by Kim Malville
August brings the Perseid meteor shower back into our skies and provides a good reason for camping without a tent. Typically, at its peak around August 12, there are 100 meteors visible per hour, when the sky is dark. This year the moon will be two days past full moon on August 12/13, so it will be difficult seeing the fainter meteors. As you can see from the figure, the Pereseid meteors begin appearing around July 28. You can identify them because they come out of the constellation of Perseus, low in the ENE sky in the evening.
Perseus is the super hero who beheaded the snake-haired, carried her head around with him, and rescued the naked maiden Andromeda (that is, naked except for her jewels) from the sea monster Cetus. Since anyone who looked directly at Medusa’s head would turn to stone, Perseus pulled it out of his bag and showed it to Cetus, who sank like a stone to the bottom of the sea.
The area of the sky where the Perseids will appear
At nightfall on August 12 the moon will be low in the east, and this should be a good time to look for a different kind of meteors, the earth grazers. It will be the time when the earth passes through the densest part of the Perseid stream. Some will fall more directly toward the earth, but there should be a few that skim across the upper atmosphere are like skipping rocks on a pond. If you see one, count yourself lucky. They are memorable and may move from horizon to horizon, lasting 15 seconds or longer.
August 1: When the moon is a small crescent, setting in the early evening, try to spot some of the early Perseids in a dark sky.
August 2: The growing crescent moon is to the right of Mars.
August 3: The first quarter moon is between Mars and Saturn.
August 6: The spacecraft launched in 2004 by the European Space Agency, Rosetta, should reach and begin to orbit the nucleus of comet Churyumov-Gerasimenko. This will be the first time a spacecraft has orbited a comet. It will continue to escort the comet through December 2015. Aboard is a spectrograph called Alice, provided by the Southwest Research Institute of Boulder.
August 10: The moon is close, the largest full moon of the year
August 12/13: Perseid meteor shower
August 23-26: Fast moving Mars passes below Saturn. Both planets will be equally bright. Mars will be the red one.
August 31: The Moon, Mars, and Saturn form a tight triangle.
Rosetta finds its comet
The Rosetta spacecraft was launched from French Guiana in March 2004 on an Ariane 5 rocket. It contains the Rosetta mother ship which will orbit the comet 67PC/G for 17 months while it dives toward the sun, and the Philae robotic lander, which will attach itself to the comet with harpoon-like tethers. Rosetta will perform the most detailed study of a comet ever attempted. The mission is controlled from the European Space Operations Centre in Darmstadt, Germany.
The probe is named after the Rosetta Stone and the lander is named after the Nile island Philae, where an obelisk was discovered with inscriptions. Together they led to the decipherment of Egyptian hieroglyphs, and now we hope for the decipherment of a comet, formed in the early days of our solar system.
Rosetta was first set to be launched in January 2003 to rendezvous with another comet in 2011. However, this plan was abandoned after a failure of a Ariane 5 carrier rocket during a disastrous communications satellite launch. These rockets were grounded until the cause of the failure could be determined. A new plan was formed to target the comet 67PC/G, with a revised launch date of 26 February 2004. After two scrubbed launch attempts, Rosetta was finally launched on 2 March 2004.
In order to reach enough speed, the spacecraft had to perform a complicated a series of sling shot flybys around Earth and Mars. The first flyby of Earth occurred on March 2005. On 25 February 2007, the craft was scheduled for a low-altitude bypass of Mars. Because of the delayed launch, Rosetta had to fly across the surface of Mars at a height of only 160 mi. During that encounter the solar panels could not be used because the craft was in the planet’s shadow, where it would not receive any solar light for 15 minutes, causing a dangerous shortage of power. This Mars maneuver was nicknamed “The Billion Euro Gamble”. Fortunately, it paid off. The second Earth flyby occurred on 13 November 2007. As it approached Earth, the spacecraft was picked up by our asteroid early warning system and was misidentified as a dangerous asteroid some 70 feet in diameter that hit the earth. Fortunately it was not shot down.
As if these adventures and tribulations of Rosetta were not enough, the most recent pictures of the comet reveal it is not a smooth, egg-like ice ball as anticipated, but it appears to consist of two different sized nuclei rotating around each other. The most recent photo was taken at a distance of 8000 miles. It was first thought to be simply an irregular object, initially described as a “rubber ducky” on NPR, but the new photos are astonishing. It is a great thrill for astronomers to discover something so surprising and unusual, but it is a huge headache for engineers figuring out where to land the probe onto the comet in November.
In any case, Rosetta will begin to orbit the comet on August 6 and will follow an awesome corkscrew path downward toward the sun. It will be slowly spiraling inward toward the comet, reaching a distance of only a few miles from the surface. During the following 17 months, the comet will be heating up, due to light from the sun, and sending off giant plumes of gas and dust. Never before have we been able to follow so closely the awakening of a dormant (and ugly) dirty ice ball into a beautiful comet with a tail extending millions of miles into space.