If all goes as planned, the Deep Impact spacecraft will release a wine barrel-sized probe on a suicide journey, hurtling towards the comet Tempel 1 - about 130 million kilometres away from Earth at the time of impact.
"It's a bullet trying to hit a second bullet with a third bullet in the right place at the right time," said Rick Grammier, project manager at Nasa's Jet Propulsion Laboratory in Pasadena.
Scientists hope the 4 July collision will gouge a crater in the comet's surface large enough to reveal its pristine core and perhaps yield cosmic clues to the origin of the solar system. The comet is about half the size of Manhattan, or roughly 28sq km.
Nasa's fleet of space-based observatories - including the Hubble, Spitzer and Chandra telescopes - along with an army of ground-based telescopes around the world are expected to record the impact and resulting crater.
The big question is: What kind of fireworks can sky-gazers expect to see from Earth?
Scientists do not know yet. But if the probe hits the bull's-eye, the impact could temporarily light up the comet as much as 40 times brighter than normal, possibly making it visible to the naked eye in parts of the Western hemisphere.
The Deep Impact spacecraft will
fire a probe at the comet
"We're getting closer by the minute," Andrew Dantzler, the director of Nasa's solar system division, said earlier this month. "I'm looking forward to a great encounter on the Fourth of July."
If the $333 million mission is successful, Deep Impact will be the first spacecraft to touch the surface of a comet. In 2004, Nasa's Stardust craft flew within 237km of Comet Wild 2 on its way back to Earth carrying interstellar dust samples.
Scientists say Deep Impact has real science value that will hopefully answer basic questions about the solar system's birth.
Comets - frozen balls of dirty ice, rocks and dust - are leftover building blocks of the solar system after a cloud of gas and dust condensed to form the sun and planets four and a half billion years ago. As comets arc around the sun, their surfaces heat up so that only their frozen interiors possess original space material.
Very little is known about comets and even less is known about their primordial cores. What exactly will happen when Tempel 1 is hit on 4 July is anybody's guess. Scientists believe that the impact will form a circular depression that will eject a cone-shaped plume of debris into space.
But not to worry. Nasa guarantees that its experiment will not significantly change the comet's orbit nor will the smash-up put the comet or any remnants of it on a collision course with Earth.
No spacecraft has touched down
on the surface of a comet yet
Discovered in 1867, Tempel 1 is a short-period comet, meaning that it moves around the sun in an elliptical orbit between Mars and Jupiter and can be sighted every six or so years.
The Deep Impact spacecraft shares the same name as a 1998 Hollywood disaster movie about a comet headed straight for Earth. Nasa says that the names for the space mission and blockbuster movie were arrived at independently around the same time and by pure coincidence.
The spacecraft blasted off from Cape Canaveral, Florida, in mid-January. In March, scientists got a scare when test images from one of Deep Impact's telescopes were slightly out of focus. The problem was fixed, and a month later, Deep Impact took its first picture of Tempel 1 from 64 million kilometres away, revealing a big snowball of dirty ice and rock.
Last week, scientists processed the first images of the comet's bright core taken from 32 million kilometres away, which should help the probe zero in on its target.
The real action starts in the early morning of 3 July (Eastern time) when the spacecraft separates, releasing an 820lb (369kg) copper probe called the "impactor" on a one-way trip straight into the path of the comet. During the next 22 hours, mission control at Jet Propulsion Lab in Pasadena will steer both craft towards Tempel 1.
Two hours before the 4 July encounter, the impactor kicks into autopilot, relying on its self-navigating software and thrusters for the rest of the journey to steer towards the sunlit part of the comet's nucleus so that space and Earth-based telescopes can get the best view.
The collision is expected to occur at about 1:52am EDT when the comet, travelling through space at 10km per second, runs over the impactor, which will be shooting some of the most close-up pictures of Tempel 1 up until its death.
"It's a bullet trying to hit a second bullet with a third bullet in the right place at the right time"
Project manager at Nasa's Jet Propulsion Laboratory, Pasadena
The high-speed collision is expected to excavate a crater that can range anywhere from the size of a house to a football stadium, and from two to 14 stories deep. A spew of ice and dust debris will likely shoot out from the newly formed hole, possibly revealing a glimpse of the comet's core.
Scientists say if the comet is porous like a sponge, the impact should produce a stadium-sized crater about 150ft (45m) deep and 650ft (195m) wide. This suggests that the comet's inside holds some of the pristine material of the early solar system.
But if the comet is packed like a snowball, the crater formed would be much smaller. Another scenario is that the comet is so porous that most of the impactor's energy is absorbed, creating an even smaller but deep crater.