October 2005 Sky from the Keeble Observatory
Last July 4th saw celestial fireworks of a different kind, as the
probe Deep Impact collided head-on with the comet Tempel 1, excavating
its own crater and revealing what was under the dark, dusty surface.
The image below shows the nucleus of the comet 13 seconds after
impact. As is often the case, new results have supported some old
models, but have challenged others.
Image courtesy of Deep Impact project team, JPL, NASA, University
Comets are believed to be leftovers from the formation of the solar
system some 4.5 to 5 billion years ago. The most broadly accepted
model is that they are “dirty snow balls” or “snowy
dirt balls,” depending on whether you think the non-volatile
component is more or less important. Prior to Deep Impact, there
were only a handful of comet probes, and none that actually made
contact with the nucleus. Spectroscopic analysis of the light from
a comet gives some notion of its composition, but the very sunlight
that causes the comet to vent gas and dust may also change the chemistry
of what’s vented.
The nucleus of Tempel 1 appears to be a dark, irregular object
about 5 km long. The material has the structural integrity of meringue,
with a density less than that of water. (Even a meringue pie would
feel “solid” if you hit it at 66 km/sec!) It also means
that the interior is well insulated from changes in surface temperature,
so that interior should be pristine material. The surface gravity
is so weak that you could launch yourself into space simply by jumping.
The number of craters is a surprise, both because of the fluffy
composition and because none of the other comet nuclei imaged have
shown any craters. That may mean that this one is unique, so we
have to be careful with generalizations.
The material ejected in the post-impact plume shows clearly that
the interior composition differs from the surface. Surface water
ice was vaporized, but the plume excavated frozen ice from the inside.
Two surprising finds in the ejected material were spectroscopic
evidence for particles of clay and carbonates. On Earth, these only
form in liquid water. Their presence in the comet nucleus is a puzzle,
but may indicate that the early solar nebula was more thoroughly
mixed than we have previously thought. There were also silicates
found, including the spectroscopic signature of olivine –
found in the beach sands of Hawaii – though in crystalline
grains much smaller than sand. They also found evidence of iron
compounds and enough carbon compounds to support the notion that
comets in the early solar system provided the carbon for Earth’s
Lunar phases for October: New Moon at 6:28 am, on the 3rd; First
Quarter on the 10th, at 3:01 pm; Full Moon on the 17th, at 8:14
am; and Last Quarter on the 24th at 9:17 pm.
This month, I thought we’d do a tour of the sky, with the
Moon as our guide. This is partly triggered by the fact that there
will be two eclipses in October, though you won’t be able
to see either one from the Center of the Universe! On the morning
of the 3rd, the Moon will pass between the Sun and Earth, though
the Moon is too far away to fully cover the Sun’s disk. This
will produce an annular eclipse, but you’ll have to be in
northeastern Africa to see it (it will be mid-day there). Two weeks
later, the Earth will pass between the Sun and Moon, casting a shadow
across part of the lunar surface. This partial lunar eclipse will
happen after the Moon sets from our skies – travel a time
zone or two west to see it.
Let’s begin with the New Moon – from here, there will
be no eclipse and the Moon will appear in the sky in the same general
direction as the Sun, in the constellation Virgo. The Sun is just
rising. The Moon rises (or sets) about an hour and 20 minutes later
every day, so on the 4th it will have moved to the east of the Sun,
and you won’t be able to see it except briefly right after
sunset. It will be a slim crescent about 4 degrees to the left of
Jupiter. By the 6th, the Moon is in the constellation Libra, about
4 degrees below and to the right of Venus, which is 20 degrees above
the southwest horizon at sunset. On the 9th and 10th, the waxing
Moon is passing through Sagittarius; through Capricorn on the 11th
and 12th. Aquarius and Pisces follow from the 13th through 17th,
with the Full Moon now rising at sunset. That bright red “star”
5 degrees from the Moon on the 18th is the planet Mars, sharing
the constellation Aries. Passing the Pleiades on the 19th, the Moon
is in Taurus until moving into Gemini for the 23rd and 24th. In
Cancer the 25th and 26th, the waning Moon is near Saturn, and near
Regulus in Leo on the 27th. You’ll have to get up before sunrise
to see it. We return to Virgo on the 30th, and the cycle continues.
For your own monthly star chart, you can direct your web browser
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.