http://www.space.com/spacewatch/050603_deep_impact.htmlIn early July, NASA’s Deep Impact spacecraft will deploy a tiny impactor to smash into the nucleus of a small comet. The idea is to excavate a sizable crater and provide valuable insight into the true nature of comets.
For skywatchers here on Earth, it should also produce a large cloud of ejected material that should cause the comet to significantly brighten enough to become visible with binoculars and perhaps even with the unaided eye.
The comet that has been chosen for the task was discovered by a Frenchman in the mid-19th century. Known as Comet Tempel 1, it already has a rather checkered history. Soon, however, it will go down in history books.
Finding the target
During June, Comet Tempel 1 will be gliding on a south-southeast course through the constellation of Virgo, the Virgin. The comet will have already made its closest approach to the Earth in early May at a distance of 66 million miles (106 million kilometers).
Although it is now moving away from the Earth, the comet is still approaching the Sun, so its overall brightness in the coming days and weeks will appear to change very little, if at all. The comet is expected to hover at around tenth-magnitude, meaning that it will glow about 40 times dimmer than a star that is at the threshold of visibility with the unaided eye.
So, to successfully locate it, you will need three things:
A star chart with the comet’s projected path plotted on it.
A good telescope and of course,
A dark, unpolluted night sky.
During the next several weeks, Virgo and the comet will be over in the west-southwest part of the sky as darkness falls and setting soon after midnight, local daylight time.
The night of impact
The Deep Impact spacecraft is expected to arrive near Comet Tempel 1 on July 4, one day before the comet reaches perihelion (its closest point to the Sun). It will have released its copper impactor about 24 hours before, while making a "deflection maneuver" to move off to a safe distance of about 300 miles (500 kilometers) from the comet.
The table-sized, 820-pound (372-kilogram) impactor is scheduled to smash into the comet’s nucleus at 23,000 mph (37,000 kilometers) per hour, creating a crater perhaps 670 feet (200 meters) wide and 50 meters deep, at around 6 hours Universal Time on July 4. That time corresponds to the late evening hours of July 3 for the west coasts of the United States and Mexico.
Along the west coast of Canada, the Sun will either be setting, or it will be twilight. Dusk will also be falling for Hawaii and New Zealand. As the Earth rotates over the next 24 hours, the rest of the world will be turned toward a view (weather permitting) of the comet.
Not to bad of a crater size.