3.05), a measure of the orbital
energy and angular momentum of the orbit, which defines it dynamically as a
Jupiter Family Comet (T = 2.0 - 3.0), not an asteroid (T > 3.0). Therefore,
in my opinion, this object is a now-dormant comet nucleus. A fragment of a
bigger object, perhaps, that after breaking up in the not-so-distant past may
have caused the gamma Piscid stream of meteoroids. If so, it should be very
dark, reflecting only 4 percent of light, and therefore be much bigger, about
530 m across. If this comet would hit Earth, it would strike at about 1 billion
tons of TNT, enough to destroy a small country.
Interestingly enough,
6344 P-L is coming closer to the Earth as we speak. The object is approaching
Earth from behind and will brighten to magnitude +18.5 in mid October, only to
pass Earth on the inside of our orbit on November 7 at a distance of 28 lunar
distances (0.072 AU). On November 17.8, it will arrive at the closest point to
the Sun.
Somewhere along this
path, the object could become weakly active and turn from an
"asteroid" into an official comet. We have a rare opportunity to
follow this process and see if the heat of the Sun can still nudge some gas and
dust off this small planet.
All of this made for a
great story, but it even got better. Just over a week later, another one of the
van Houtens' discoveries, 5025 P-L, was recovered by the automated Spacewatch
II asteroid survey program. 5052 P-L is a Near Earth Object, coming to within
0.2 AU from Earth's orbit, not quite as dangerously close. The news was
announced on MPEC 2007-T58.
Brian Marsden wrote to me
about the circumstances of this recovery on October 11: "An object
reported to us by Spacewatch on Tuesday and placed on the NEO Confirmation Page
and followed up yesterday was found here also to have been observed in NEO
surveys on Sept. 10 and 30. This was good enough to reveal that 5025 P-L
fitted, as well as a Spacewatch single-nighter in 1999. This kind of
thing is what should have happened last month with 2007 RR9 leading to 6344
P-L, but you came to the rescue instead." He added: "All the
same, it's a curious coincidence that the two old P-L NEOs should be recognized
just over a week apart."
Indeed, after 6344 P-L,
this was the remaining P-L object waiting recovery. I also knew it, because
5025 P-L was linked in the past tentatively to the Taurid shower. Its initial
orbit was based on only three nights of observations and was extremely
uncertain. The original solution had it move far beyond Jupiter. Now, with T =
2.988, it falls among a whole group of other similar potential dormant comets,
reaching to just inside the perihelion distance of Jupiter. Whether or not some
of our Taurids originate from this object can now be investigated.
Brian further wrote:
"The oldest lost NEO is now 1972 RB. Actually, with a 49-day arc the
orbit is quite good. It's just that there hasn't been a chance to look at it
since discovery. It comes to a distance of 0.1 AU in 2013 and should then be
mag 16-17 or so."
In honor of the work by
the van Houtens and Gehrels,
the objects 6344 P-L and 5025 P-L will keep their designations, waiting to be
numbered and receive an official name. Now the orbits of these objects are
precisely determined, we can be certain that they will not impact Earth in the
foreseeable future.
Dr.
Peter Jenniskens is author of "Meteor Showers and their Parent
Comets," published by Cambridge University Press in 2006, which provides
many more background stories on other potential parent bodies and our meteor
showers.
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