After four years of brainstorming, custom tool development and training, two astronauts plan to venture outside the International Space Station Friday for the first of four spacewalks to repair a $2 billion cosmic ray detector. The excursions are considered the most challenging since work to service the Hubble Space Telescope.

The first of up to four Vega rocket launches planned this year lifted off Thursday night from French Guiana carrying the Italian PRISMA hyperspectral Earth-imaging satellite.
The solid-fueled launcher blasted off from the Guiana Space Center at 0150:35 GMT (9:50:35 p.m. EDT).

Mostly clear skies and brisk westerly winds are forecast at launch time Friday afternoon for SpaceX’s first resupply mission to the International Space Station in nearly a year.