Visiting the Centre Spatial Guyanais and (if you're lucky) seeing a rocket launch are two of French Guiana's biggest highlights. Both need to be arranged in advance though, so plan carefully. Just to visit the CSG you'll need to apply online or by phone with your passport information at least 48 hours in advance. The three-hour bus tours of the space center, which include a visit to the massive launchpad, are fascinating; witnessing a launch is simply unforgettable.
In 1964 Kourou was chosen to be the site of the Centre Spatial Guyanais because it's close to the equator, is away from tropical storm tracks and earthquake zones, and has a low population density. Three launchers are now in service, increasing the number of liftoffs to more than a dozen per year; this frequency makes it that much easier to coordinate your visit with a launch.
The launch site is the only one in the world this close to the equator (within five degrees), where the earth's spin is significantly faster than further north or south; this means that the site benefits from the 'slingshot effect,' which boosts propulsion and makes launches up to 17% more energy efficient than those at sites further away from the equator. Since 1980 two-thirds of the world's commercial satellites have been launched from French Guiana. The center is run by Centre National d'Études Spatiales (CNES; www.cnes.fr) in collaboration with the European Space Agency (ESA; www.esa.int) and Arianespace (www.arianespace.com).
Visit the ESA website to find out the launch schedule and reserve a space at one of the observation points within the space center. Email firstname.lastname@example.org well ahead of time, providing your full name, address, phone number and age. It's free, but children under 16 are not permitted at sites within 6km of the launchpad and those under eight are not permitted within 12km. You can watch it, reservation-free, with locals at Kourou's beaches or at the Carapa Observation Site, 15km west of the city center.
Whether you're attending a launch or just visiting the space center for a tour, be sure to bring your passport. Tour guides at the space center sometimes speak English or German; ask when you book. Note that the space center is closed on the day before, the day of and the day after a launch. Be aware also that if a launch is postponed, then days when the space center is shut will be extended too. This makes attending a launch and visiting the CSG for the tour quite hard to combine during a short stay, so plan ahead and give yourself plenty of time to do both.