From Sky And Telescope Contributor Kelly Beatty..."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 (from ELPALLSKY...not according to this and other meteor/fireball reporting sites) 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
(from ELPALLSKY...SIGNIFICANT!) 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 26½ miles (42½ 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."
