[meteorite-list] Son of Rosetta?

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Fri, 16 Nov 2007 00:06:22 -0800
Message-ID: <GOEDJOCBMMEHLEFDHGMMMEMBDGAA.mojave_meteorites_at_cox.net>

Hi Larry and List,

Just when the "asteroid" Rosetta case of mistaken identity was finally
starting to settle down, things took a turn for the bizarre earlier
today. Another object has been discovered on a very similar trajectory
trailing Rosetta (see Minor Planet Electronic Circular MPEC 2007-V119
for object 2007 VF189).
Link: http://cfa-www.harvard.edu/mpec/K07/K07VB9.html

It was picked up November 12th at Mt. Lemmon. Compare the orbital
elements of it with those of Rosetta when it was mistakenly reported
as minor planet 2007 VN84 (designation now retired):

2007 VF189 Earth MOID = 0.0014
AU
Epoch 2007 Oct. 27.0 TT = JDT 2454400.5 MPC
M 302.93114 (2000.0) P Q
n 0.75186100 Peri. 84.69614 -0.72209792 -0.68599714
a 1.1977817 Node 51.95276 +0.58124996 -0.67166613
e 0.3857965 Incl. 6.51444 +0.37513076 -0.27977230
P 1.31 H 28.3 G 0.15 U 6

Orbital elements:
Rosetta Earth MOID = 0.0001
AU
Epoch 2007 Oct. 27.0 TT = JDT 2454400.5 MPC
M 302.66563 (2000.0) P Q
n 0.76181070 Peri. 79.69236 -0.65353221 -0.75645089
a 1.1873297 Node 51.14851 +0.68070296 -0.60243373
e 0.3412776 Incl. 1.91562 +0.33096698 -0.25466771
P 1.29 H 26.3 G 0.15 U 9

The main difference between these two is ~4.6 degrees in orbital
inclination. Nevertheless, the coincidence was a bit of an eye-opener
when the MPEC was issued this morning: two objects that passed inside
the Moon's orbit on similar trajectories in the space of a few hours!

One possibility that was initially considered was that the trailing
object could be the Ariane 5 upper stage that deployed Rosetta in
March 2004. But the chances of this are extremely remote, given
that Rosetta had undergone two gravity assist fly-bys (one of earth,
one of Mars) ~prior~ to the most recent earth flyby this past Tuesday.

In order for the object to be associated with Rosetta, it would have
to have been shed more recently -- certainly after the first earth
flyby in 2005, and probably after the Mars gravity assist earlier
this year. And yet, if it had, the inclinations would match
better.

So, crazy as it sounds, the second object appears to be just a
"rock" ... or maybe it's a probe launched by the curious inhabitants
of 67P/Churyumov-Gerasimenko... ;-) --Rob

-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com]On Behalf Of
lebofsky at lpl.arizona.edu
Sent: Tuesday, November 13, 2007 7:39 AM
To: Sterling K. Webb
Cc: lebofsky at comcast.net; Meteorite List; mexicodoug at aol.com
Subject: Re: [meteorite-list] Rosetta gravity assist flyby


Hi Sterling, et al.:

The "asteroid" (Rosetta) was discovered near midnight on November 7 and
was confirmed the next night at 2 other sites.

I did a calculation of size vs magnitude for the "asteroid" at discovery.

Its H magnitude (how bright it would be at 1 astronomical unit, 1 AU, from
the Earth) was 26.3. That would make it 23 meters in diameter with a 10%
reflectivity (gray). The darkest asteroids reflect 5% of the light the
hits them which would give a diameter of about 30 to 35 meters.

At the time of discovery, it was 0.04 AU from the Earth (about 6,000,000
km) was magnitude 19.7 (about 1,000,000 times fainter than the faintest
stars one can see with the naked eye) and was moving at a little less than
2 arc-minutes a day (mostly north to south).

The diameter of the Moon is 30 arc-minutes (1/2 degree) for comparison. It
turns out that the main belt asteroid Ceres was in the same area of the
sky and was moving about 1/2 as fast north to south, but 15 times faster
west to east at this time. Why the difference? Ceres is moving in its
orbit around the Sun while Rosetta was aiming right at the Earth (nearly
so), so even though is was much closer to Earth, it was going almost
directly toward us! (if an object is getting brighter but with no apparent
motion, duck!)

The whole idea behind discovering Earth-approaching asteroids is to find
them not when they come by the first time (not much you can do about them)
but to get an "early warning" for when it might be coming by the next
time, as in the case of Apophis. For comparison, I think that Apophis was
moving at several degrees a day at the time of discovery. At that point
you have a chance to do something about it (beyond just running for
cover).

So, to answer Sterling's question, VN84 was not discovered because of its
fast motion OR brightness, but more for how slow it was moving west to
east relative to its north-south motion! I will try to track this down.

Larry
Received on Fri 16 Nov 2007 03:06:22 AM PST