Comet 73P/Schwassmann-Wachmann 3 Shows That Breaking Up Is Not So Hard
To Do
Don Yeomans
Manager, NASA/JPL Near-Earth Object Program Office
April 24, 2006
Periodic comet 73P/Schwassmann-Wachmann 3 has broken into more than 30
different pieces as it approaches the sun. This comet was discovered on
1930 May 2 by Arnold Schwassmann and Arno Wachmann at the Hamburg
(Germany) observatory. It was the third periodic comet discovered by
this pair and the 73rd comet to be recognized as periodic.
Because of poor observing conditions, the comet was not recovered
during
its next return to perihelion in 1935-36. As a result, the comet's
preliminary orbit was rough and the comet's close passes by Jupiter in
1953 October (0.9 AU) and 1965 November (0.25 AU) further degraded the
orbital predictions. One Astronomical Unit (AU) is about 93 million
miles. The comet was re-discovered on Aug. 13, 1979, missed during the
next return to perihelion in 1985-86 - again because of poor observing
conditions - and then observed during its returns in 1990, 1995-96,
2000-2001, and the current 2005-2006 return. In late 1995, the comet
began to breakup into various fragments. The main comet (fragment C)
was
seen to give rise to two new fragments, A and B.
In 2001, the main comet (fragment C) was observed along with the old
fragment B and a new fragment E.
During the 2006 return to perihelion, which for the main fragment C
takes place on 2006 June 6 (just inside the Earth's orbit), the comet
began to fragment into more than 30 additional pieces. All of the
observed fragments in 2006 will pass relatively close to the Earth
during the interval May 12 through May 28 but none will pass closer
than
5.5 million miles. These passages of the fragments past the Earth offer
astronomers an excellent opportunity to examine the cometary breakup
process and hopefully these observations will shed some light on just
why some comets disrupt. Apparently some comets have very weak internal
structures and perhaps rapid rotation or the pressure of vaporizing
interior ices, as the comet approaches the warming sun, causes these
breakup events.
In addition to ongoing ground-based visual observations, the Hubble
Space Telescope and the Spitzer infrared space telescope are continuing
to observe the comet and the Arecibo and Goldstone planetary radars
will
begin observations on April 30, 2006.
For predictions of where on the night sky these fragments will appear
(i.e., ephemeris predictions), go to:
ssd.jpl.nasa.gov/horizons.cgi
and request information for comet
"73P." The main comet, which is also the brightest, is fragment C.
Deirdre Kelleghan
Irish Astronomical Society
www.irishastrosoc.org
  