How to watch for a comet’s fireworks

By Joe Rao

Night sky columnist

updated 4:32 p.m. ET June 3, 2005

In 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 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 comet will have already made its closest approach to Earth in early May at a distance of 66 million miles (106 million kilometers).

Although it is now moving away from 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 the 10th 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.
• 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 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 0600 GMT on July 4. That time corresponds to the late evening hours of July 3 for the west coast 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 Earth rotates over the next 24 hours, the rest of the world will be turned toward a view (weather permitting) of the comet.

At the moment of impact, Comet Tempel 1 will be situated about 3½ degrees to the east-northeast of the bluish first-magnitude star, Spica. For comparison, your fist on an outstretched arm covers about 10 degrees of sky.

So what will we see? That’s the $64,000 question. Nobody can really say for sure.

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