By Jonathan Amos
Science reporter, BBC News, Kourou
The Planck telescope is now very close to flight
Europe's Herschel and Planck space telescopes have finally come together.
The satellites now share a common cleanroom at the Kourou spaceport in French Guiana, from where they will be despatched into orbit on 16 April.
The observatories have been produced as part of a joint programme that has taken more than 10 years to develop and which is worth some 1.9bn euros.
Their arrival in the S1 preparation hall at Kourou marks the first time the pair have come face to face.
For space science, Herschel-Planck is definitely the biggest launch in our history
Jacques Louet, Esa
"They were once at our technical centre in Holland at the same time, but there was a wall between them so they didn't actually come together," observed Thomas Passvogel, the Herschel-Planck project manger with the European Space Agency (Esa).
"It's great to have them finally in the same room," he told BBC News.
The satellites will ride atop a single Ariane rocket into orbit. In doing so, they will become the most valuable payload dedicated to pure science that Europe has ever attempted to put in space.
Herschel - the bigger of the two at 7m in height - is a far-infrared and sub-millimetre telescope which will investigate how stars and galaxies form and how they evolve.
HERSCHEL SPACE TELESCOPE
Herschel's main mirror will be the biggest anyone has put in space
Can probe clouds of gas and dust to see stars being born
Will investigate how galaxies have evolved through time
Instrument detectors will be cooled close to absolute zero (-273C)
Planck will survey the Cosmic Microwave Background - a "fossil light" that pervades the entire Universe and is detectible at radio wavelengths. It should provide new insights into how the cosmos came into being, and why it looks the way it does now.
The telescopes are undergoing a final round of testing to check nothing has been damaged in transit from Europe.
Their sensitive mirrors have been covered to protect them from contamination.
Herschel's giant 3.5m primary mirror is obscured by a silver drape. Planck's reflectors are hidden under an orange film.
The covers will not now be removed until just before launch.
One by one, the observatories' instrument teams are signing off their work. The UK group that built Herschel's SPIRE instrument left Kourou this week, stating there was nothing left for them to do.
"It's been many years of work to build and test the instrument. And to see it here with its telescope, constructed and ready to go - it's wonderful," said Tanya Lim, from the STFC-Rutherford Appleton Laboratory.
Provided no last-minute problems are identified, the satellites will be moved shortly to the spaceport's fuelling hall where their tanks will be pumped full of hydrazine. This will power the thrusters the spacecraft will use to make orbital corrections.
ARIANE - EUROPE'S 'WORKHORSE'
The solid boosters are now attached to Ariane's core stage
Heavy-lift rocket can launch two satellites on same flight
Herschel will be released first; Planck soon after
Orbit injection very different from standard Ariane satellite flights
And because both observatories' instruments are required to operate at extremely cold temperatures, their cooling systems will also need to be primed. In the case of Herschel, this means loading its cryogenic tank with more than 2,000 litres of super-fluid helium.
The 4m-tall Planck will sit underneath its larger cousin at the top of the Ariane. The pair will be protected during their rapid ascent through the Earth's atmosphere by a 17m-long fairing.
The Ariane vehicle is itself very close to completing its integration phase. The solid rocket boosters have been attached to the central core stage, and the whole structure will be moved next week to the final assembly shed to await the arrival of the passengers.
"Herschel and Planck will be assembled on payload adaptors, and then the fairing will be closed and hoisted on top of the launcher," explained Jean-Jacques Auffret, from rocket operator Arianespace.
"After that there will be tests to ensure that all electrical links are OK. And then we roll out."
Herschel and Planck take many weeks to get to their observation stations
Gravity conditions at L2 allow for cheap (in fuel terms) orbital corrections
Environmental conditions (heat & radiation) are more stable than at Earth
L2 takes its name from its discoverer, Joseph-Louis Lagrange (1736-1813)
The launch represents another first for Ariane. Never before has it sent spacecraft to the second Lagrangian point (L2) - a gravitational "sweet-spot" that lies some 1.5 million km from Earth in the direction opposite the Sun.
PLANCK SPACE TELESCOPE
In Planck's mirror (above), it is possible to see Herschel's reflection
Planck will survey the famous Cosmic Microwave Background
This ancient light's origins date to 380,000 years after the Big Bang
It informs scientists about the age, evolution and fate of the cosmos
Planck's measurements will be finer than any previous satellite
It will mean the satellites having to make fewer orbital corrections over their lifetimes.
To get them there, Europe's workhorse launcher will put Herschel and Planck on an initial, highly elliptical trajectory, running from a perigee (closest approach to Earth) of 270km to an apogee (most distant point) of 1.2 million km.
The satellites will then thrust themselves off this path to take up station around the L2 point.
It is sure to be a nervous few moments when the Ariane lifts clear of the pad - and not just for the scientists and engineers who have invested so much time in the double mission.
For Esa officials, too, there will be some anxiety as they wait for vindication of their decision to fuse the Herschel and Planck programmes.
Only Envisat, Esa's flagship Earth-observing mission, had been to the pad with a bigger price tag, said Jacques Louet, the head of science projects at the agency.
"Envisat was 2bn euros for the space segment, including the launcher; and 300 million for the operations over five years," he told BBC News.
"But for space science, Herschel-Planck is definitely the biggest launch in our history."
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