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 of Maryland.
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 to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.