September 2007 Sky from the Keeble Observatory
Recall last spring’s pollen, coating every surface with a layer of yellow dust.
If you forgot to clean your windshield, seeing to safely drive was incredibly difficult.
As we noted last month, the two rovers on Mars have been coping with a massive dust
storm over most of the Martian southern hemisphere. Instead of yellow pollen, they
are dealing with red dust. Although the pollen is of biological origin and the dust
is not, the underlying cause for both phenomena is the change of seasons.
Earth’s seasons are caused by the tilt of our equator to the plane of the orbit.
In the northern hemisphere we experience summer when the tilt is towards the Sun,
winter when tilted away. We see this as the Sun higher or lower in the sky, and
by the longer periods of sunlight in summer, shorter in winter. Our orbit is nearly
circular, so there is minimal effect from changing distance – we’re actually closer
to the Sun in January than in July. Mar’s tilt is almost the same as Earth’s, and
with a year twice as long, the seasons last longer, too. However, the orbit is more
eccentric, so the changing distance matters. Southern hemisphere temperature changes
are more extreme, with Mars closer to the Sun during southern summers. The extreme
temperature changes cause fierce winds which entrain huge amounts of fine dust and
can lead to nearly planet-wide dust storms.
The rovers, dubbed Spirit and Opportunity, rely on solar power. There is already
a penalty because Mars is 50% farther from the Sun, and the intensity of sunlight
falls off with the square of distance. While Earth receives about a kilowatt per
square meter, Mars only gets about 450 watts. Add the fact that solar cells are
only about 10% efficient, so that the peak power available to the rovers would be
about 45 watts per square meter.
Now add the dust. We measure the absorption of light in dimensionless units called
“optical depth” – designated by the Greek letter tau. Increasing optical depth causes
an exponential decrease in the intensity of light. At an optical depth of one, the
available power is down 37%. Tau equal two brings power down to 14%, at three we’re
down to 5%. The Martian storms increased the optical depth to 5 – less than 1% power
available, or about half a watt per square meter, and that for several months.
The rovers can run on batteries, but they also rely on the solar arrays to keep
the batteries charged. Battery power is used at night to run the instruments, computers,
and telemetry, and also to operate on-board heaters to keep the electronics warm
enough to work properly. Controllers initiated special low power protocols for several
weeks in order to save the rovers until the storms settled. Optical depth has now
decreased to about 3, but a further problem is that the dust is now settling directly
on the solar arrays, further blocking their ability to generate electrical power.
Hopefully the winds will blow this dust away, and both rovers can resume their exploration.
Lunar phases for September: Last Quarter on the 3rd, at 10:32 pm; New Moon on the
11th, at 8:44 am; First Quarter on the 19th, at 12:48 pm; Full Moon on the 26th,
at 3:45 pm.
Last month’s Full Moon was accompanied by a total lunar eclipse, with Moon entirely
within Earth’s shadow. This month, the New Moon marks a partial solar eclipse –
however, it will not be visible from here! You’ll need to be in the southern half
of South America, or all the way south to Antarctica to see it.
Predawn planet watchers will have Mars high to the south all month. Look for the
red planet above and to the right of the familiar shape of Orion. Early in the month,
Venus rises to the east about an hour before sunrise. Saturn follows, but may be
too close to the horizon for an easy view. By month’s end, they’re both rising earlier
and will be higher at sunrise, with Venus about three hours ahead of the Sun and
Saturn about two.
Evening planet watchers have fewer opportunities this month. Jupiter is low (about
30 degrees from the horizon) to the south and drifting lower and towards the western
horizon as the month goes on. Look for it above the bright red Antares. Mercury
begins the month low to the west at sunset, probably not out of the horizon clutter
and haze. It will be a little higher by month’s end, but the plane of the ecliptic
meets the western horizon at a shallow angle, so visibility will not be improved.
An overhead view about 2 ½ hours after sunset finds the Milky Way bisecting the
sky from northeast to southwest. The constellation Cygnus is almost at zenith, marked
by the bright star Deneb. At this time of year it clearly resembles its namesake,
the swan. About 15 degrees toward the west we find the bright blue Vega, in the
constellation Lyra, and to the south lies Altair, in Aquila. These three stars will
likely be the first to emerge from evening twilight, accompanied by bright Jupiter.
To the northeast, the familiar W of Cassiopeia is tilted slightly backward. Below
and to the right you may see the relatively nearby Andromeda Galaxy – best seen
on a moonless night, and with averted rather than direct vision. To the northwest
is the familiar shape of the Big Dipper – formally Ursa Major – with the two stars
at the end of the “bowl” pointing up toward Polaris, the Pole Star. Polaris is not
extremely bright, but its importance lies with its position almost directly over
the north pole. From the northern hemisphere, this star is always above the horizon.
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.