Astronomers at Australia's national radio and optical observatories will watch as a probe released from a spacecraft slams into a comet about 133 million km away at a speed of nearly 37,000 km/h (10.2 km per second).
The cosmic demolition derby takes place about 4pm AEST on 4 July when the comet, Tempel 1, will be most easily seen from the mid-Pacific.
The 370 kg probe, carried by NASA's Deep Impact spacecraft, has been travelling toward the comet for 173 days and has travelled over 431 million km.
At the time of the collision the comet will be travelling at 108,000 km/h. The probe will be travelling in almost the same orbit at 80,000 km/h, and will hit the comet at an angle.
The impact may gouge out a crater up to 200 m across and 50 m deep, and could lead to a flow of gas and dust from the comet's interior lasting for months. This outflow is what ground-astronomers will be looking for.
The comet will appear to be near the star Spica, the brightest star in the constellation Virgo, and also near the planet Jupiter. By the time the sun sets for eastern Australia it will be high in the sky, almost due north.
Before the impact the comet will not be bright enough to see with the unaided eye. The impact may brighten it, but by how much is unknown.
When the copper-fortified probe hits the flying iceberg the crash will be observed by the Deep Impact spacecraft, the Hubble Space Telescope, and ground-based observatories around the world.
In Australia CSIRO's radio telescopes and the optical telescopes of the Anglo-Australian Observatory will be watching.
CSIRO's radio telescopes can work in daylight and so scientists will be able to watch the comet at the time of the impact, and for the following seven hours.
The Anglo-Australian Observatory's telescopes will be able to see the comet about two hours after the impact, after the sun has set in eastern Australia.
Australian telescopes are better placed than telescopes in Europe and North or South America to see any changes in the comet a few hours after the impact.
“Australia has excellent radio telescopes,” says Dr Paul Jones, a Visiting Fellow at the Australia Telescope National Facility, who leads the team using CSIRO's telescopes.
“We'll be looking for specific molecules erupting from the comet—molecules you can most easily detect in the radio.”
“It's not every day you get take part in an experiment like this,” says CSIRO's John Sarkissian, who will be observing with the Parkes telescope. “We probably won't get another chance for some time.”
Because the comet is moving across the sky faster than the background stars, astronomers will have to use special observing techniques.
“We're going to have to drive the telescope manually at a predetermined rate,” says the Anglo-Australian Observatory's Dr Rob Sharp, who is observing with the AAO's UK Schmidt telescope at Siding Spring Observatory.
“The Schmidt hasn't been used in this way for quite some time, so we'll be relying on the expertise of the telescope operators.”
Smashing into the comet will give astronomers access to the pristine material of the comet's interior. Comets preserve material from the early solar system but their surfaces have been chemically and physically altered by the Sun's radiation.
The impact of the probe into the comet has been likened to a mosquito running into a 767 airliner.
The impact will not appreciably modify the comet's orbital path and the comet poses no threat to Earth now or in the foreseeable future.
CSIRO's Parkes telescope (near Parkes NSW) will be used to look for OH molecules, which are a sign of water in the comet. CSIRO's Mopra telescope (near Coonabarabran NSW) and Australia Telescope Compact Array (near Narrabri NSW) will be used to look for HCN molecules, which are a general marker for a range of carbon-based molecules.
The Anglo-Australian Observatory's UK Schmidt Telescope (near Coonabarabran NSW) will be used to measure chemical abundances at many spots on the comet simultaneously. The Anglo-Australian Telescope (near Coonabarabran NSW) will study several compounds containing carbon and nitrogen, to calculate isotope ratios for those elements.
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