The 820-pound copper probe was on course to intercept the comet Tempel 1 and smash a hole in it so scientists could get their first peek at the heart of one of the icy celestial bodies.
Comets are leftovers building blocks of the solar system, which formed when a giant cloud of gas and dust collapsed to create the sun and planets. Because comets were born in the system's outer fringes, their cores still possess some of the primordial ingredients and studying them could yield clues to how the solar system formed 4.5 billion years ago.
The "impactor" probe separated from the Deep Impact spacecraft late Saturday and began a 500,000-mile dive toward the sunlit section of Tempel 1, a pickle-shaped comet half the size of Manhattan and 83 million miles away from Earth.
The mothership, meanwhile, fired its thrusters to slightly change course and stake out a front-row seat 5,000 miles from the high-speed collision, expected to occur at 1:52 a.m. EDT Monday.
On Sunday, scientists released the first picture of the solo probe taken by the mothership shortly after separation showing the probe as a bright, distinct dot surrounded by blackness.
The probe was to designed to shoot close-up pictures at a relative speed of 23,000 mph until hitting the comet. After that, the mothership was to take over recording the scene through its high-resolution telescope.
"We anticipate a little bit of a bumpy ride," said systems engineer Jennifer Rocca.
Among challenges workers in mission control at NASA's Jet Propulsion Laboratory in Pasadena were facing was the probe's switch to autopilot two hours before Sunday night's encounter, relying on computer software and thrusters to steer itself into the path of the onrushing comet.
About 15 minutes after the probe's impact, the mothership was to make its closest flyby of the comet nucleus, approaching within 310 miles. Scientists expected it would be bombarded with flying debris and will stop taking pictures, turning on its dust shields for protection.
NASA's brigade of space-based observatories, including the
Hubble Space Telescope, Chandra X-ray Observatory and Spitzer Space Telescope, also were pointing toward the comet to record the impact.
Little is known about comet anatomy, so it wasn't clear what exactly would happen. Scientists expected the collision to spray a cone-shaped plume of debris into space with a resulting crater ranging anywhere from the size of a large house to a football stadium and be between two and 14 stories deep.
Deep Impact blasted off in January from Cape Canaveral, Fla., for its six-month, 268 million-mile journey. In what scientists say is a coincidence, the spacecraft shares the same name as the 1998 movie about a comet that hurtles toward Earth.
Discovered in 1867, Tempel 1 moves around the sun in an elliptical orbit between Mars and Jupiter every six or so years.
In April, the 1,300-pound spacecraft took its first picture of Tempel 1 from 40 million miles away, revealing what amounts to a celestial snowball. Last month, still 20 million miles away, scientists saw the solid core of Tempel 1 for the first time.