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Spacecraft is coldest object out in space

The coldest known object out in space isn't a frozen comet or some distant, chilly gas cloud. It's a spacecraft: the Planck probe, designed to study the faint afterglow of the big bang.
Image: Planck
An artist's conception shows light entering the European Space Agency's Planck telescope.ESA
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The coldest known object out in space has now been announced by scientists. It's not a frozen comet or even some distant, chilly gas cloud. Rather, it's a spacecraft.

On July 3, the European Space Agency's Planck spacecraft reached this frigid extreme as part of a key step in the satellite's mission to observe the remnant radiation of the big bang.

Since its launch on May 14 (accompanied by its sibling spacecraft Herschel), Planck has been traveling to its final orbit at the second Lagrange point of the sun-Earth system, L2, and cooling its instruments down to their operational temperature of minus 459.49 degrees Fahrenheit (minus 273.05 Celsius). This temperature is just 0.1 degree Celsius above absolute zero, the coldest temperature theoretically possible in our universe.

"It is indeed both the coldest spot in any spacecraft that we know about, and also the coldest known object in space, including dust, gas etc.," Planck project scientist Jan Tauber wrote in an e-mail. "Of course in a laboratory on Earth, colder spots can be made."

Such low temperatures are necessary for Planck's detectors to study the cosmic microwave background, or CMB — the first light released by the universe, only 380,000 years after the big bang — by measuring its temperature across the sky.

Over the next few weeks, mission operators will fine-tune the spacecraft's instruments. Planck will begin to survey the sky in mid-August.

Planck is equipped with a passive cooling system that brings its temperature down to about minus 382 F (minus 230 C) by radiating heat into space. Three active coolers take over from there, and bring the temperature down further to an amazing low of minus 459.49 F (minus 273.05 C).

The CMB can be detected in all directions of the sky at an average temperature of 2.73 Kelvin (minus 450 degrees Fahrenheit or minus 270 degrees Celsius). Previous space-based missions have found that the temperature of the CMB varies ever so slightly in different areas. Planck is built to resolve these variations in greater detail.

Planck's detectors will look for variations in the temperature of the CMB that are about a million times smaller than one degree — this is comparable to measuring from Earth the heat produced by a rabbit sitting on the moon.

The photons in the CMB could also tell astronomers when and how the first stars formed. They could also shed light, so to speak, on how galaxies, galaxy clusters and the large voids in space came to be.

After a crucial maneuver, Planck also reached the L2 point last week. At the start of the maneuver, Planck was located 0.89 million miles (1.43 million kilometers) from Earth.

"Everyone here is quite happy to see Planck getting into its operational orbit," said Chris Watson, Spacecraft Operations Manager.

Planck is slated for a 15-month mission, time enough for two full surveys of the sky.

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