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 navigation
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
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
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 the glare.
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
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.