Herschel and Planck launched together on an Ariane 5 rocket from French Guiana in South America (Image: ESA)

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The largest space telescope ever launched, called Herschel, took to the skies on Thursday, along with a companion called Planck that will make the most precise measurements yet of the radiation left over from the big bang.

The Herschel and Planck space telescopes launched together aboard an Ariane 5 rocket at 1312 GMT from Kourou, French Guiana.


Now the pair, which together cost more than €2 billion, will make their way separately to L2, one of Earth’s five Lagrangian points, where the gravitational tugs from the Earth and the Sun cancel out. L2 is situated some 1.5 million kilometres from Earth – four times the distance from Earth to the moon.

Herschel, which boasts a 3.5-metre-wide mirror, is almost four times as big as its rival Spitzer. Over the coming years, it will scan the sky’s infrared light, allowing it to study cool celestial objects, from comets and asteroids in our own solar system to some of the universe’s most distant galaxies.

By orbiting near L2, Herschel will be naturally cooled to some 80 °C above absolute zero. That will minimise the heat radiating from the telescope itself, and liquid helium will cool its detectors even further, to just 0.3 °C above absolute zero. The telescope is expected to begin its scientific observations in September, after several months of testing out and calibrating its instruments.

Space-time ripples

Herschel’s launch companion, Planck, will map the cosmic microwave background (CMB), relic radiation from the big bang. It is expected to have some 10 times the sensitivity and three times the angular resolution of NASA’s WMAP satellite, which was launched in 2001.

With detectors cooled to just 0.1 °C above absolute zero, Planck will spin around its own axis, which will always point towards the sun. With each rotation, it will record a strip of sky, measuring the temperature of the CMB with an accuracy of about one millionth of a degree.

If all goes well, Planck could slash the uncertainties in astronomers’ measurements of the curvature of space-time and the relative ratios of dark matter, dark energy, and ordinary matter in the universe.

It could also find the first evidence of gravitational waves that are thought to have rippled through space-time as a result of inflation – a period just after the big bang when the universe rapidly expanded in size. The gravitational waves would have left a signature in the electromagnetic alignment, or polarisation, of CMB photons, that Planck may be able to detect.

Planck is expected to begin its scientific observations in July.

See a gallery of the most important telescopes in history