ESA Science & Technology - ESA's Planck satellite shapes up

Planck's conceptual design drives the specifications that European industry will have to follow in the construction phase, and identifies the most difficult parts of the project. It demonstrates that the sensitivity required for Planck pose a real technological challenge. "There's no previous experience at these levels of sensitivity. We are really facing an interesting challenge here!", says the Planck project scientist, Jan Tauber.

Planck's conceptual design shows the best way to integrate the scientific instruments (the payload) and the other components in the spacecraft, taking into account the scientific, technological and budgetary requirements. It is the result of a one-year study by the French company Alcatel Space, which has worked in close contact with other European industry (e.g. Dornier in Germany) and with the scientific groups building the payload instruments.

The design remains at the conceptual level, details will be dealt with during the detailed design phase (often called 'phase B'), but it does impose constraints on how each component of the satellite should be and is thus a key tool for ESA in defining the specifications for Planck. These specifications will form the basis of the competition that ESA will start next autumn to choose Planck's prime industrial contractor.

The general dimensions and shape of the spacecraft components are now clear. The whole satellite will be about 3.8 metres high and 4.5 metres wide. The payload consists of a 1.5 metre telescope that will focus the radiation from the early Universe onto the detectors. Below this so-called 'payload module' is the 'service module', containing all the systems to operate the satellite. Three conical aluminium layers at the base of the paylaod module protect the instruments from the heat coming from the service module: one of the instruments has to be kept at almost -273 °C, while the service module is several hundreds of degrees hotter.

To determine the best design for Planck, Alcatel engineers started from a baseline design and then used mathematical models to simulate what would happen to that theoretical spacecraft during the launch and when in orbit.

"Once you have the baseline design you start the analysis. You simulate the mechanical conditions during the launch, the temperature that will be reached when in orbit, the electromagnetic behaviour... if something doesn't fit with the requirements we change the design and start again", explains Alcatel project manager Jean Jacques Juillet.

Simulated noise

The studies leading to the final design must also identify critical areas of the project. In this case, one of the most complex problems is how to simulate the noise signals that may hide or confuse the true scientific data. Planck's goal is to analyse a radiation emitted shortly after the Big Bang that is still filling the Universe, known as the 'cosmic microwave background radiation'. Planck has to measure the temperature of this radiation all over the sky, with enough sensitivity to detect variations a million times smaller than a degree. The problem is, many astronomical objects and even the satellite itself, emit radiation that is much more intense than the cosmic background, and that can therefore hide its signature altogether.

Since astronomers have to subtract this spurious noise from the total measured signal, they want to know in advance, what the level and spectrum of the noise will be: to find this out they have to simulate it. This kind of simulation is difficult and laborious to make for the levels of sensitivity required by Planck.

"We are working with levels of sensitivity several orders of magnitude above average. We have to verify that those parts of the spurious signals generated by the spacecraft are at a level where the in-flight measurements themselves provide enough information to allow their subtraction. Unfortunately it is impossible to fully measure these signals on the ground. One of the things we have to do now is to find a good combination of measurement and analysis that will give us confidence that the satellite will achieve its objectives", says Tauber.

Mock-up arrival

As a way to demonstrate that all parts of the satellite can be integrated, Alcatel has also built a full size mock-up of Planck. It arrived at ESTEC just before Christmas and will remain in its two big containers for about one month. It will be mounted this month in the Erasmus building at ESTEC. Made of wood and aluminium, it will allow Planck scientists to have a glimpse of what the satellite will look like when it is finished, in the year 2005.

Footnote about Planck
The Planck satellite, one of ESA's main missions for this decade, has been designed to help answer key questions for humankind: how the Universe came to be and how it will evolve. To fulfil its mission Planck will examine the first light that filled the Universe after the Big Bang, the so-called Cosmic Microwave Background radiation. It will carry two arrays of highly sensitive detectors that are now being built by more than 40 institutes, most of them European and some from the United States. Planck will be launched in 2007 together with ESAs far-infrared space telescope, FIRST. They will separate shortly after the launch and will be operated independently at similar orbits located about 1.5 million kilometres away from Earth.