Day of the comet

By Peggy Mihelich

Thursday, June 30, 2005; Posted: 4:07 p.m. EDT (20:07 GMT)

ATLANTA, Georgia (CNN) — Six months after it blasted off from Earth, the Deep Impact spacecraft is poised to meet its cosmic fate — in a hyper-speed smashup with a comet.

In the early morning hours of July 4, NASA scientists will steer a probe about the size of a washing machine directly in the path of a comet about half the size of Manhattan, and watch the two collide.

What comes out of the collision scientists hope will unlock the inner workings of comets.

“There’s a certain amount of nervousness at present. … It’s a harsh environment out there and this is not easy,” said Don Yeomans, manager of NASA’s Near Earth Objects Program and a co-investigator of the mission.

“We don’t really know what to expect, frankly. … It could be anything from a crater the size of a football stadium to something that’s far more modest. Or the spacecraft could simply bury itself into the comet.”

Such a complex mission carries its fair share of risks — like the effects of flying a spacecraft through a wash of dust and ice kicked-off by the comet’s tail.

“It could sandblast some of our imaging lenses, we are worried about that a little bit,” Yeomans said.

But worries will be put aside in the hopes of a big scientific payoff — a glimpse beneath the comet’s surface.

Icy dirt ball

Comets are the trailblazers of the heavens — rushing through space from the far reaches of the solar system and back toward the sun in long oval orbits.

They are made of ice, dust and gas left over from when the sun and the planets formed.

Scientists believe comets may hold the keys to the birth of the solar system and perhaps to the birth of life itself.

The target of Deep Impact is Tempel 1, a jet-black, pickle-shaped icy dirt ball traveling at 6.3 miles per second.

Since its launch on January 12, NASA’s Deep Impact spacecraft has been racing to catch up with Tempel 1 while observing it along its journey through the solar system.

With a cost of $330 million, Deep Impact is the eighth mission in NASA’s Discovery Program, which supports low-budget science missions.

Among the program missions — the Near Shoemaker mission that landed a spacecraft on asteroid Eros; the Mars Pathfinder mission; and the solar wind collection spacecraft Genesis, which crashed into the Earth when its parachutes failed to open on descent.

The Deep Impact spacecraft is composed of two probes mated together — “flyby” and “impactor.”

Flyby is about the size of a small car and will monitor the impact. It carries two cameras — a high-resolution one, which will be tightly focused on the crater, and a medium-resolution camera, which will take wider views.

The impactor is an 820-pound copper-fortified probe designed to produce maximum wallop when it hits the comet. It also carries a medium-resolution camera that will record the probe’s final moments before it collides with the comet.

Because of the spacecraft’s distance from Earth — currently 83 million miles — communications are delayed, making it impossible for crews on the ground to react in the moments before impact.

“It takes 7 1/2 minutes for a signal to get from Earth to the spacecraft and another 7 1/2 minutes to get back. So, we can’t joystick this spacecraft like a video game,” Yeomans said.

Which is why scientists designed both probes with self-navigational guidance systems.

“The spacecraft has to be smart enough on its own to observe the comet, determine whether it’s headed in the right direction, if not, make its own course correction, and then fire its thrusters to achieve that course correction,” Yeomans said.

Independence Day

Staging the fireworks show begins 24 hours before impact, when mission scientists at NASA’s Jet Propulsion Laboratory will release the impactor from flyby.

Scientists will spend the day steering flyby into position for observing, while aligning impactor for its rendezvous with Tempel 1.

Tempel 1 is traveling through space at about 23,000 mph (37,100 km/h) — the equivalent of traveling from New York to Los Angeles in less than 6.5 minutes.

At these speeds, impactor has to be in the right place at the right time to intercept the speeding snowball.

“It’s a bullet trying to hit a second bullet with the third bullet,” Rick Grammier, Deep Impact project manager at JPL, told a press conference in June.

Two hours before impact, scientists will turn the controls over to impactor, which will maneuver itself into the path of the comet.

If all goes well, at 1:52 am ET on July 4, Tempel 1 will run into impactor, busting a hole in the comet and revealing its inner core.

“It will be all over in the blink of an eye,” Grammier said.

Until its death, the impactor will record images and gather data while flyby passes 310 miles (500 kilometers) away, observing the impact, the ejected material, and the structure and composition of the comet’s interior. Most of the data will be stored on flyby and radioed back to Earth after the encounter.

Mission scientists hope to have the first images on the Deep Impact Web site within 20 minutes of the encounter.

“We get one chance lasting 800 seconds to take all of the key data from impact until we’ve flown past,” said mission science chief Michael A’Hearn of the University of Maryland in Baltimore.

It’s not just Deep Impact that will be observing. Every space and ground-based telescope large enough to do the job will be watching.

The Hubble, Chandra, Spitzer, Galex and SWAS space telescopes will all be recording the event. The Rosetta spacecraft, a European probe on its way to another comet will also observe.

On the ground, more than 100 professional astronomers at 60 observatories and a small army of amateur astronomers will also turn their telescopes in Tempel 1’s direction.

“We’re trying to understand the structure of a comet and also its composition. Once we hit it, we will open up surface areas that are exposed to the sunlight and see new subsurface ices for the first time,” Yeomans said.