Deep Impact

Deep Impact will rendezvous with Comet Tempel 1 on July 4, 2005. An impactor will blast a crater in the comet while the mother ship watches from a safe distance. Astronomers on Earth will be watching too.

Courtesy Pat Rawlings and NASA/JPL.

When NASA's Deep Impact slams an 800-pound projectile into Comet Tempel 1 at 23,000 miles per hour, the collision should create quite a splash. Cameras and other instruments on the main spacecraft will watch this first-ever comet excavation from a safe distance, and astronomers on Earth will be looking on with powerful mountaintop telescopes.

Countless amateur stargazers will be watching too. If you have a telescope or binoculars, you can watch to see if the comet brightens after the probe's kamikaze plunge. Contrary to optimistic predictions, you're very unlikely to see anything of this event with your unaided eye. But you can at least see the place in the sky where it's happening.

Your location will be important. The crash is set to happen at 10:52 p.m. Pacific Daylight Time on July 3rd (1:52 a.m. Eastern Daylight Time on the morning of July 4th). At that moment, the comet will be well placed in a dark sky from the western United States, especially the Southwest. But for the eastern United States and Canada the comet will have already slipped below the horizon.

Anyone well west of the Mississippi River has a chance of seeing the comet when Deep Impact slams into it. Observers in southwestern Canada, Mexico, and Central America will have good seats too. From other locations the comet will have set or the Sun will have risen by the time of the impact.

Comet Tempel 1 Finder Chart

On the night of July 3–4 and for several evenings immediately afterward, Comet Tempel 1 will be situated in the southwestern sky not far from the bright star Spica. Even after Deep Impact makes its splashy arrival, the comet will likely not be visible to the unaided eye. Instead, try looking in the location marked X with good binoculars or a small telescope. The comet will look like a faint, fuzzy star.

Night Sky illustration by Steven A. Simpson.

Here's where to look. After dark, find Jupiter shining high in the southwest. It's the brightest "star" in that part of the sky (brighter Venus sets in the west-northwest during dusk). Off to Jupiter's left, by somewhat more than the width of your fist seen at arm's length, is the fainter star Spica. The impact with the comet will happen a couple of finger-widths above Spica, as shown in the illustration at right.

Even during and after impact the comet is expected to remain faint, and telescope users will need to use a detailed star chart like the one below. Suitable charts also appear in the June 2005 issue of Sky & Telescope, the July-August 2005 issue of Night Sky, and online at Anyone who's not already familiar with how to use star charts with a telescope will need to follow the beginner's instructions in the online article.

Comet Tempel 1 Finder Chart

Comet Tempel 1 is positioned near bright Jupiter and Spica in the evening sky. But it's very faint; you'll have to use the faintest stars plotted here to pinpoint the exact position to examine with your telescope.

Night Sky illustration by Steven A. Simpson.

Some scientists speculate that Comet Tempel 1 will stay bright long after it gets hit, so you may be able to spot it for several nights after July 3rd. If your evening sky is clear on July 6th or 7th, there's an easy way to find Comet Tempel 1 with your telescope. Just center Spica in your lowest-power eyepiece, then let the sky drift by (turn off the scope's tracking motor if it has one). Wait exactly 20 minutes, and Comet Tempel 1 should be in the field of view.


Don't fret if you can't observe Comet Tempel 1 during the impact, either because you're not in a suitable location or your skies are cloudy or it's too faint. Several organizations plan live webcasts of the event.

NASA has produced an excellent animation of what the encounter and impact might look like up close. Much more about the mission is at NASA's Deep Impact Web site.

Several articles about the Deep Impact mission and its target comet appear in the June 2005 Sky & Telescope.


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