A 370kg projectile was shot into the surface of Comet Tempel 1 in 2005 Nasa's Stardust spacecraft is about to sweep past Comet Tempel 1.
The encounter early on Tuesday (GMT) will give scientists unique information on how these great balls of ice and dust change over time.
Tempel 1 was visited by another probe back in 2005. It fired a projectile at the body to disturb the surface.
Stardust's images will reveal the extent of the impact crater and any other alterations that may have occurred on the 14km-wide object.
The spacecraft is expected to get to within about 200km (120 miles) of the comet nucleus.
It will take more than 70 high-resolution images; its dust analysis instruments will also investigate the environment around the object.
The event is occurring at an enormous distance from Earth - approximately 336 million km (209 million miles) away.
Pete Schultz Brown University“One idea is that there were two proto-cometary bodies that collided at very low speeds and smooshed together to form a comet like a stack of pancakes”
Stardust will be moving past its target at about 10km/s, with the moment of closest approach timed for 0437 GMT.
"The spacecraft is currently opposite the Sun from the Earth," explained Tim Larson, the mission's project manager at Nasa's Jet Propulsion Laboratory.
"That means that when we send a command up to the spacecraft and wait for the confirmation that the command arrived and was executed properly - that round-trip light-time is about 40 minutes. So there's nothing we can do to command the spacecraft or control it real-time during a flyby like this. Therefore, everything has to be programmed ahead of time, put onboard the spacecraft and sequenced; and everything must happen autonomously."
All the data acquired during the flyby will be stored on the spacecraft until an hour after the pass.
Stardust will then re-orientate itself to begin to beam back the pictures and other information.
The spacecraft is on what Nasa calls a "bonus mission". Stardust was launched back in 1999 with the primary goal of visiting another comet altogether - Wild 2. This it did in 2004, capturing dust particles from around the comet nucleus that it then returned to Earth in a capsule for study. But with sufficient fuel supplies still in its tanks, the probe was re-tasked by the US space agency to visit Tempel 1.
This extended mission has been dubbed Stardust-NExT, which is short for "New Exploration of Comet Tempel 1".
The "new" element relates to the fact that Tempel 1 has already been seen up-close by the Deep Impact spacecraft.
During that encounter in 2005, a washing-machine-sized block was fired at the comet to kick up surface material to study its composition.
But Deep Impact's swift passage across the face of the comet meant it never got to see the crater made by the projectile. Stardust will.
What is more, Stardust will be able to see what else has changed on Tempel 1 in the two trips it has since made around the Sun.
The closer a comet gets to our star, the more material it loses as ices vaporise and dust particles are carried away into space.
"Deep Impact saw only about a third of the surface and we'd like to see more," said Joe Veverka, the Stardust-NExT principal investigator from Cornell University
"And we'd like to see more of the areas that Deep Impact saw, including the smooth flows which apparently suggest that comet nuclei are not only modified by processes from the outside but also by internal processes."
Scientists observed a series layered terrains on Tempel 1. They hope the new data can help explain presence of these features and whether they have something to do with the comet's original construction.
"One idea is that there were two proto-cometary bodies that collided at very low speeds and smooshed together to form a comet like a stack of pancakes," speculated Pete Schultz, a mission co-investigator from Brown University.
This is the second comet rendezvous in just four months. In November, Deep Impact, itself also re-tasked by the agency, encountered Comet Hartley 2.
In November last year, another Nasa spacecraft got a close-up view of Comet Hartley 2 To date, spacecraft have flown close by seven comets - Tempel 1, Hartley 2, Borrelly, Wild 2, Halley, Giacobini-Zinner, and Grigg-Skjellerup (the missions to Giacobini-Zinner and Grigg-Skjellerup did not return pictures).
"Comets preserve some of the most faithful information about what happened when the Solar System formed," explained Professor Veverka.
"We know that comets preserve interesting molecules, some of which could have been involved in the origin of life on Earth. So, the overall objective of these studies is to get to the point where we can return sizeable samples of cometary material for chemical analysis to answer the question, 'are we comet stuff or not?'
"Stardust will not be returning samples but by imaging the crater, we will learn more about the mechanical properties of the surface so that when there is a mission that tries to land on the surface, there'll be data on how difficult it will be to remove material from the surface to bring back to Earth."
This article is from the BBC News website. � British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.
Source: http://www.bbc.co.uk/go/rss/int/news/-/news/science-environment-12433138
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