Why do space missions like JUICE never travel in a straight line?

From the JUICE mission to Artemis, space missions always travel in zigzags and not in a straight line. It is the result of a combination of several factors.

It is a constant in the history of space conquest. From  Sputnik 1  to  Artemis, the program that will return humans to the Moon for the first time since 1972, most space missions do not have a straight trajectory. Despite the immense technological progress made, space agencies generally rely on journeys punctuated by several round trips, even if they are longer.

This will be the case with the JUICE mission, which successfully launched to the moons of Jupiter on April 14, 2023. Yet wouldn’t it be easier to travel in a straight line the 628 million kilometers that separate the Earth from Jupiter? It is not so simple.

The trajectory of a space mission is one of the many elements taken into account during its preparation. It plays an important role in the duration of the mission, its cost, and its scientific contribution. To define this trajectory, scientists take a lot of data. Gravity, distance, speed, or even atmospheric pressure are thus taken into account. But the most important thing is the impact that celestial bodies can have on a space mission, thanks to their gravity.

A story of gravity

This is called gravitational assistance, as Elisabet Canalias explained in a CNES program on April 6, 2023: “Gravitational assistance is to take advantage of the mass, the energy of a body to change the speed of the probe without expending fuel, which costs additional weight. It makes mission concepts possible that otherwise wouldn’t be possible”. 

As part of the JUICE mission, for example, the probe left Earth on April 14, 2023. But before arriving on Jupiter, the probe will first pass close to Venus, in August 2025. The gravitational force of this planet will allow it to relaunch itself in space to return… to Earth. And yes, in 2026, it is from the gravity of our planet that the JUICE probe will propel itself again, before returning once again in 2029.

To reach its final objective, the Juice probe will have to make a good number of round trips // Source: CNES

This will be its last return to Earth, as it will then definitely head for Jupiter, where it is supposed to arrive in July 2031. Once there, the JUICE probe will this time take advantage of the gravity of the 35 icy moons around Jupiter to propel itself between July 2031 and November 2034, before arriving in Ganymede a month later. In total, it will therefore take more than 10 years of travel for the probe to reach its final objective, taking full advantage of the gravity of the planets.

This path is not a surprise: it was defined by scientists for very specific reasons. Used properly, gravity allows space missions to use the attractive force of the planets to speed up or slow down, saving fuel and energy. Data that can be crucial in the case of a mission like JUICE.

Of the 6,100 kg that will be propelled, 3,600 will be devoted to fuel or 60% of the total weight of the space mission. Without the gravity assist, the probe would have been much heavier, since it would have required even more fuel, as explained on the Stack Exchange forum: “It takes enormous amounts of energy to get into Earth’s orbit, leave it and go into orbit around the Sun. It also takes a lot more energy to raise (or lower) the orbit to match that of another planet. Finally, more energy must be used to orbit the target planet”. 

Moreover, the economy is not only about energy. If the average cost of a mission like Mars 2020 is estimated at 2.5 billion dollars (about 2.3 billion euros), the JUICE mission costs just over 1.6 billion euros. Admittedly, these missions are of a different level, and it is true that NASA and ESA have not provided the same resources. But the difference in budget remains significant and the savings made on fuel by avoiding a straight trip play an undisputed role in this saving.

Everything also depends on the mission, according to Elisabeth Canalias, whom we also interviewed: “The technology (to travel in a straight line) exists. But it’s always a compromise between the weight you want to launch, the launcher you have, and its price. The Americans flew direct missions to Jupiter. But these are different constraints and each mission has its specificities.”

The contingencies of space

In addition to these economic and technical data, scientists must also take into account the extremely dangerous environment that is space, in particular for space missions. Even if they are designed with materials capable of withstanding a good number of unforeseen events, it is still complicated to get out of them intact in the event of a collision with space debris or by taking radiation and solar storms in full face.

These possibilities, which the scientists are aware of, therefore oblige them to draw the safest possible trajectory, to avoid incidents that could upset the mission. So, if a straight-line trajectory can be faster, it only increases the risk of problems.

In space, there are thousands of space debris // Source: Pixabay

During its flight, a space mission can also find itself confronted with atmospheric pressure or solar winds. Elements that can disrupt flight paths and require corrections along the way.

Clearly, even if a space mission can travel in a straight line, the unforeseen circumstances of the road would necessarily force it to change its trajectory, at one time or another. To avoid these dangers as much as possible, space missions are therefore forced to follow a more complex trajectory, but which makes it possible to avoid the risk zones.

Taking time is good

With such constraints, it is difficult to justify space travel in a straight line. Economically and energetically, it would not be interesting for a space agency. Even NASA, which is the one with the biggest budget, rarely carries out missions with a straight trajectory.

Especially since most of these missions are not intended to go quickly. On the contrary, the longer they are, the more scientifically interesting they are, as Elisabeth Canalias explains: “If we are in a hurry to get results and save time, that (a mission with a straight trajectory) can be good. But if what counts is to carry out the mission with precision, to arrive at a given time with the payload, that is to say with the scientific instruments that one wishes to use, and if one does not is not constrained by time, there is not a huge advantage in the straight line“.


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