Re: What is 1991 VG?

Jim Scotti (jscotti@LPL.Arizona.EDU)
Wed, 17 Jul 1996 15:54:09 -0700 (MST)

comment....  For those who don't remember what 1991 VG was, I discovered
this object on 1991 November 6 with the Spacewatch Telescope on Kitt Peak. 
It was a magnitude V=20.7 object moving with rates typical of a Near-Earth
Asteroid (NEA), namely 0.695 degrees/day of longitude westward and about
0.25 degrees/day of latitude southward.  That's about 0.3 deg/day faster
in longitude than the fastest one would expect a "run-of-the-mill" Main
Belt (MB) asteroid to move, so as soon as I confirmed that object was
real, I knew it was an object of interest - just how interesting came
later!  Since it was moving pretty fast, I assumed it must be somewhat
close (we often see NEAs near aphelion in the asteroid belt but they move
at rates closer to, but still significantly different than the MBs), so on
November 7, I planned to get 2 observations of the object, one set early
and another late to make use of the parallax caused by the Earth's
rotation.  I didn't find it in the first data set where I thought it would
be using both a linear extrapolation of its motion and a Vaisala
preliminary orbit technique, but I assumed I'd find it the next day near
that prediction and went ahead later with the second set of observations. 
Again in the 2nd observatoni set I didn't find it, but luck was going my
and I found what appeared to be a 2nd object about half a degree away with
similar rates.  The following day, I didn't find anything near the
predictions for 1991 VG in either data set, but I did find my new object
in the first data set at least.  I then set out to try and get both
objects on November 8.  My predicted positions for each were close
together in declination and about 10 minutes of Right Ascension apart, so
I could try and get both at the same time, giving plenty of leeway in RA
assuming my first nights object had accellerated.  I had also noted a
change in the rates of motion of my 2nd object, but I assummed that was
just due to an error in the measurements.  When I didn't find anything at
either of the 2 objects predictions, I was very confused!  During the
night, I started playing around with orbits for the 2nd object (since it
had about a 4-5 hour arc), but when nothing productive came of that, I
played a hunch and tried linking my 2 objects and VOILA! a resonable
(though very earthlike) orbit.  I looked at the location it predicted for
my 3rd night's observation and there it was!  Turns out that the object
was only 0.022 AU away and since it was moving along roughly with the
Earth, most of the 0.695 deg/day motion was induced by my own motion
around the Earth. 
   Once we had a preliminary orbit, we found that the object would pass
within about 0.003 AU of Earth in early December!  We were immediately
suspicious that the object might be manmade.  On November 8, I already
suggested to Brian Marsden that perhaps it might be a Saturn IVB stage,
but it was only a wild guess at trying to explain the Earth-like orbit. 
The disconcerting thing is that an object in such an orbit would not last
very long - how could it be natural?  One possibility we discused was tht
it might be a recently escaped Earth-Sun trojan asteroid.  I soon noted
that the archive of last known state vectors for Apollo hardware was not
very helpful in determining where they might be now.  Soon, the orbit was
good enough to say with some precision where it had been over the last 30
years and it.  Marsden integrated the orbit backwards, hoping to improve
the predictions for an attempt to observe 1991 VG with radar by linking it
to a manmade spacecraft.  He found that it had been in the vicinity of
Earth last in about 1973 or 1974, but only got within about 0.07AU.  That
suggested the Helios A spacecraft booster, a Centaur upper stage. 
Jonathan McDowell contacted General Dynamics and found out that that
booster was put back into a Geocentric orbit after dumping the Helios
  So, the debate is still alive, and may continue to be so until someone
can recover it next time it comes back around and then someday we can go
out and take a close look.  Does it have a regolith or a rocky surface or
does it have "USA" or "CCCP" painted on the side?  My guess is that it is
indeed a natural object, but if it is manmade, perhaps it is a Saturn IVB
stage from one of the early Apollo missions.  As I recall, at least one
left the 3rd stage in a high Earth orbit that would have eventually been
perturbed into solar orbit. 
   BTW, we were able to follow 1991 VG until just before its closest
approach on December 5, then we were able to recover it and observe it in
April 1992 when it came back past Earth.  From the photometry we did we
can say that if the objects' albedo is low, around 5%, it is about 19
meters in diameter.  If it is around 20%, then it is about 9 meters in
diameter.  If it were a perfect mirror it could be as small as about 4
meters in diameter. 


On Wed, 17 Jul 1996, Larry Klaes wrote:

> Has anyone yet determined if Object 1991 VG was a rocket body or
> small planetoid?
> Larry

Jim Scotti                              
Lunar & Planetary Laboratory 
University of Arizona                520/621-2717 
Tucson, AZ 85721 USA