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Planck’s mirror in testing. Image credit: ESA
Image credit: ESA
Planck Telescope Tested in Vacuum: ESA’s Planck space telescope was removed last week from the Large Space Simulator (LSS) at ESTEC, ESA’s research and technology centre in Noordwijk, the Netherlands, after a thorough two-week test in temperatures down to -178 degrees Celsius. The test is an important milestone towards launch in 2008.

Once in space, Planck will investigate cosmic background radiation: the remnants of the Big Bang over fourteen billion years ago. The telescope will make observations in the far-infrared spectrum; this can only be achieved using super-cooled instruments. It is essential to test the telescope at very low temperatures – a task highly suited to ESA’s Large Space Simulator.

Cold vacuum
“Central to this test is exposure of the mirrors and their structural frame to a very cold vacuum”, explains ESA scientist Philippe Kletzkine. “The telescope is built at room temperature and then cooled to way below the freezing point. Even though the materials were chosen carefully, this makes each individual component of the telescope shrink to some small but not negligible extent. We need to know whether the resulting changes in shape match our predictions. We have to be spot on, so the mirrors line up properly.”

Videogrammetry is used to get a clearer picture of the changes in shape. Thousands of photographs, taken from many different angles, are used to build up a three-dimensional image of the mirrors, their structural frame and the place where the telescope’s cameras will be mounted. This process is repeated at several temperatures.

The telescope has been returned to a cleanroom at ESTEC. This completes the hands-on part of the measurement – now it is time for some arithmetic. Kletzkine: “Over the next few weeks we will be busy interpreting the test results. According to our first impressions the telescope has behaved well.”

Earlier this year, Planck’s mirrors were tested individually without the telescope structural frame at Alcatel, in Cannes, France. The results of their videogrammetry tests met expectations. Alcatel Alenia Space France is Prime Contractor to ESA for the Planck spacecraft.

Preparations for the mission continue at ESTEC. All the components needed to complete the spacecraft will be delivered and tested together in the coming months. Eventually the flight model will undergo vibration testing, acoustic testing, high and low temperature testing, as well as various tests of the computers and communications equipment on board.
(Source: ESA News Release)
Artist illustration of STEREO spacecraft. Image credit: NASA
Image credit: NASA
STEREO Spacecraft Set for Launch: At the end of this month, NASA is scheduled to put two eyeballs in orbit around the sun to provide the first stereoscopic views of the immense magnetic explosions on the sun’s surface that fling particles at Earth and create storms in space.

The twin spacecraft, called the Solar TErrestrial RElations Observatory (STEREO), will explore these massive explosions, or coronal mass ejections, which erupt as billowing magnetic storms that can dwarf the sun. Often more than 6 million miles across - the sun is 860,000 miles in diameter - they can throw out a cloud of gas equivalent to the mass of Mount Everest at speeds of 5 million miles per hour.

This gas reaches Earth and clashes with the planet’s own magnetic field, disrupting radio communications and threatening satellites and astronauts while producing beautiful, Kodachrome auroras - the Northern and Southern lights.

The spacecraft is scheduled to launch from Kennedy Space Center, Fla., on Thursday, Aug. 31, for a two-year mission. One STEREO craft will precede and one will follow Earth in its orbit around the sun to find out what the solar shock wave looks like elsewhere when Earth experiences an onslaught of charged particles.

“With STEREO, we have an unprecedented opportunity to make simultaneous measurements at several points along Earth’s orbit, to find out what coronal mass ejections look like at different locations and different times. We have never had that before,” said Janet Luhmann, a research physicist at the University of California, Berkeley’s Space Sciences Laboratory and a co-principal investigator on the mission.

Luhmann led a team that built a suite of instruments for STEREO that measures the energy of electrons and ions from the sun and the intensity of the sun’s magnetic fields. Called the In-situ Measurements of Particles And CME Transients (IMPACT), it is one of four instrument packages aboard the nearly identical spacecraft. Together, they provide data that will help pin down how and where the electrons and ions are accelerated in the sun’s corona and atmosphere and how coronal mass ejections propagate through and interact with the steady solar wind.

“By taking a multipoint perspective, imaging as well as in situ measurements with IMPACT of coronal mass ejections, STEREO is supposed to give a definitive answer to the questions: What are these coronal mass ejections? How are they shaped? How do they evolve? Where do they come from?” Luhmann said.

As an experiment, UC Berkeley scientists also will turn the data sent back by IMPACT into stereophonic sound.

“It will provide a sound track to any movies that come out of STEREO images,” said Luhmann. “The sound is not just a gee whiz thing, but it conveys a sense of the physical processes in space, which are invisible.”

The “sonification” project is both a test to see whether researchers’ ears can detect patterns in the measurements not obvious from visual or other analyses, and a way to engage the public in experiments that don’t produce pretty pictures. Space Sciences Laboratory scientists have produced an educational and public Web site about the sonification project and IMPACT measurements.

IMPACT incorporates seven instruments that will measure the energies of the solar wind “plasma” electrons, ranging from the slower ones produced by flares to the high-speed electrons produced by coronal mass ejections; the most energetic of the ions - protons, helium and iron nuclei; and the local magnetic field. Electron and magnetic field detectors are mounted on a 15-foot boom that points away from the sun.

“We might find, for example, that the Earth would experience a huge storm if it had been at the position of the head STEREO spacecraft, but there is nothing there at the position of the Earth,” Luhmann said. “We don’t really have a good feeling for how broad these disturbances are. I think that with current modelling capabilities for space weather, combined with these multipoint measurements, we will finally sort this out and at the end be better able to forecast space weather.”

“In terms of space-weather forecasting, we’re where weather forecasters were in the 1950s,” said Michael Kaiser, STEREO project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “They didn’t see hurricanes until the rain clouds were right above them. In our case, we can see storms leaving the sun, but we have to make guesses and use models to figure out if and when they will impact Earth.”

The twin STEREO spacecraft will be launched aboard a Delta II rocket and immediately slip into slightly different orbits near Earth. Then, two months after launch, a close flyby of the moon will sling one of them into a 388-day orbit that causes it to lag behind the Earth by 22.5 degrees. A month later, the second spacecraft will fly near the moon and be sling-shotted into a 346-day orbit at a position 22.5 degrees ahead of the Earth. Each year, these differing orbital periods will cause the spacecraft to drift farther apart - by 45 degrees each year - and farther from the Earth, until they eventually reach a point behind the sun from Earth’s perspective.

Each STEREO observatory, which is about the size of a golf cart, carries 16 instruments in all, including imaging telescopes for optical photos, equipment to measure solar wind and more energetic particles, magnetometers and radio antennas, which also were built at the Space Sciences Laboratory under the direction of Stuart Bale, assistant professor of physics.

The United States, the United Kingdom and several European countries provided the various STEREO instruments. The instruments were integrated with the observatories by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. NASA’s Goddard Space Flight Center in Greenbelt, Md., is responsible for the project management. The NASA Launch Services Program at Kennedy Space Center and Boeing are responsible for the launch. The total U.S. cost of the mission is $478 million, with an additional $60 million coming from European contributions.
(Source: Universe Today)
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