July 2012 Sky from the Keeble Observatory
It would be hard to find someone who is not fascinated by the planet Mars. It has
figured prominently in popular culture at least since the time of H. G. Wells and
Edgar Rice Burroughs. Italian astronomer Schiaparelli thought he saw straight lines
on the planet during the 1877 opposition, which he drew and labeled as canali
or channels. These were then “seen” by other observers, some of whom mistranslated
the label into English as canals, and the whole mythos of Martians came to be.
We now know, of course, that there are no canals, though there are rift valleys
and dry river beds. But we also know that Mars was once much warmer and wetter,
with a denser atmosphere and the possibility that life may have been possible there.
We also know that it’s not a trivial assignment to safely land a probe on Mars to
look at the geology and search for evidence of water. Amazingly, NASA has a pretty
good record for soft landings extending back to the 1976 Viking probes, both
of which were successful using retrorockets. 1997 saw Pathfinder safely land
on the 4th of July, deploying a small rover called Sojourner,
which was the size of a small microwave oven. The landing mechanism deployed parachutes,
and then released the probe surrounded by airbags to bounce to rest.
That same landing technique was successfully applied to the larger (desk size) rovers,
Spirit and Opportunity in 2004. We lost contact with Spirit in 2011,
but Opportunity is still working over 21 miles from its landing site and over 8
years into its 90 day mission! Curiosity is the next rover, and it’s even
larger – about the size of a small SUV – and too big and massive for the air bag
trick to work, and even too big for retrorockets. In both cases, the necessary rockets
or airbags would weigh far more than the probe itself.
The planned landing scenario for Curiosity is ambitious, and since it has never
been tried, it’s also risky. Engineers on the project refer to “7 minutes of terror”
as Curiosity slams into the thin Martian atmosphere and begins braking. The entry
capsule is similar in shape to the Apollo spacecraft which safely carried astronauts
to the Moon and back – roughly the shape of a gumdrop. This capsule will protect
the rover from the intense heat of reentry until a large supersonic parachute can
be deployed to further slow the lander. However, since the atmosphere is less than
1% as dense as Earth’s, even a very large parachute cannot slow it enough – it will
still be going 100 meters per second when the parachute reaches terminal speed.
The next step is to deploy a hydrazine thruster powered “sky crane” which will further
slow the descent and will lower the rover to the surface wheels down. When touchdown
is confirmed, the sky crane will sever cables and fly away to crash a safe distance
from the rover itself. If all goes well, a safe landing followed by first images
and data from Curiosity will take place early on August 6th. Next month
we’ll talk about the rover itself, its nuclear power supply, and the exploration
mission it hopes to carry out.
Lunar phases for July: Full Moon on the 3rd, at 2:52 pm; Last
Quarter on the 10th, at 9:48 pm; New Moon on the 19th at 12:24
am; First Quarter on the 26th at 4:56 am.
Now that we’re into summer, having passed the solstice on June 20th,
it may surprise some to know that on July 4th we will be at aphelion
– our greatest distance from the Sun. The seasonal temperature variations are due
not to the Earth-Sun distance, but to our planet’s 23½ degree equatorial tilt relative
to the plane of the ecliptic.
At the end of the month, you may get a chance to see the delta Aquarid meteors.
It’s not typically a busy meteor shower, but the first quarter moon sets early enough
that pre-dawn observers may have a chance to see up to 10-15 per hour. The radiant
of the shower will be to the southwest around 3:00 am. This shower doesn’t have
a real “peak” so look for them from the 29th through 31st.
Pre-dawn planet watchers will strain to see Venus and Jupiter low to the east early
in July, but they climb higher as the month progresses. Venus is closer to the horizon,
and will be much brighter than Jupiter. Look for them below the Pleiades in Taurus.
Emerging from evening twilight, you’ll see Saturn to the southwest and Mars to the
west-southwest, both about 30 to 40 degrees above the horizon. They’ll move closer
together on the sky, and be separated by about 5 degrees by month’s end, appearing
above the western horizon as the sky darkens.
At mid-month about 3 hours after sunset, the constellation Lyra and its brightest
star Vega mark the zenith sky. To the southeast, about 40 degrees from Vega (50
degrees above the horizon) is Altair, also known as alpha Aquilae – the brightest
star in the constellation Aquila, the Eagle. Deneb in Cygnus is to the northeast
about 60 degrees above the horizon. These three stars mark the Summer Triangle,
and they are visible virtually all night as the sky appears to rotate over our part
of the globe. Ursa Major is to the northwest, with the familiar “pointer stars”
in the bowl of the “big dipper” allow your eye to follow their line up to Polaris,
which marks the approximate north pole of the celestial sphere. The “handle” of
the dipper can be followed back to the west to find Arcturus, in the constellation
Bootes. To the northeast Casseiopia looks like a crooked W on the sky. As you notice
Mars to west, you’ll also see its “rival” Antares to the southwest in the constellation