocchia and Giovanni B.
          Valsecchi

          ================================================
          Andrea Milani Comparetti
          Dipartimento di Matematica
          Piazzale B. Pontecorvo 5
          56127 PISA ITALY

          tel. +39-050-2213254 fax +39-050-2213224
          cellular phone +39-349-4482751
          E-mail: milani@...
          WWW: http://adams.dm.unipi.it/~milani/
          ================================================





          [Non-text portions of this message have been removed]
        • Bill J Gray
          Hi Andrea, This is an interesting point! Have there been any previous natural objects (except planetary satellites) that needed J2? The effect is indeed quite
          Message 4 of 5 , Jul 5, 2011
            Hi Andrea,

            This is an interesting point! Have there been any previous
            natural objects (except planetary satellites) that needed J2?

            The effect is indeed quite large. By default, I have J2,
            J3, and J4 "turned on" (for the earth, Mars, and gas giants;
            I've never needed them for asteroids, but have needed them
            for natural and artificial satellites.) That gave me an RMS
            of .505 arcseconds for 2011 MD. Prompted by your post, I
            recompiled with J2 disabled, and got 1.139 arcseconds, with
            lots of five-arcsecond outliers. So it matters a lot for
            this object, at least. And that's without even trying to fit
            in radar data.

            Luca, you're right that having two-thirds of the observations
            from one observatory is apt to bias things. I dunno quite how
            to handle that issue. Seem as if the Thing To Do would be to
            assign a lower weight to observatories that contribute massive
            amounts of astrometry, but I don't know what that weight should
            be. And if that observatory took 1000 observations on one night
            and three ten nights later, the first set of observations should
            be assigned a lower weight than the second set. I _think_ OrbFit
            handles this somewhat; I should probably borrow some ideas from
            that software, as I have before.

            I also think OrbFit handles the concept of uncertainty in the
            time of observation, which would be really important for 2011 MD.
            Most of the near-perigee observations have huge errors along-track,
            with small errors cross-track.

            -- Bill
          • De queiroz Jose
            a short animation of 2011MD . 72 frames at each 1 sec. http://www.youtube.com/watch?v=UL4d9fyLvxg&feature=digest a happy weekend to all of you cheers jose
            Message 5 of 5 , Jul 9, 2011
              a short animation of 2011MD . 72 frames at each 1 sec.

              http://www.youtube.com/watch?v=UL4d9fyLvxg&feature=digest

              a happy weekend to all of you
              cheers jose
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