[neo_followup] 2021 GK1 = possible junk

[Bill Gray]

Hi folks,

    A follow-up to this post.  We did get two more
tracklets on it on May 4 and 5,  thanks to (703) Catalina
and (Y00) SONEAR (thank you!)  That helped.  It looks as
if there may have been encounters in the late 1960s.
More data would help.  The object is currently going
around at perigee,  headed to lower elongations,  and
will soon be effectively invisible for a while.

    That may be especially true if it's an artsat;  those
seem to get faint more quickly at high phase angles than
rocks do (not always,  but often).  One small incentive for
me to revise Find_Orb to determine G and/or G1 and G2
magnitude parameters is that "oddball" slope parameters
might hint at non-rock-ness.

    I mention this because if you've tried for 2021 GK1
and failed,  or _do_ try for it and don't see it,  I'd
like to hear from you;  it would be added evidence that
this is indeed not a rock.

    A few weeks after perigee,  it'll come back out where
the big scopes can get it at faint magnitudes.  But some
data now,  near perigee,  would fill in a gap in the
orbital arc that might give us a more solid value for
non-gravitational parameters,  and perhaps a good enough
orbit to identify the object.

    Should note that I really ought to have sent this
e-mail yesterday.  The object is down at mag 19.5 already
(if it's a rock),  and will quite soon be at low enough
elongations to make observing difficult to impossible.

Thanks!               -- Bill

On 5/5/21 3:36 PM, Bill Gray wrote:
> Hello all,
> 
>     This is a slightly "slow" object relative to us,  the
> sort of thing that could be either space junk or lunar
> ejecta.  At present,  it's a coin toss.  If it's junk,
> it's rather large (H=27.1,  possibly bigger),  but still
> within the realm of possible junk sizes.
> 
>     The current astrometry solves to an area/mass ratio
> of 0.0054 +/- 0.0046 m^2/kg,  which is just right for a
> returning booster.  But then again,  a rocklike area/mass
> of essentially zero would also fit about one sigma away.
> 
>     Sometimes,  I can run a trajectory backward and say
> something like "this hasn't been in our neighborhood since
> Sputnik 1 went up" or "this passed Earth at a date/time
> suspiciously close to thus-and-such mission."  We have a
> decent arc on this object,  but the uncertainties as to
> where it was become huge after you go back a few decades.
> So I can neither prove nor disprove that it's junk that way.
> 
>     If it had a small MOID with Mars or Venus,  I'd just try
> to run ephems backward to see if there was an encounter
> with one of those planets at the time a mission arrived
> there.  No such luck (and the fact that it doesn't seem
> to be "going anywhere" argues a bit in favor of it being
> lunar ejecta).
> 
>     I've been keeping an eye on the DOUs for more data as
> it gets closer to us.  There hasn't been any new data
> since 1 May,  and we're now at the point where a few more
> observations might bring the AMR uncertainty down quite a
> bit.
> 
>     It's now at about mag 19,  and should be at least somewhat
> visible for another week or so.  It won't require much
> observation,  but if you can get a few precise measurements
> of it during that time,  we might be able to settle the junk
> vs. lunar ejecta question before it leaves our neighborhood.
> 
> Thanks!            -- Bill