July 2005 Sky from the Keeble Observatory
Two probes are currently of interest. One fizzled. The other may provide us with
some fireworks for the 4th of July!
Cosmos 1 was intended as an orbital test of a “solar sail.” Launched from a Russian
submarine on a converted ballistic missile, the satellite was to deploy a set of
reflective vanes which would use the pressure of sunlight in place of thrusters
to change its orbit. The project was a joint undertaking by Russia and the United
States, with substantial private funding from the Planetary Society. Unfortunately,
the booster failed about 83 seconds after launch, and the mission crashed into the
Deep Impact is a comet probe, on track as this is written to fly by the comet Tempel
1 on July 4th. Actually, only part of the probe will fly by. The rest will crash
into the comet’s surface at 23,000 miles per hour. The goal is to find out what
the interior composition and structure of a comet are really like.
The best current model is to consider a comet nucleus to be a “dirty snow ball,”
mostly ices of water, ammonia, methane, etc. The rest is thought to be an aggregate
of dust and gravel. We don’t know for sure what will actually happen, except that
there is zero chance the collision will send the comet careening into a collision
with Earth. The probe is roughly a half-ton, about a meter tall and a meter in diameter.
The nucleus of Tempel 1 is roughly shaped like a 14 km long potato.
On the 3rd of July the impactor is scheduled to separate from the main probe. On
board computers will steer it into the path of the comet nucleus while its on-board
cameras return images of its target. The main part of the probe will pass 500 km
from the comet, while its cameras and other instruments monitor the collision and
its aftermath. The chemical composition of the vaporized comet material, the size
and depth of the crater, and coordinated observations with ground-based observatories
and the Hubble Space Telescope will all be used to analyze the nature of the target.
If there is more rock than ice, the crater may be much smaller than the football
field size some are predicting. If the comet lacks structural integrity, the probe
could blast all the way through without leaving a crater at all!
Lunar phases for July: New Moon at 8:02 am, on the 6th; First Quarter on the 14th,
at 11:20 am; Full Moon on the 21st, at 7:00 am; and Last Quarter on the 27th at
Earth is at aphelion – farthest distance from the Sun - on the 5th, about 152 million
kilometers (95 million miles). Even though we are further from the Sun now than
in mid-winter, the high position above the celestial equator (and above the southern
horizon) maximizes the solar heating of the ground. Southern hemisphere folks are
heading into winter.
At the beginning of the month, Saturn is still in the early evening sky, setting
about an hour after sunset. Mercury and Venus are above Saturn, about 20 degrees
off the western horizon, within a degree of one another. Venus is very bright, and
may be the first “star” to emerge from twilight. Jupiter is about 45 degrees above
the horizon to the southwest. Mercury will disappear into the Sun’s glare by the
end of June, but Venus will remain in the early evening sky. Jupiter will settle
to about 30 degrees off the horizon by August.
Predawn planet watchers will find Mars which high above the southeast horizon at
sunrise. Saturn returns to the morning twilight at month’s end, rising about ½
hour before sunrise.
At mid-month, an overhead view about 2 hours after sunset still gives us a little
more to cheer about than last month. Although Hercules is not a constellation dominated
by extremely bright stars, use binoculars to find the spectacular globular cluster
M 13 (13th in the Messier catalog). You’ll have a better chance early in the month,
when there is less interference from the moon. This sits at the edge of the faint
luminous band of the Milky Way, which stretches from the northeast to the southern
horizon at this time, curving gently toward the east.
From zenith to the southwest, you’ll find the bright star Arcturus in the constellation
Bootes, about half way to Jupiter. To the south you can find the red star Antares,
in Scorpio. Antares means “rival of Mars” – a tribute to its red color, though Mars
is not visible in the evening sky this month. From Hercules toward the eastern horizon
you’ll find the bright blue star Vega, in the constellation Lyra. With binoculars
you may find the faint Ring Nebula just south of Vega. Below Vega, the next bright
star you’ll notice is Deneb, marking the “tail” of the swan in Cygnus. This constellation
lies in the direction towards which the Sun orbits in the plane of the Galaxy. It
is also the direction toward which the Voyager 1 probe is making its exit of the
At the “head of the swan” we find the beautiful binary Albireo. The imaginary line
from Deneb to Albireo marks the approximate plane of the Milky Way. If we follow
that line to the south we’ll note Altair, the brightest star in Aquila, and then
the familiar “tea pot” shape of Sagittarius, which marks the direction toward the
center of our Galaxy.
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.