CAPE CANAVERAL, Jan 13: A Nasa spacecraft lifted off on Wednesday on the first leg of a six-month journey to blast a hole in a comet and study what lies beneath.

The Delta 2 rocket took off from Cape Canaveral Air Force Station as planned with a spacecraft the size of a small car tucked inside its nose cap.

The probe, named Deep Impact, was put on a path to encounter Comet Tempel 1 on July 4 more than 132 million kilometres from Earth. "We have ignition and lift off of a Delta 2 rocket, carrying Deep Impact - NASA's journey to unlock the mystery of the solar system's origin," said NASA launch commentator Jessica Rye.

While most space probes passively study the cosmos or conduct microscopic tests, Deep Impact will experiment with a planetary body itself. Scientists find comets alluring because their frozen cores contain pristine samples of materials used in the formation of the universe.

While telescopes and spacecraft have collected volumes of data about comets - one probe is slated to deliver the first samples of comet dust to scientists on Jan. 15, 2006 - nothing is known about comets' interiors.

As the icy bodies near the sun, dust and particles boil off, forming the comets' distinctive tails and fuzzy comas. "The key goal of Deep Impact is to relate all these measurements of the surface material to what the deep interior material is, (material) which is preserved from the beginning of the solar system," said Jay Melosh, a University of Arizona researcher and a mission co-investigator.

Exposing a comet's core is no easy task. Not only are comets relatively small - Tempel 1, for example is believed to be about eight to 14 kms long and about one-third that in diameter - they are moving very quickly.

Deep Impact, for example, will have to blast across space at 10kms per second to overtake comet Temple 1, drop off a stubby-nosed, copper-capped projectile in its path and scramble to a safe distance to observe the mega-explosion.

The comet is expected to strike the coffee-table sized projectile with the force of 4.5 tons of dynamite, scientists said. Complicating the orbital ballet are several unknowns, including the exact size of the comet, its slow tumble and the fact that its interior could be as loosely structured as a bowl of corn flakes or packed as tightly as concrete on a sidewalk.

In addition, Melosh said in a pre flight briefing, the impact must take place on the sunlit side of the comet, as well as in the line of sight of the mother ship, which will be stationed about 480kms away. Otherwise, scientists will get no data back on Earth.

Still, principal investigator Mike A'Hearn said simulations give the team more than a 99.9 percent chance of a good hit. The team wants to carve as big a crater as possible into the comet's core to get images and infrared data about its inner materials and structure.

The information is expected to help answer some basic questions about the formation of the solar system, including what role comet impacts played in the development of Earth. "We know comets delivered water," said A'Hearn. "but what else?" -Reuters