August 2004 Sky from the Keeble Observatory
Slipping just inside Saturn's weirdly braided F-ring, Cassini successfully
completed its orbital insertion maneuver on June 30th and began a planned 4-year
mission orbiting the second largest planet in our solar system. The spacecraft is
named for the 17th century Italian-French astronomer who first observed an apparent
gap in Saturn's rings. We now understand that the gap, and others less prominently
seen from Earth, is not empty. Rather, it contains less bright ice than the more
prominent rings ... Cassini transited the ring plane with its shielded antenna
forward to protect the probe from collisions with dust and small pebbles.
Early images of the rings are at much higher resolution that we obtained in the
1970s from two Voyager flybys ... so spectacular that some mission scientists
thought that their colleagues were fooling them with simulations rather than real
data. (See for yourself at http://saturn.jpl.nasa.gov/home/index.html
) Braiding, "scallops," and assorted "spiral density waves"
give the rings the appearance of an exotic CD (or phonograph record, for those of
my generation). These features represent the consequences of gravity acting between
the ring material and various of Saturn's moons, some barely larger than the ring
The bright rings are a swarm of icy chunks, ranging in size from snowflake to iceberg,
and reflect sunlight efficiently. The darker gaps, analyzed in infrared and ultraviolet
light, now seem to consist of smaller particles with more rocky dust. They've been
described as akin to frozen mud. Analysis also shows that the composition of the
rings is less ice and more mud as you get closer to the planet. The puzzling exception
is the "Cassini division" - the apparent gap - which splits the bright
icy rings with a band of darker material. Good thing they didn't target the probe
to pass through that gap!
Next month we'll discuss Saturn's larger satellites, including Titan. The only moon
in the solar system with a substantial atmosphere, Titan is one of the prime targets
of the mission. It will receive a Christmas present, when Cassini drops into its
atmosphere a probe named Huygens, after the early 17th century Dutch scientist who
first correctly identified the rings.
Lunar phases for August: Last Quarter on the 7th, at 6:01 pm EDT; New Moon on the
15th, at 9:24 pm; First Quarter on the 23rd, at 6:12 am; Full Moon on the 29th,
at 10:22 pm.
Evening planet watching will continue to disappoint! At sunset early in August,
you'll find Jupiter only 20 degrees above the western horizon, setting about two
hours later. Mars and Mercury are lower still, approximately midway between the
position of the Sun and that of Jupiter. Unfortunately, summer in central Virginia
usually means haze and humidity, so you may not even see Jupiter. Mercury will completely
disappear into the Sun's glare, re-emerging into the morning sky by the end of the
month. Morning sky-watchers will have a better array of planets. Saturn begins August
about 17 degrees above the ENE horizon as the Sun rises, but climbs to nearly 40
degrees by month's end, rising then more than three hours before the Sun. Venus
is already high in the predawn as the month begins, about 35 degrees ahead of the
Sun. By month's end, you'll find it high and brilliant right next to Saturn.
At about 10:00 pm, at mid-month you will find the bright star Vega, in the constellation
Lyra - the lyre - at zenith. Binoculars should enable you, on a clear moonless night,
to pick out the faint Ring Nebula. Also, seeing doesn't have to be perfect for binoculars
or a small telescope to resolve the "double-double" - the modestly bright
star next to Vega - as a pair of close binaries. Hercules, directly overhead at
this time last month, has shifted about 30 degrees to the west. You should still
be able to see the globular cluster M13 in Hercules - it will look like a faint
dandelion puff, composed of perhaps a million stars.
Cygnus, the Swan, is now to the east, about 56 degrees from the horizon, with 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.
As mentioned last month, binoculars or a small telescope will reveal this as a beautiful
binary, one star blue, the other orange. It's not an extremely bright star, but
it's nearly alone in that patch of sky.
Altair, to the southeast about 55 degrees above the horizon, marks the downward
apex of a triangle with Vega and Deneb. Low to the southwest, that reddish star
is Antares, in Scorpio. It's name means "Mars' rival" - but with Mars
lost in the evening twilight, Antares has no competition to worry about.
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.