The problem: how to direct a spacecraft from Colorado 268 million miles through the solar system at 43,000 miles an hour, then smash it into a comet of unknown size moving even faster.
The answer: practice.
It took thousands of computer runs – billions of calculations – to refine the program that should drive a copper bullet into a comet just before midnight on July 3.
Earlier this week, navigation engineers beamed up what is most likely the last version of the software directing the craft, built by Ball Aerospace & Technologies Corp. in Boulder.
NASA’s $333 million Deep Impact mission is filled with unknowns: Scientists don’t yet know the size or shape of their target – a mysterious comet called Tempel 1. They can’t predict where their bullet will strike, and they’re not sure what kind of debris the spacecraft will encounter.
It’s those uncertainties that make the mission intriguing, said Rick Grammier, the National Aeronautics and Space Administration’s Deep Impact project manager at the Jet Propulsion Laboratory in Pasadena, Calif.
“This is extremely challenging, technically,” Grammier said. “We’ve tested and tested and tested, but in the end, we’re going to a place we’ve never been, in an environment we’ve never seen.”
It’s worth the risk, NASA officials said, for a possible glimpse at a comet’s interior. Scientists believe the icy insides of comets are little changed since the solar system’s birth about 4 billion years ago.
Deep Impact’s bullet will have to automatically pilot itself to a good landing spot – because everything will be moving so fast.
“We’ve got this land mass the size of Washington, D.C., and we’re hitting it with a 1-meter (about 3-foot) barrel, at a closing speed of 23,000 miles per hour,” Henderson said. “A fighter pilot could not do it.”
It took years of simulations to develop the computer code that can.
Last year, engineers crowded around a computer screen in Pasadena, watching two small dots streak toward each other.
One represented the mysterious Comet Tempel 1, the other, part of Deep Impact – now hurtling toward the real comet at 43,000 mph.
When the two objects on screen collided, the engineers cheered.
“They’d say, ‘Yeah! That was a good one,”‘ said Monte Henderson, deputy program manager with Ball in Boulder.
When the computer-generated objects missed, the engineers groaned and dug back into their code, Henderson said.
By now the navigation system honed in those simulations never misses, and it is theoretically far better than it needs to be, Grammier said.
Since scientists don’t know what Comet Tempel 1 looks like, engineers challenged their software with bizarrely shaped objects, deeply pocked with dark valleys or arched like a boomerang.
In three tests in mid-June, the bullet landed “dead center on a lit area,” Henderson said. For scientists to watch the impact, it must happen in sunlight.
Andy Dantzler, director of NASA’s solar-system division, said he believes mission planners have thought of everything. “It is a very daring, interesting mission … that’s going to try to uncover some secrets of our solar system,” he said.
Staff writer Katy Human can be reached at 303-820-1910 or khuman@denverpost.com.
