Earth and Jupiter can approach each other at a minimum distance of about 600 million kilometers. At the time of writing this article, more than 5 months have already passed since the launch of the European probe JUICE, which is heading towards Jupiter. However, although the probe has already traveled a distance of 370 million kilometers through space, it has covered only 5% of the entire journey. Why is it taking her so long? The answer lies in many factors that the flight dynamics experts at ESA’s control center have at their fingertips. They have data on the amount of propellants that will be used by the launch vehicle, on the weight of the probe, but also on the position of the planets in our system. Based on this (and much more) information, experts design the path of the probe. The world of orbital mechanics may seem counterintuitive to the layman, but with a little patience and a lot of planning, the field allows a lot of science to be done with just a little propellant, as we’ll describe today in an article published on the ESA website.
Whether you look at the sky with your naked eyes or through a telescope, or watch animations of the orbits of cosmic bodies, you will notice that the planets, moons, stars and galaxies are in constant motion relative to each other. When a mission launches from Earth to another cosmic body, it is not as if the probe bounces off a stationary planet, because the globe orbits the Earth at a speed of approximately 30 km/s. As a result, the probe launched from Earth has a significant amount of “orbital energy”, which is the main value that matters when we determine the shape of the orbit around the central body. Just after launch, the probe is more or less in the same orbit around the Sun as our planet.
To leave this orbit and take the shortest possible (direct) path to Jupiter from Earth would require a gigantic rocket with an enormous amount of propellants. However, let’s assume that we could build it. But we would run into another problem. Such a hypothetical probe launched from Earth on a direct path to Jupiter would need much more propellant to not only fly past Jupiter, but to slow down and enter its orbit. Jupiter and Earth are in constant motion relative to each other. The furthest they can be is 968 million kilometers, which is when the Sun is between them. On the contrary, the shortest mutual distance between the two planets is the roughly 600 million kilometers already mentioned in the introduction. But this maximum approach lasts only a moment, and then the planets begin to move away from each other again. In other words – they never maintain a fixed distance from each other.
This is due to the fact that the planets move around the Sun in different orbits at different speeds. Try to imagine that you are sitting in a moving car and you have the task of throwing a ball and hitting another moving object with it. Engineers are in a similar situation and must therefore calculate the ideal moment to make a “jump” from Earth’s orbit, aiming for where Jupiter will be when the probe arrives there. So it is not heading to where Jupiter is at the time of launch from Earth. With a bit of imagination, this situation can be compared to a hockey or football pass that goes into a free space where there is no teammate yet, but the passing player knows that he will run or drive there.
Let’s try to imagine that we have a very powerful rocket and at the right moment, when the planets are correctly aligned, we will go on the shortest trajectory. How long would it take? Older space probes such as Voyager and Pioneer covered this distance in less than two years, and the fastest object that went to Jupiter was the New Horizons mission. It launched on January 19, 2006 and made its closest approach to Jupiter on February 28, 2007, so its trip around the giant planet took just over a year. However, all these missions did not stay at Jupiter, they only flew past it and headed on. So they’re good examples of how long it takes you from launch to fly past Jupiter on your way to somewhere else.
In order for the probe to settle in the orbit of the giant planet and be able to study it from all sides and over time, or to get into orbit around its moon, it is necessary for it to get rid of part of its energy. Such a deceleration will require a lot of propellants for the maneuver that will put the probe into orbit around Jupiter. So if you don’t want to start with a huge amount of propellants, you have to take a not so direct route with a flight time of 2.5 years.
Here we see how the mass of the probe has a key effect on the time it takes for the probe to get somewhere. Engineers must carefully monitor the probe’s weight, balancing the amount of propellant and scientific instruments it must carry to accomplish its mission. The more mass a probe has, the more fuel it must carry. As a result, the probe becomes heavy and it is more difficult to take it out. This brings us to the properties and capabilities of the launch vehicle. At launch, the probe must reach sufficient speed to leave Earth’s gravity and travel to the outer regions of the Solar System. The more powerful the rocket used, the shorter the flight.
The JUICE probe was one of the heaviest interplanetary probes ever launched, weighing in at just over six tons. It carries on board the largest set of scientific instruments ever headed for Jupiter. Even the massive Ariane 5 launch vehicle was unable to push the JUICE probe so that it could fly directly to Jupiter. For this reason, current probes such as JUICE and Europa Clipper have to rely, just like earlier probes Galileo or Juno, on so-called gravitational maneuvers, during which they gain extra speed during flybys around cosmic bodies. The bigger the push, the easier the journey.
The dwarf planet Pluto is very far from the Sun and must travel a much greater distance in its orbit than Mercury, the innermost planet of our system. Although Pluto moves much more slowly relative to the Sun than Mercury, its orbital energy is significantly higher than that of Mercury. In order to get a probe into orbit around another planet, we need to match the probe’s orbital energy to the planet’s orbital energy. When the BepiColombo mission was launched, its orbital energy was comparable to that of Earth. The probe therefore had to get rid of this anergy in order to fall closer to the center of the Solar System. It got rid of excess orbital energy during close flybys of neighboring planets.
If we reverse this principle, we find that it also works for travel to the outer regions of the Solar System. In order to get to a much larger orbit that lies further from the Sun, the JUICE probe must take some orbital energy from Earth or Venus. Depending on the relative direction of motion of the planet and the probe, the gravitational maneuver can either speed up, slow down, or change the direction of the probe. The probe, on the other hand, will also affect the planet a little, but it is such a small change that we can safely ignore it. However, there is no violation of Newton’s third law of motion: “Every action provokes an equally large opposite-oriented reaction.“
JUICE will use a series of flybys around Earth, the Earth-Moon system, and Venus, putting it on a path that will ensure its rendezvous with Jupiter in July 2031. That’s when the team behind the probe’s flight will begin the most challenging period as JUICE explores the complex system around a giant planet. The probe’s complex trajectory during the entire mission includes several gravitational maneuvers on the way to Jupiter (including the first-ever flyby of the Moon and Earth). However, once the probe reaches Jupiter, it will undergo an impressive 35 flybys of the so-called Galilean moons – Europa, Ganymede and Callisto. The final point of interest will be Ganymede, into whose orbit JUICE is to enter. It will thus become the first orbiter of an extraterrestrial moon.
If we were to look for the most important maneuver in the entire mission plan that the control center will oversee, it would be the ignition that slows the probe down by about 1 km/s just 13 hours after the gravity maneuver at Ganymede for JUICE to enter orbit around Jupiter. Getting into an orbit that will allow a probe to orbit any cosmic body is not easy. The probe must arrive at the correct angle by controlling the speed and then perform a key and distinctive maneuver at the right moment, the scope of which must be accurately captured and during which it must be correctly oriented in space. If it arrives too quickly or slowly, too sharply or bluntly, starts the maneuver at the wrong moment, its ignition lasts longer or shorter, or is poorly oriented in space, then it is very likely that the mission will end in failure. Either the probe will fly away into space, or it will end up on such a different path that it will never reach the one originally planned, because it will not have enough propellants to correct it.
But if everything works out, it will be worth it. JUICE will get close to Jupiter’s moons to get photos and measurements like never before. Could life be hiding beneath the icy crust of Ganymede, Europa and Callisto? What will we learn about the formation of planets and moons throughout the universe? Thanks to the wonders of orbital mechanics, we’ll find out (relatively) soon.
European Space Agency (ESA): BepiColombo Launch Media Kit. 2018.