July 2004 Sky from the Keeble Observatory
Cassini is safely in Saturn orbit, and already returning spectacular data! We'll
have more to say next month.
Last month, I said that I'd write more about the Messier Catalog, but with your
indulgence I'd prefer to put that off for a while. Rather, I'll continue our discussion
of comets with new information returned by the Stardust probe.
Stardust has been in space since February 1999, when it was launched into orbit
around the Sun. Its first task was to collect dust in an aerogel sample for return
to Earth in 2006. Separate collections were performed in 2000 and again in 2002.
Completion of its first solar orbit gave it a gravity assist from Earth, which boosted
it towards last January's rendezvous with the comet Wild 2 - pronounced "vilt
too." Comets are named after their discoverer, and the number indicates that
this was the second comet found by its namesake. The comet encounter was flawless,
and included collection of dust from the comet's tail and 72 images from the probe's
Some findings were no surprise. The comet ejects gas, dust, and small rocky fragments
from its sunlit side. Some of the larger fragments struck the spacecraft, but did
not penetrate its protective covering, which is similar to body armor worn by our
troops in Iraq. The nucleus is small, an oblate ellipsoid (think of M&M candy)
roughly 5.5km by 4.0 km by 3.3 km. (1 mile = 1.6 km) It is also very dark, reflecting
only about 5% of the sunlight that strikes its surface. It is similar in appearance
to Saturn's moon Phoebe, imaged in June by the approaching Cassini probe.
But there were also surprises. Previously imaged comet nuclei (Halley's famous comet
was studied in 1987 by several space probes, comet Borrelly was studied by the Deep
Space 1 probe in 2001) have been prolate ellipsoids (think of potatoes). Although
the resolution was not as fine as obtained for Wild 2, those larger comets do not
appear to have the same kinds of surface craters. The lack of sharply defined craters,
coupled with the tidal breakup as it passed close to Jupiter of Comet Shoemaker-Levy
9 in 1994, led many to assume that comet nuclei were essentially loosely bound rubble
with little structural integrity. Deep craters with steep sides provide clear evidence
that this comet is not just a flying gravel pile. If the surface were constantly
being peeled away every time the comet approaches the Sun, craters should not be
persistent features. Further, there are on its surface a number of steep "spires"
jutting hundreds of meters toward space. Again, these are evidence for material
cohesion, but they also resemble eroded plateaus in the desert southwest.
The speed and intermittent flux of dust from the comet also suggest that the gas
and dust are being released explosively, perhaps by the release of pressure from
subsurface gas pockets. The previously mentioned observations of comets Halley and
Borrelly seemed to show nearly continuous jets contributing to the comets' tails.
Perhaps the best lesson from this encounter is that all comets are truly not alike.
Some may be flying rubble piles, others giant "dirty snowballs," others
still like Wild 2. We'll know better the details of its composition when the samples
are returned in 2006.
Lunar phases for July: Full Moon on the 2nd; Last Quarter on the 9th; New Moon on
the 17th; First Quarter on the 24th.
Evening planet watching in July, as it was for June, is largely Jupiter watching.
After sunset, Jupiter emerges from twilight about 30 degrees above the west-southwest
horizon. It sets about 1:00 am. Mercury and Mars are above the west-northwest horizon
at sunset, but probably lost in the horizon clutter and haze. The "morning
star" is Venus, which will rise to the northeast about two hours before the
Sun at mid-month. At sunrise, look for Venus to be very bright about 30 degrees
above the eastern horizon. Saturn rises just before the Sun, but will be lost in
Two hours after sunset, at mid-month you will find the constellation Hercules directly
overhead. With binoculars, you should be able to see the globular cluster M13 at
zenith. About 20 degrees toward the east, you'll find the bright star Vega, in the
constellation Lyra - the lyre. Also in Lyra, binoculars should enable you, on a
clear moonless night, to pick out the faint "smoke ring" of the Ring Nebula.
This remnant of a star is what will happen to the Sun in another five billion years:
the outer envelope of the star will be ejected as an expanding shell of ionized
gas, leaving a hot white dwarf remnant at the center. It's called a planetary nebula,
not because it has anything to do with planets, rather because it shows a small
disk on the sky.
Below Lyra lies the familiar cross shape of Cygnus - the swan. The long axis is
parallel to the horizon, with bright Deneb at the northern end of the cross (the
tail of the swan). At the other end is a modest star called Albireo. Binoculars
or a small telescope will reveal this as a beautiful binary, one star blue, the
other orange. Cygnus also marks the plane of the Milky Way, and lies in the actual
direction towards which the Sun orbits in the Galaxy.
From zenith to the east you will find the bright star Arcturus, in Bootes - the
herdsman. This direction is out of the plane of the Galaxy, rather sparse in bright
stars, but rich in external galaxies. Ursa Major - the big bear, more commonly the
Big Dipper - lies high to the northwest.
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.