European space scientists will this week attempt one of the most daring operations ever undertaken in interplanetary flight. On Wednesday, they will direct their Jupiter Icy Moons Explorer (Juice) to make a flyby of Earth and its moon and carry out the first double gravity-assist manoeuvre in space.
The delicate, high-risk exercise is vital to the success of the European Space Agency (Esa) mission and is aimed at taking the €1.6bn (£1.4bn) robot craft to its target, Jupiter, by July 2031. There it will begin exploration of two of the giant planet’s moons, Europa and Ganymede, in a bid to find signs of life that may lurk in their ice-covered oceans.
The manoeuvre will require extraordinarily accurate navigation, however. The slightest mistake could take Juice off course and doom the mission, Esa has warned. “It’s like passing through a very narrow corridor, very, very quickly: pushing the accelerator to the maximum when the margin at the side of the road is just millimetres,” said Juice’s spacecraft operations manager, Ignacio Tanco.
The craft’s delicate celestial dance will begin on Wednesday as it passes close to the moon and then flies on to Earth, exploiting their gravitational fields to change its speed and direction as it sweeps in a slingshot motion over the two worlds and into the inner solar system. A flyby of Venus will then take place next year, followed by two further slingshots past Earth in 2026 and 2029 before Juice finally heads off to Jupiter.
It is an extraordinary interplanetary waltz that will require Juice to travel at exactly the right speed, time and direction for each encounter. However, without such precise manoeuvring, space engineers simply could not explore the Sun’s more remote planets, Esa says.
To fly straight to Jupiter would require Juice to carry 60,000kg of propellant, an unfeasible load. In addition, it would need more fuel to slow down so that it could enter into orbit around the planet. That means that the scenic route, using the inner planets to gain gravitational assists to reach its target, is the only way to get to the outer solar system, say scientists.
It is an approach that will also be adopted by Nasa later this year when it launches its own Jovian moon mission, Europa Clipper. Its trajectory will sweep the probe over Mars and then back to Earth for a second flyby to boost its velocity. Although launched more than a year after Juice, it will reach Jupiter in 2030 and will focus its attentions on Europa while its European counterpart will make Ganymede its prime target.
“We know Europa has an ocean beneath the ice on its surface and we are pretty sure Ganymede has one as well,” said Prof Emma Bunce, director of the Institute for Space at Leicester University. “That makes them extremely interesting targets for our attention.”
Juice and the Europa Clipper should both make vital contributions to the hunt for life in our solar system and that means a great deal depends on how well Esa and Nasa space engineers cope with the their flyby encounters – beginning with Juice’s manoeuvres this week.
“It is going to be very exciting and a bit hair-raising,” added Bunce. “Nevertheless, this manoeuvre is going to be vital to the mission. The more precisely it is carried out, the less fuel we will need to use to make future course corrections and we will have more to use to explore Jupiter and its moons.”
Bunce, who has been closely involved with the construction of two of the instruments that have been fitted to Juice, added that the probe was not designed to detect life on Jupiter’s moons directly. “It is going to address the question of habitability, by studying the properties of the subsurface oceans. It will tell us whether or not life could be present. Actually detecting that life will be much harder.”
The idea that we might find alien life on ice-coated moons around planets in deep space would have seemed ridiculous a few decades ago. It was assumed that planets nearer the Sun, in particular Venus and Mars, offered the best hopes.
But Venus has been found to have a surface temperature of 475C, while its atmosphere has a crushing, unbearable pressure that has pulverised robot probes landing there. In addition, it was discovered that Mars had lost its atmosphere and surface water billions of years ago. Efforts to find evidence of life below the surface has so far produced no results.
In contrast, probes launched several decades ago have revealed that three of Jupiter’s main moons – Ganymede, Callisto and Europa – are worlds of ice that are covered in vast oceans of liquid water, the one prerequisite for the existence of life on Earth. “If ever there was a next-best place to look for life, it’s here,” says the US astronomer Neil deGrasse Tyson.
The trouble is they are very difficult to reach. Journeys to Mars take about eight months. Juice – which was launched from Esa’s spaceport in Kourou in French Guiana last year – will take eight years to reach its target, thanks to all those planetary flypasts it will require – starting this week.