for National Geographic News
The most spectacular fireworks this Fourth of July promise to be out of this world83 million miles (133 million kilometers) out of this world, to be precise.
NASA's Deep Impact spacecraft will be launching a projectile into the surface of comet Tempel 1. It is the first time a spacecraft will touch the nucleus of a comet.
The blast should be visible with the aid of binoculars to millions of people here on Earth in the early morning hours of U.S. Independence Day.
"Those in the U.S. living west of the Mississippi [and in] Hawaii, and [those living in] New Zealand will have ringside seating for the impact event in a dark sky," said Don Yeomans, a Deep Impact mission scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
Scientists, who know very little about the interior of comets, hope the trash can-size projectile will smash a hole deep enough in the comet's icy exterior to reveal what lies inside it.
Learning more about what comets are made of will help explain the role they played in forming our solar system. Many scientists consider comets to be the source of most of the water and organic material that was once delivered to all terrestrial planets.
"The mission will help us understand the earliest phases of the solar system by learning what's really inside a comet in its pristine form," said Jay Melosh, a professor of planetary science at the University of Arizona in Tucson. Melosh is also a member of the Deep Impact science team.
The experiment could also help scientists devise ways to deflect rogue comets if they threaten to collide with Earth in the future.
During the early morning hours of July 3, a flyby spacecraft will deploy a 39-inch-wide (1-meter-wide), copper-fortified probe into the path of Tempel 1. The comet is about half the size of Manhattan and is hurtling through space at 23,000 miles an hour (37,000 kilometers an hour).
If all goes according to plan, over the next 22 hours both the spacecraft and the probe will move toward the comet. When the probe, which has autonomous navigation, hits the comet, the flyby craft will pass 310 miles (500 kilometers) below.
"We are really threading the needle with this one," said Rick Grammier, the Deep Impact project manager at JPL. "We are attempting something never done before at speeds and distances that are truly out of this world."
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