Jupit
Explainer

Why Juice needs eight years to reach Jupiter

ESA's Jupiter probe left Earth in April 2023 and will not arrive until July 2031. The long way round is what makes the mission possible.

Told by Otto Perihelion, who is made up. Checked by the Jupit editorial desk,

Light from the Sun reaches Jupiter in about 43 minutes on average. The Jupiter Icy Moons Explorer, Juice for short, is taking a little over eight years. It is not lazy. It is heavy, and it is being careful with its fuel.

Too heavy to go direct

Juice left Kourou on an Ariane 5 on 14 April 2023 weighing around six tonnes, more than half of it propellant it needs to keep for the far end of the trip. The rocket could not give that much mass the speed for a straight shot to Jupiter. So Juice collects speed from planets instead, one pass at a time. It is the long way round, and it is free.

The route

  • August 2024, Moon then Earth. The first time any spacecraft used the Moon and Earth together in a single manoeuvre. It bent the path inward, towards Venus.
  • August 2025, Venus. A pass that added speed and sent Juice back out towards Earth.
  • 28 September 2026, Earth. Juice passed 8,640 km above the Indian Ocean. ESA reports the flyby turned its path by 20 degrees and added 3.5 km a second.
  • January 2029, Earth (planned). The last push, and the one meant to throw Juice out to Jupiter.
  • July 2031, Jupiter (planned). Arrival.
13 au acrossDrag the map sideways, or press the arrow keys on it, to move timeOrbits and distances are to scale. I draw the Sun, planets and spacecraft larger than life until you zoom in. One au is the distance from Earth to the Sun, about 150 million km.

Drag the map sideways and watch how long Earth and Venus take to come back round. Each flyby has to wait for the planets to be in the right place, which is why the tour takes years. Planets do not hurry for anyone.

What the patience buys

The propellant saved on the way out gets spent at Jupiter. Juice is due to make dozens of flybys of Europa, Ganymede and Callisto, then settle into orbit around Ganymede, planned for late 2034. If that works it will be the first spacecraft to orbit a moon in the outer solar system.

NASA's Europa Clipper launched eighteen months after Juice and is planned to get there more than a year earlier, in April 2030, on a shorter route past Mars and Earth. If both keep to their plans, the two missions will be working in the Jupiter system at the same time. I can hardly wait. I shall have to.