* Astronomy

Shortly after Deep Impact was launched Jan. 12, engineers found a focusing problem in the spacecraft's main camera-telescope, the High Resolution Instrument.
The problem has been traced to a mirror used when the HRI was tested at Ball, the space agency said. While flat at room temperatures, the mirror unexpectedly developed a slight curvature during testing at ultra-cold temperatures. Ball engineers didn't detect the curvature at the time.
"It was a mirror used in the test setup, not part of the instrument itself. Because of the curvature in the test mirror, we set the focus (on the HRI) slightly incorrectly." - Harold Reitsema, director of program development at Ball.

A worldwide network of observers, both professional and amateur, is part of the Deep Impact project. Within the global network of space and Earth telescopes for this unprecedented astronomical event, Europe plays a significant role.
Two ESA spacecraft, ESA’s Rosetta comet-chaser and its XMM-Newton space observatory, together with the NASA/ESA Hubble Space Telescope, will monitor the comet before impact, and then watch the impact and its aftermath.
ESO’s Very Large Telescope (VLT) facilities in Chile will observe the event in a big observation campaign.
ESA’s optical ground station at Tenerife, Spain, will also look at the impact.



Rosetta is in the most privileged position in space to watch this unique event, and will be able to monitor the comet continuously over an extended period.
Rosetta is likely to be one of the key observatories of this event because of its set of powerful remote-sensing instruments.
The Deep Impact experiment will be the first opportunity in time to study the crust and the interior of a comet.
As the material inside the comet’s nucleus is pristine, it will reveal new information on the early phases of the Solar System.
It will also provide scientists with new insight on the physics of craters formation, and thereby give a better understanding on the crater record on comets and other bodies in the Solar System.

The scientific outcome of the experiment depends crucially on pre-impact and follow-up observations. Before the impact, it is necessary to find out as much about the comet as possible, such as size, albedo (reflectivity) and rotation period.
It is essential to have a good set of observations before the impact to unambiguously distinguish the effects of the impact from the natural activity of the comet.

Due to the currently limited understanding of the structure of these dirty ‘snowballs,’ it is not known what the effect of the impact will be. Some scientists predict the ejection of a plume and the creation of a football stadium sized crater. Others think that the comet could simply swallow the impactor with hardly any visible effect, or that it may eventually break up.
To prepare for the Deep Impact event, two teams of astronomers have already used ESO’s telescopes over several months to do pre-impact monitoring, taking images and spectra of the comet both in the visible and mid-infrared wavebands.
These teams make observations typically once per month, using either the 3.6m or the 3.5m New Technology Telescope (NTT) telescopes at La Silla.



ESO’s telescopes will also be used in the post-impact observations. As soon as the comet is visible after the impact from Chile, all major ESO telescopes – the four Unit Telescopes of the Very Large Telescope Array at Paranal, as well as the 3.6m, 3.5m NTT and the 2.2m ESO/MPG telescopes at La Silla – will be observing Tempel 1, in very close collaboration with ESA and the space mission’s scientific team.

Fifty-nine days before going head-to-head with comet Tempel 1, NASA's
Deep Impact spacecraft successfully executed the second trajectory correction manoeuvre of the mission.
The burn further refined the spacecraft's trajectory, or flight path, and also moved forward the expected time of the July 4th comet encounter so that the impact would be visible by ground- and space-based observatories.
The 95-second burn - the longest remaining firing of the spacecraft's motors prior to comet encounter -- was executed on May 4.
It changed Deep Impact's speed by 18.2 kilometres per hour.



"Spacecraft performance has been excellent, and this burn was no different. It was a textbook manoeuvre that placed us right on the money." - Rick Grammier, Deep Impact project manager at NASA's Jet Propulsion Laboratory, Pasadena, US.
Right on the money is where Deep Impact has to be to place a 1-meter-long (39-inch) impactor spacecraft in the path of a comet about as big as the island of Manhattan that is bearing down on it at 37,100 kilometres per hour. At the same time, from a very comet-intimate distance of 500 kilometres, a flyby spacecraft will be monitoring the event. This all occurs i at 9:52 GMT (July 3,10:52 p.m. Pacific time) -- at a distance of 133.6-million kilometres from Earth.
"With this manoeuvre our friends working the Hubble Space Telescope are assured a ringside seat. Their observations, along with space telescopes Chandra and Spitzer and numerous ground-based observatories, will provide us with the most scientific bang for our buck with Deep Impact." - Dr. Michael A'Hearn Deep Impact Principal Investigator.
Deep Impact is comprised of two parts, a "flyby" spacecraft and a smaller "impactor." The impactor will be released into the comet's path before a planned high-speed collision on July 4.
The crater produced by the impact could range in size from the width of a large house up to the size of a football stadium, and from 2 to 14 stories deep.
Ice and dust debris will be ejected from the crater, revealing the material beneath.
The Deep Impact spacecraft has four data collectors to observe the effects of the collision. A camera and infrared spectrometer, which comprise the High Resolution Instrument, are carried on the flyby spacecraft, along with a Medium Resolution Instrument. A duplicate of the Medium Resolution Instrument on the impactor will record the vehicle's final moments before it is run over by comet Tempel 1 at a speed of about 37,100 kilometres per hour.

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The Deep Impact mission team closed the Commission Phase of flight with the completion of the impactor projectile's checkout activities. During that phase, the team verified the basic health of all subsystems and tested the operation of the science instruments.
The mission now moves to Cruise Phase.
The Deep Impact spacecraft successfully photographed
Comet Tempel 1, from a distance of 39.7 million miles.

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