(sci.space readers can skip the first paragraph)

  Yesterday, in response to Henry Spencer's question about the 
temperature of a blackbody in interstellar space, I said "Dust grains
acts as blackbodies, and they're at 40-150 K."  Well, I was dead
wrong.  Our local interstellar dust expert, Bruce Draine, has
informed me that dust grains _aren't_ good radiators in the far IR,
which is why they are so warm; actually, the ambient radiation field
from distant stars can bring a true blackbody to only 3 or 4 Kelvin.
Sorry, Henry, and anyone else I misled.  Obviously, time for me to
take another ISM class :-(

  In other news, Alan Stern of the Southwest Research Institute gave
a talk on the Pluto-Charon binary system yesterday.  He gave a brief
overview of the currently-accepted system parameters (volume ratio of
about 8:1, mass ratio about 15:1 or so, plus lots more...) and then
gave his thoughts on the formation of Pluto-Charon.  His idea is 
that there were lots and lots of small planetesimals in the outer
solar system, with masses distributed as a power law of some kind;
over time, the planetesimals accreted into larger bodies.  Most got
scattered out of the solar system by close encounters with Jupiter
and Saturn, but many accreted into the gas giants, especially
Uranus and Neptune.  A large planetesimal was captured by Neptune -
we call it Triton [captured how?  Perhaps by a collision with a smaller,
already-existing Neptunian moon, perhaps by a very close passage through
Neptune's atmosphere - mondo aerobraking!].

  He notes that the two recently discovered "Kuiper Belt" objects,
1992 QB1 and 1993 FW, plus Chiron and Pholus, are all about the same 
mass, and he identifies this group as one-accretion-down from the
larger bodies of Triton and Pluto/Charon.  Pluto/Charon, he thinks,
formed when an impacting body hit proto-Pluto, knocking some material
into a ring around Pluto which later accreted in Charon; similar to
ideas about the formation of Earth's moon.  There is good evidence
from spectra that the surfaces of Pluto and Charon are very different
(Pluto has methane frost, Charon doesn't), which can be used as evidence
for the impact theory.

  He believes that there may be around 1000 Pluto-to-Chiron-sized objects
remaining in a relatively stable dynamical zone just outside Neptune's 
orbit, beyond 35 AU or so.  1992 QB1 and 1993 FW are the first members
of this population to be found, in his model.  Note that such bodies
will be very dark, since if their surfaces are covered with methane 
frost, it will have photolyzed into very dark, long-chain hydrocarbons
by now.  The reason that Pluto has such a high albedo (around 0.5, I think)
is that its surface warms up JUST enough around perihelion to sublimate,
and when the atmosphere freezes out again, thirty years later, it forms
bright, new frost.  So any bodies much farther away than 30 AU are going
to be very hard to see.

  I hope I haven't made any errors in the transcription; if you see
a howling mistake, it's undoubtedly mine, not his.

  By the way, he's one of the top guns behind the Pluto Fast Flyby
mission (I think), and said that the current plans are to use a
Titan 4 to send the probe on "just about a rectilinear trajectory"
to Pluto (we were speaking loosely at the time...).  He'd like
to use a Proton, which gives a slightly smaller velocity but costs
MUCH less.  His figures: $500 Million for 2 Titan 4 launches (there
will be two separate probes, launched separately), or $120 Million
for 2 Proton launches.  He told a story about how the Soviets originally
offered to sell Proton launches for $30 Million each, but were forced
to increase their prices in the US in order to be allowed in the
marketplace.

  I'm just telling you what he said.

                          Michael
