Sir

Your well-written and pertinent News Feature (Nature 411, 880–881; 2001) “Raking through the embers” omits to mention the Sky Polarization Observatory (SPOrt), which is a space mission currently under development that is precisely aimed at a sky-polarization survey.

The programme was proposed to the European Space Agency and was selected in 1997, and is now fully funded by the Italian Space Agency (see http://sport.tesre.bo.cnr.it). SPOrt will be on board the International Space Station for at least 18 months from 2005. It is designed to directly measure Stokes' Q and U parameters at the microkelvin level, providing an accurate measurement of the polarization of the cosmic microwave background (CMB) on a large angular scale (>7°) from four independent channels in the frequency range 22–90 GHz. More than 80% of the sky will be scanned under the most favourable conditions for testing re-ionization models.

Complementary to the MAP and Planck projects discussed in your feature, which will map the CMB anisotropy with unprecedented accuracy, SPOrt should also provide the much-desired galactic foreground polarization maps so far unobserved at frequencies above a few gigahertz, and study the correlation between the available COBE and MAP temperature maps and polarization patterns over large sky regions.