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Telescopes and space missions

Telescopes and space missions

Europe launches cosmic explorers

14 May 2009
The Ariane-5 taking off

Two groundbreaking missions to map the geometry of the universe and study the formation of the earliest galaxies have successfully launched onboard an Ariane–5 rocket from French Guiana. The Herschel and Planck satellites, which have been built by the European Space Agency (ESA), took off at 13:12 local time from the Guiana Space Centre in Kourou.

Their destination is an area in space some 1.5 million kilometres further out from the Sun beyond the Earth. Known as Lagrange point L2, it is where a space probe can usefully hover, little disturbed by stray signals from home and without having to use much fuel to keep it in position.

First to arrive, in roughly two months’ time will be Planck – a microwave observatory like NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), which is also at L2. Planck will probe the geometry and contents of the universe by finely measuring the cosmic microwave background (CMB) radiation – a remnant of the Big Bang.

“Planck will provide a big jump in knowledge,” says Nazzareno Mandolesi of the Institute of Space Astrophysics and Cosmic Physics in Bologna, principal investigator for one of Planck’s two instruments, which will together measure the CMB at frequencies between 27 GHz to 1 THz.

More than a month later, Herschel, named after the German-born astronomer who in 1781 discovered Uranus, will join the group in a much wider orbit around L2 than Planck. This far-infrared and submillimetre telescope will study the universe’s coolest objects, from the era when the first stars and galaxies were formed to the present day.

For the first time we will get a proper sense of star formation in [other] galaxies Michael Rowan-Robinson, Imperial College London

“Herschel is the first really big infrared telescope,” says astrophysicist Michael Rowan-Robinson of Imperial College London. “For the first time we will get a proper sense of star formation in [other] galaxies.”

Cosmic echo

The Planck mission has a more focused goal than Herschel: to map out the CMB in the finest detail yet. The CMB was created 400,000 years after the Big Bang, when primordial protons, neutrons and electrons formed neutral atoms that allowed photons to finally move freely. The photons have continued to do so ever since, being stretched to microwave frequencies due to the expansion of the universe.

The European Space NASA’s Cosmic Background Explorer (COBE) set the field alight in 1992 when it revealed that the CMB is not uniform but has slight variations that carry information about the early universe.

“It transformed the field completely,” says astrophysicist Pedro Ferreira of the University of Oxford. Researchers set to work on a raft of new instruments, ground-based, airborne and in orbit, including WMAP and Planck.

[Planck will be able to] distinguish between different theories of inflation and decide what theories are actually viable Pedro Ferreira, University of Oxford

The value of Planck and other CMB experiments is that they provide some of the only hard data about the very early universe. Cosmologists believe that the nascent universe underwent a period of extremely rapid growth called inflation and Ferreira says that Planck will be able to “distinguish between different theories of inflation and decide what theories are actually viable”.

The Degree Angular Scale Interferometer, sited at the South Pole, found the first evidence that the CMB photons are polarized; and Planck will measure that polarization in more detail than was possible before.

The big challenge for Planck will be to detect a so far unobserved type of polarization known as “B-modes”, which date back to the period of inflation and are determined by the density of primordial gravitational waves.

“This is a signal that has gone unobstructed since the Big Bang,” says Ferreira. If they could be detected, thinks Ferreira, such waves might tell us what mechanism generated them in the universe’s first moments, what caused inflation, and even if there was something before the Big Bang.

Eye in the sky

Herschel has two goals: to study star formation in our galaxy; and galaxy formation across the universe. It is hard to see star-forming regions at visible wavelengths because they are usually shrouded in gas and dust that block visible light. Infrared light pierces this veil and Herschel has the resolution to reveal the details of how clouds of cool atoms and molecules coalesce into stars.

As water vapour in the atmosphere absorbs much of the infrared radiation from space, astronomers have long been trying to get telescopes above the atmosphere. IRAS, a US- UK-Netherlands mission in 1983, was the first to map the entire sky, followed by ESA’s Infrared Space Observatory (ISO) in the 1990s and NASA’s current Spitzer Space Telescope.

All of these missions, however, are limited by their cooling systems. Because any warm object emits radiation in the infrared, these telescopes and their detectors must be chilled close to absolute zero using liquid helium.

Helium is heavy to hoist into orbit, which limits the size of any mirror that can be launched to less than 1 m, in turn restricting angular resolution. Moreover, helium eventually boils off, thereby limiting mission lifetimes to just a few years.

As Herschel is viewing slightly longer wavelengths than previous infrared missions, it can get by with its mirror and telescope just being “passively” cooled to 80 K by the coldness of outer space, leaving just the detectors bathed in liquid helium.

This allows Herschel to have a mirror that is 3.5 m across, the largest yet deployed in space. The satellite will investigate light with wavelengths of 55–670 µm. On a larger scale, Herschel will look back to the early universe to see galaxy formations that are invisible to the likes of the Hubble Space Telescope because of gas and dust.

“We will find out how all the galaxies we see today came into being,” says Matt Griffin of the University of Wales, Cardiff, principal investigator on one of Herschel’s three instruments. It will also probe the planet-forming regions around stars in our Milky Way, the gas giants of our solar system, and comets and objects in the Kuiper Belt.

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