[meteorite-list] Asteroid 2011 MD Flyby Yields New Thinking

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 9 Jul 2011 14:32:08 -0500
Message-ID: <B2CFBFB2E03E441582CC9DC54351509E_at_ATARIENGINE2>

Melanie, List,

At no point was 2011 MD "captured" by the Earth.
Strongly "tugged at" but never "captured." If it had
been at any point, it would never have escaped.

> why Earth doesn't have any natural satellites
> other than our moon?

A satellite in an orbit outside the orbit of the Moon
would be long-term stable with respect to the Moon's
influence, but since the Moon is a distant satellite,
relatively speaking, to the Earth, orbits further out
run a high risk of having that satellite stripped away.

OK, you say, let's make it CLOSER to the Earth
than the Moon.

The Moon is a big enough object to dominate the
interior of its orbit. And interior object in a different
orbital plane would be tugged and tugged until its
orbit was within or very near to the plane of the
Moon's orbit, and every time the satellite passed
"close" or its closest to the Moon, the Moon's gravity
would tug it outward.

These transfers of energy would "pump up" the
eccentricity of the satellite's orbit. It would rapidly
become more elliptical, coming closer to the Moon
at its apogee and closer to the Earth at its apogee.

I think you can see where this is heading. That can't
keep progressing. Sooner or later, the satellite will
get so close to the Earth that it reaches the "Roche
Limit," where the gravitational tides are greater than
the material strength of the satellite and it breaks
apart into rubble -- The Earth now has a Ring which
will decay and re-enter, unless the disrupted satellite
was large, in which it will be disrupted and dispersed
as well.

If, on the other hand, it were far enough from the
Earth, the increasingly eccentric satellite would eventually
collide with the Moon. New crater. Or, if the satellite
were larger, new basin, or if it were really big, new
mare sea of lava. When the Moon was accreting after
the Giant Impact we now think formed it, this mechanism
is the one that would have cleaned up all the pieces.

One has only to look at the "front," or Earth-facing
hemisphere, and compare it to the "back," non-Earth-
facing hemisphere, to see where most large objects
orbiting interior to the Moon ended up. Stuff from
exterior orbits would strike the sides (Mare Orientale).
Stuff from interior orbits would batter the "face." The
back would be strikingly different than the front...
and it is.

The Moon is a good housekeeper and does not allow
orbiting vermin, not even "dust bunnies."


Sterling K. Webb
--------------------------------------------------------------------------
----- Original Message -----
From: "Melanie Matthews" <miss_meteorite at yahoo.ca>
To: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>; "Meteorite Mailing List"
<meteorite-list at meteoritecentral.com>
Sent: Saturday, July 09, 2011 1:30 PM
Subject: Re: [meteorite-list] Asteroid 2011 MD Flyby Yields New Thinking


Interesting that it was momentarily captured by our planet's gravity...
though
wonder why Earth doesn't have any natural satalites other than our moon?
I've
read some online claims that Earth might have in the past?


Cheers

 -----------
-Melanie "MetMel" - avid meteorite collector/enthusiast from Canada!
IMCA#: 2975
eBay: metmel2775


I eat, sleep and breath meteorites 24/7.



----- Original Message ----
From: Ron Baalke <baalke at zagami.jpl.nasa.gov>
To: Meteorite Mailing List <meteorite-list at meteoritecentral.com>
Sent: Fri, July 8, 2011 5:02:21 PM
Subject: [meteorite-list] Asteroid 2011 MD Flyby Yields New Thinking


http://www.skyandtelescope.com/news/125041789.html

Asteroid Flyby Yields New Thinking
Kelly Beatty
Sky & Telescope
July 5, 2011

It was refreshing to see the news media show general restraint when
asteroid 2011 MD zipped 7,600 miles from Earth on
June 27th. I didn't spot any over-the-top headlines or crazy reporting
about potential collisions with Earth. Instead, this rogue rock passed
by uneventfully and put on a pretty good show for amateur astronomers
equipped with good scopes and blessed with dark skies.

Even though 2011 MD never got brighter than about 11th magnitude, its
close flyby did trigger some interesting changes.

First, the asteroid's orbit was yanked around quite a bit. Not only did
it pass very close to Earth - well inside Earth's ring of geosynchronous
satellites on its outgoing leg - but the asteroid also sped by
relatively slowly. This put it within our planet's gravitational grip
long enough to bend its trajectory significantly, causing the orbit to
expand outward, as shown at right.

Steven Chesley, a member of the Jet Propulsion Laboratory's team of
solar-system dynamicists, calculates that 2011 MD's trajectory was bent
by 130 degrees <http://neo.jpl.nasa.gov/news/news172.html>. "I don't
recall
ever seeing such a large turning angle for any other object," notes
JPL's Paul Chodas. The close pass also reoriented the orbit's tilt by
more than 5??, according to Andrea Milani, a near-Earth asteroid (NEA)
specialist at the University of Pisa.

But a second consequence of the close pass has more to do with how
Chesley, Chodas, and Milani do their computations - and showed that a
little tweaking was in order.

Soon after the flyby, as the asteroid receded into the depths of space,
observers noticed that 2011 MD wasn't exactly following its calculated
escape route. In some cases the positional mismatch was as great as 20
arcseconds - shockingly bad, given the all the precise positional data
reported by professional and amateur observers worldwide.

It didn't take long to track down the error's cause. "The passage of
2011 MD was such a close approach that the orbit was significantly
affected by the shape of the Earth," Milani explains. Our planet isn't a
perfect sphere but instead is slightly oblate - squashed pole to pole
by about 26?? miles (42?? km) relative to its equator, about one part in
300. This slight out-of-roundness causes, in turn, slight deviations
from a perfectly spherical gravitational field, which geophysicists
adjust for using a fudge factor known as J_2 .

Once dynamicists recalculated 2011 MD's trajectory with J_2 included,
the positional errors reported by observers largely disappeared. So why
weren't the calculations done this way to begin with? "The answer is
that it is an very insignificant term for almost all objects," Chodas
explains, "and yet it would add somewhat to the computational load. The
object has to make an extremely close approach to the Earth for this
term to make a difference, say, within 10 Earth radii," or about 40,000
miles.

"Never before 2011 MD has an asteroid passed at a few Earth radii and
been observed both before and after the encounter," Milani points out.

So even though June's interloper never posed a threat to Earth (nor will
it in the foreseeable future, according to both JPL
<http://neo.jpl.nasa.gov/risk/2011md.html> and NEODyS
<http://newton.dm.unipi.it/neodys2/index.php?pc=1.1.8&n=2011MD>), its
visit taught the world's asteroid watchers a useful lesson that will pay
dividends during future close calls.

As a consequence, the NEODyS asteroid-tracking system
<http://newton.dm.unipi.it/neodys/> maintained by Milani and others has
been tweaked. "We have implemented a model of Earth's gravity field
including oblateness," he reports, "which kicks in only when the
distance from the geocenter is less than 0.001 astronomical unit," or
about 90,000 miles. The JPL modelers will likewise invoke J_2 as needed.

"Our work with NEA orbits and impact monitoring is research work, not
routine, even though we have been doing it for more than a decade,"
comments Milani. "These cases in which we have to upgrade the software,
although not frequent, keep happening - and we do not expect they will
stop, because we are certainly still in the learning phase."
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Received on Sat 09 Jul 2011 03:32:08 PM PDT