Ariane 5 was designed primarily to launch two satellites together. With a single, relatively low-mass payload (approx. 6 tons), the thrust of the main stage Vulcain 2 engine had to be adapted at the end of its burn to reduce the launcher’s acceleration to remain within the rocket stage’s qualified envelope.

To set the probe off on its interplanetary cruise, Ariane 5 had to accelerate JUICE to Earth-escape velocity (11 km/s). This meant calculating the best trade-off between the mass of propellant in the spacecraft and the maximum performance capacity of the rocket.

The launch took place at exactly 9:14 am, local time at the Kourou spaceport in French Guiana, and therefore rocket and payload were exposed to direct sunlight.  This poses a substantial potential risk: solar radiation could damage JUICE’s 10 sensitive scientific instruments and also negatively alter the state of the propellants used by the HM7B upper stage engine. To avoid overheating, the launcher was put in “barbecue” mode – rolling continuously, so that no side was exposed to the sun for too long.

Following a to-the-second lift-off, the penultimate Ariane 5 rocket carried its precious passenger on a trajectory due east out over the Atlantic Ocean. After nearly half an hour together, the JUICE spacecraft separated smoothly from Ariane 5 to begin its eight-year voyage to the Jupiter system – and a thrilling new chapter in space exploration.