[neo_followup] C1971M1 = 1964-054A = OGO-1

[Bill Gray]

Hello all,

    (Just got back from a couple of days of camping with my wife,
outside the range of Internet availability.)

    This is definitely 1964-054A.  I should have posted something
about it;  we've known for a few years now that it would re-enter
the earth's atmosphere sometime around 2020 Aug 31 :

https://www.projectpluto.com/pluto/mpecs/64054a.htm

    Note that the prediction was made in early 2018,  based on
observations from 2016 to 2020.  I do have further observations,
mostly from Goran Gasparovic,  for as late as August 18.  Based
on those,  it was still difficult to tell when it would re-enter;
there were a few "skimming" passes causing the apogee to drop,
but there was a lot of uncertainty as to how much.

    The problem is that Find_Orb computes an area/mass ratio for
the object based on how much it's pushed around by sunlight.
This dodges around effects caused by the object shape and
thermal properties (a really shiny object that did a good job
of reflecting most sunlight back where it came from would
appear "lighter" than a similar object that absorbed most
of the sunlight,  then re-radiated it in all directions.)

    The object then dips into the earth's atmosphere,  and
then I start modelling drag effects as if it's a sphere with
that AMR,  and using a "standard atmosphere model".  The
overall effect is about right,  and you see the object lose
speed and spiral in after a couple of orbits,  but the
specifics are not very specific.

    We've had a couple of other objects do the same thing
(such as 1977-093 a year or two ago,  mentioned at the
above URL),  and there are a few others expected to re-enter
in the next year or two.

    If you look at the plot of the perigee height of 1964-054A
since launch in 1964 to the present...

https://www.projectpluto.com/pluto/mpecs/64054a.pdf

    ...you'll see that the perigee (red line) came really
close to scraping the upper atmosphere in late 1980,  and
had various rises and falls over the subsequent decades.
This most recent "peri-perigee" was just a little too low.

    I'll post the rest of the data for this object at the
above URL and further comments once I catch up on various
things that erupted during my vacation.

-- Bill