The final review meeting at ESA on 14 April 2003 concluded the project a resounding success. With the successful completion of the test and measurements program on the Nulling Instrument, a technological breakthrough has been achieved that will allow astronomers in the future to detect small planets orbiting other stars, something which has not been possible with the current observational technology.

As more planets become visible, the technology will also have a role to play in the search for extraterrestrial life. But it is not only space telescopes of the future that stand to benefit. The technology can also help medical science.

Over recent years, astronomers have discovered more than 100 large planets outside our solar system. However, these planets are only observable indirectly and cannot be seen by large telescopes like the Hubble Space Telescope and the Very Large Telescope in Chile.

To be able to see planets directly, telescopes require a much greater resolution. And to detect extraterrestrial life, it is essential to be able to look at planets that orbit close enough to a star to be in the "habitable zone".

Which creates an additional problem: light. The light from a star is some million times brighter than the light from the planet. It's like trying to see a candle next to a lighthouse from a distance of 10,000 km!

Artificial solar eclipse The solution to this problem is to combine the signals from two or more telescopes using a technology known as 'Nulling Interferometry'. This technology is able to create an artificial solar eclipse, allowing the orbiting planet to become visible.

In order to detect extraterrestrial life, the atmosphere of the planet has to be studied. This can be done using Infrared Spectroscopy across a broad waveband (from wavelengths of 6 to 18 microns). Nulling Interferometry across such a waveband requires some state-of-the-art devices such as an 'Achromatic Phase Shifter' or Periscope Phase Shifter, Beam Splitters and Beam Combiners.

Astrium + TNO = top results Astrium and TNO TPD have, with support from ESA, TU Vienna and the NIVR, developed these highly critical devices and applied this technology successfully in a Near Infrared Nulling instrument (at a wavelength of 1.5 microns).

A stable broadband rejection ratio of 36,000 has been achieved. Translated into a wavelength of 12 microns, this produces a rejection ratio of 2,000,000! A further increase to a value of 40million is possible when determined from a stable narrow band rejection ratio of 400,000 at 1300nm.

Long-term stability of the null depth, important to be able to collect the light from a faint planet, has been demonstrated over periods of several hours.

Technology for telescopes and medical science The technology developed by Astrium and TNO TPD can be applied in future Nulling instruments for terrestrial and space telescopes, such as the GENIE instrument for the Very Large Telescope in Chile, the ESA DARWIN mission and the NASA Terrestrial Planet Finder (TPF).

So maybe these up and coming projects will soon be able to provide an answer to one of the most compelling questions for mankind: are we alone in the universe?

But the technology developed for the space telescopes of the future has a broader application. It can also be used for medical purposes, in optical tomography, for instance, to detect cancer at an early stage.

Detailed information on the Nulling instrument and obtained results was by Astrium and TNO TPD last week at the "Toward Other Earths" conference in Heidelberg, Germany, April 22-25.

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