[neo_followup] C26FED2 = interesting object

Sun Feb 16 10:11:04 EST 2020

On 2/16/20 1:20 AM, Sam Deen wrote:
> I'm leaning on rock, personally.

    I expect to remain in the "insufficient data" camp for
quite a while.  It's unlikely that we'll get enough
astrometry to tell.

> The current orbit gives an Area/mass ratio of 2.8 +/- 8.0 m^2/kg, which obviously is very far from conclusive, 

    Extremely inconclusive.  The object could have the effectively
zero AMR of a rock.  Or it could have the 5.6 m^2/kg of an empty
trash bag object,  and the residuals would be the same.  Or
anywhere in between and still fit the available data.

    We're up to 19 observations as I write this,  which has
brought the AMR to 0.13 +/- 0.51.  If it's artificial,  an
AMR of,  say,  0.02 would probably be in the right ballpark
for a piece of junk this size.  I don't think it's an ETBO,
because something that light would have a tough time staying
in orbit around us;  solar radiation pressure would push
it out into an heliocentric orbit... actually,  then into
interstellar space;  ETBOs are that light.

but seems to point towards the object not being artificial. For comparisons, here are a few AMRs according to Bill's distant artsat pseudo-mpec catalog:

    Yes,  but we also have,  for example,  ZTF00V9 = ZTF00Vv
= ZTF00Y5 = ZTF00Yq,  with an area/mass ratio of 107 +/- 3
m^2/kg :

https://www.projectpluto.com/pluto/mpecs/ztf00v9.htm#etbo

    and many in between.  I've learned,  mostly over the last
couple of years,  that artsats come with a fairly bewildering
range of area/mass ratios.  (I'd previously assumed such high
AMRs must be bogus,  and that I had bad data or data for
two or three different objects,  or that it was maneuvering.
Once I started looking for ETBOs,  I realized they were
everywhere.  One,  'Multijunk',  has been a "discovery" on
NEOCP at least two dozen times.)

> Im not sure how to use find_orb to force-fit test AMR values,

    You can apply a constraint of,  say,  "A=0.005",  much as
you would constrain "e=1" if you thought it was a comet or
"a=5.2,e=0" for a circular Jupiter Trojan orbit.

  but I suppose that even with a poorly-defined orbit, being able to demonstrate its AMR is over 0.05 m^2/kg would make it most likely not an artsat in my opinion at least.

    If you find,  say,  AMR = 0.020 +/- 0.001,  _then_ you've
got evidence it's an artsat,  with the uncertainty in the
value significantly less than the value itself.  Proving
something is _not_ an artsat is a lot harder.  Tell me
you've got AMR = 0.001 +/- 0.010,  and I'll just say that
an AMR of 0.01 would fit within that uncertainty,  so it
_could_ be an artsat,  roughly as likely as being a rock.

    Getting an accurate AMR for a rock has happened... in
fact,  for 2006 RH120 (only confirmed temporarily captured
rock),  we got AMR = (98.5 +/- 2)E-5 m^2/kg.  That object,
however,  did three orbits over most of a year,  and we got
a radar observation on it.  Unless we get far more precovery
data than I expect,  astrometry isn't going to tell us if
this is a rock or not.  Radar data might -- for example,
with 2018 AV2,  we not only measured a high AMR;  Arecibo
got a stronger radar echo from it than would be expected
for a rock.  Or spectroscopy,  as happened initially with
J002E3 (it had the spectral signature of titanium dioxide
paint).  Or infrared data;  if the albedo is near 100%,
you start to think about an object with white TiO2 paint.

> 
> Additionally, the aphelion of this thing is seriously impressively large. Its aphelion is recorded at 0.0148 +/- 0.0005 AU, which is larger than the radius of Earth's hill sphere at ~0.010004 AU. I assume find_orb wouldn't be giving a geocentric orbit if it was orbiting faster than escape velocity, though.

    It would,  but with e > 1.  (In Settings,  you can
choose the reference object.  The default 'automatic'
setting gets you a geocentric orbit if the object is
within the earth's sphere of influence,  a selenocentric
orbit if it's with the moon's sphere of influence,  etc.
But you can,  for example,  insist on a Neptune-centric
orbit for an object that is nowhere near Neptune.)

  Running the orbit forwards shows it won't be staying in geocentric orbit for long, at least nominally. The radial velocity will reach its minimum around April 7, at which point I assume the gravitational hold of the sun will start to take over, and it will move back into heliocentric orbit. Running the orbit backwards nominally says it's been in orbit since February 13 2018, putting it as having orbited us for around 2 years by now. Of course, that's massively premature to conclude with only half a day's worth of observations. All I can say for sure is that it's orbiting us
> right now, and was almost surely orbiting us since the November perigee.

    I think there may have been some previous perigees,  but
yeah,  we don't have enough data yet to say.

> 
> In regards to ephemerides:
> 
> C26FED2 would have been above magnitude 23 since February 10, although with a low enough elongation that would have made detection improbable. The first time it really would have been detectable was on February 13 when it peaked around magnitude ~16-17 at an elongation of 124 degrees. It has been slowly dimming ever since, and will remain above magnitude 23 until February 26.
> 
> I would give it good chances of at least a couple other observatories spotting it over the coming days. It's a shame the orbital period is so long, though. Once it stops being visible on February 26, it won't be coming back.. So, the next few days are all we have to figure out more about this weird object. Hopefully some larger scopes will get pointed at it in that time!
> 
> ~Sam
> On Saturday, February 15, 2020, 5:59:52 PM MST, Bill Gray <pluto at projectpluto.com> wrote:
> 
> 
>      With follow-up from (L01) Višnjan,  it's confirmed that this
> is a temporarily captured object.  Whether it's natural or junk
> is unknown.  I _really_ hope someone gets more data for it.  If
> it's a temporarily captured rock,  that's a Really Big Deal.
> 
> -- Bill
> 
> On 2/15/20 1:09 PM, Bill Gray wrote:
>  > Hi folks,
>  >
>  >     This object might just be a satellite.  If so,  it's a
>  > somewhat unusual one,  in a very elongated orbit.  It may be
>  > lunar ejecta passing by us,  perhaps even a temporarily captured
>  > object a la 2006 RH120 = 6R10DB9.  Or it could be heliocentric
>  > junk passing by us,  similar to J002E3 or 2018 AV2.  Further
>  > observations would be a really excellent idea.
>  >
>  > Thanks!               -- Bill
> 
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