JULY 2005

by Deane Morrison

This year July 4 delivers double fireworks. On that day we get not only the usual show, but a historic crash landing of a NASA probe on a comet.

The dual spacecraft of NASA's Deep Impact mission is now heading for Comet Tempel I deep in space. At 1 a.m. CDT on the 4th, about 24 hours after separating from the flyby spacecraft, the washing machine-sized impactor spacecraft will crash into the comet nucleus at 23,000 miles per hour, gouging out a crater and throwing up a spray of cometary dirt, gas and ice. Pictures of the comet's "spilled guts" and other data will be recorded, not only by the flyby spacecraft but by the Hubble and Spitzer (infrared) space telescopes and telescopes on Earth. The comet's innards contain clues to the composition of the primordial soup from which the solar system formed. The comet, which is roughly half the size of Manhattan, dates from the solar system's earliest and chilliest era, about 4.5 billion years ago.

The collision will take place near the comet's point of closest approach to the sun: about one-and-a-half times the Earth's distance from the sun, near the orbit of Mars. The comet won't be visible from the Midwest during the encounter, but you can read all about it at http://www.deepimpact.umd.edu.

One object you can see from your back yard is Mars, which is waxing ever brighter as we move toward a close encounter this fall. Mars rises after midnight, glowing orangish as it moves eastward through the stars of Pisces. If you have a telescope, you may be able to pick out some martian features, especially if you look near dawn, when Mars is highest. On the 17th, Mars reaches perihelion, its closest approach to the sun in its orbit. After that, Mars drifts farther from the the sun-and us. Even so, it will still be fairly close when Earth passes it in early November-only 8 million miles farther away than it was during a similar close encounter in August 2003. But this time Mars will be much higher in the sky than it was in 2003.

Low in the west, Venus opens the month in company with Mercury, a much dimmer light glimmering about two degrees below the queen of planets. On the 8th, a crescent moon hangs above Venus. Mercury soon drops out of sight, but Jupiter, a bright beacon to the east, slides westward toward Venus as it falls behind Earth in the race around the sun. Saturn, having passed Venus in late June, disappears into the sun's afterglow in the first few days of the month.

July's night sky is packed with constellations, including many that actually look like what they're supposed to represent. In the south, Scorpius curves up from the horizon and menaces dim Libra, the scales, to the west. The jewel of Scorpius is Antares, the smoldering red heart of the scorpion. Just to the east, the Teapot of Sagittarius tips as if to pour its contents on the scorpion's tail. Above the Teapot hangs the tiny Teaspoon. High in the east floats the Summer Triangle of bright stars. Easternmost is Deneb, in Cygnus, the swan. Altair, in Aquila, the eagle, forms the southern point of the Triangle, and forming the northwest point is Vega, the brightest of the three, in Lyra, the lyre of Orpheus.

On the night of the 17th, the moon passes barely south of Antares. The next night, a nearly full moon rises in the middle of the Teapot. The full moon of July, called the thunder moon or the buck moon (for the velvety antlers of buck deer that start to grow at this time of year), arrives at 6 a.m. CDT on the 21st, just half an hour after moonset.

Earth reaches aphelion, its farthest distance from the sun, on the 5th. On that day Earth swings out to 94.3 million miles from the sun, about 3.4 percent farther than at its closest approach (perihelion) in January.