Scientists are now just days away from their first glimpse of the inside of a comet, when 'Deep Impact' finds its target, writes Dick Ahlstrom.
Spectacular fireworks are planned for July 4th, but they might be a bit difficult to see. They take place 133.6 million kilometres away on that day when a copper-clad projectile slams into a mountain-sized comet at more than 37,000 kilometres per hour.
It has taken 173 days for Nasa's Deep Impact spacecraft to reach its intended target, comet Tempel I, and scientists are now only days away from their first glimpse of the inside of a comet.
The cylinder-shaped impactor to be released by Deep Impact is one metre long and one metre in diameter. Weighing 372kg, it will in effect be "run over" by the advancing comet, which is zipping along at a startling 10.3km per second.
Cameras, spectrographs and other equipment will in turn watch the collision and will give information about the physical properties and the ingredients that go into the making of a comet.
It is tough enough to get a satellite up to a fast-moving comet, but controllers at Nasa will also have to set the impactor in the comet's path. This process will begin the day before the collision when Deep Impact releases the copper bullet. Thrusters on the impactor will then be fired to position it in front of the advancing Tempel I.
This might sound an easy thing to do given that the irregular-shaped comet, at about 14km by 4.6km by 4.6km, is something close to half the size of Manhattan Island. The high speeds give no room for error, however, and if the impactor can't be placed accurately enough, the comet will whiz by untouched.
"We are really threading the needle with this one," according to Rick Grammier, Deep Impact project manager at Nasa's Jet Propulsion Lab in Pasadena. "In our quest of a great scientific payoff we are attempting something never done before at speeds and distances that are truly out of this world."
Scientists are particularly interested in comets. Some are hurled into our solar system from a place far beyond Pluto, known at the Oort Cloud. Others dip into our solar system from the Kuiper Belt.
The assumption is that comets are cosmic leftovers, the stuff from which our sun and planets were made. Study a comet and you may be looking back at our solar system's origins.
The question is, will the impactor sink like a stone in snow or hit Tempel I with a bang? Scientists have long referred to comets as akin to "dirty snowballs", loose constructs of silicate dust (about 25 per cent), organic compounds such as tars (25 per cent), ices including water ice (40 per cent), and other material (10 per cent).
The Deep Impact mission will finally answer this question, at least in terms of Tempel I's content. The crater produced could vary in size from a large house to the size of Croke Park, from two to 14 storeys deep.
Deep Impact's cameras will be watching the show to see what kind of hole the impactor makes, with image-capturing reaching a peak as the spacecraft reaches a point about 700km away from Tempel I.
Unfortunately, during the critical time, just as the impact is about to take place, the spacecraft will have to go into "shield mode" to protect itself from the comet's coma, the halo of dust and debris that surround the comet itself. There is a 50-50 chance Deep Impact will be knocked out of service by this wave of particles.
The spacecraft will get to within 500km of the nucleus about 14 minutes after impact, but with shields up it won't be able to watch the impact or its immediate aftermath with its on-board telescope.
About 35 minutes after closest fly-by, mission controllers will drop the dust shields and turn the instruments back towards the comet, which will begin to pull away from the spacecraft because of its greater speed. The spacecraft's telescope should be able to detect the site of impact and an infrared spectrometer will analyse the material thrown out by the copper bullet.
The fly-by spacecraft is about the size of a large jeep. It will continue to relay images and data back to Earth for about 30 days after its close encounter with Tempel I.
Despite the energy generated by the impact, there will be little change in the comet's trajectory. Mission scientists equate the net impact on the comet's motion to a mosquito being run over by a passenger aircraft.
"The impact simply will not appreciably modify the comet's orbital path," states Deep Impact mission scientist Don Yeomans.
Nor will the encounter change the comet's route to make it a danger to Earth. "Comet Tempel I poses no threat to the Earth, now or in the foreseeable future," says Yeomans.