Voyager Casts off the Hydrogen-Tinted Spectacles
This week a team of researchers in France, Russia and the USA have reported that Voyager Spacecrafts 1 & 2 - launched 34 years ago - have made it far enough out of the solar system to detect Lyman-alpha emission. Lyman-alpha emission is an ultraviolet radiation generated when an electron moves between the first and second energy levels of hydrogen, and it is thought to be an indicator of regions where star formation has occurred. But in order to look at Lyman-alpha radiation you have to move far away from the clouds of hydrogen gas that are present in our solar system. It is these hydrogen particles (called a heliospheric hydrogen glow) which can also scatter and disrupt Lyman-alpha radiation just as city lights can interfere with your view of the stars. Both Voyagers have now travelled far enough away from the sun to look back and see through this hydrogen glow. Previously Lyman-alpha emission has only been predicted with equations but, writing in the journal Science, Rosine Lallement and colleagues from The University of Paris Diderot can confirm that this emission has been detected and that it does have its origin in the regions surrounding newborn stars. We can therefore use this information and look at other galaxies, as well as the Milky Way, in order to understand where a star formed. Sadly, Voyager 2's spectrometer has had to be switched off to conserve power, and the spectrometer on Voyager 1 will probably be turned off soon, too. This means that those spacecraft will not be able to collect much more data on Lyman-alpha sources. It may be that the New Horizons Spacecraft (due to arrive at Pluto in 2015) can complete the data collection and look at the emission from other galaxies.