PARIS, Aug 19 (Reuters) - European scientists were due

to attempt a first in orbital gymnastics late on Monday, tapping

into the gravity of the Moon and then the Earth in quick

succession to guide the JUICE probe towards Jupiter in the

first-ever double slingshot manoeuvre.

Just over a year after it was launched, the European Space

agency's Jupiter Icy Moons Explorer (JUICE) is returning towards

Earth on Aug. 19-20 and will use the braking effect of its

gravity to take a shortcut to Venus and onwards to Jupiter.

In a novel double manoeuvre, the Airbus-built JUICE

probe will first use the gravity of the Moon to swing towards

Earth on exactly the right trajectory.

That's risky because the slightest error at that stage would

be amplified by the second part of the routine which involves

using Earth's gravity to slow down. Scientists warn that could

derail the eight-year odyssey to reach Jupiter and its moons.

"Inherently this is a bit tricky, because you would need to

correct any error, and you would need propellant for that,"

Nicolas Altobelli, JUICE Mission Manager, said in an interview.

Scientists have used the "gravity assist" method for decades

to navigate the solar system while saving propellant.

It involves brushing past a planet or moon and using the

power of its gravity to speed up, slow down or alter course.

But this week's lunar-Earth fly-by involves the first ever

attempt to carry out two such manoeuvres back-to-back.

If successful, it will put JUICE on course to reach Jupiter

and its three large ocean-bearing moons - Callisto, Europa and

Ganymede - in 2031 with the help of three further single gravity

assists: Venus in 2025, and then Earth again in 2026 and 2029.

ESA scientists had weighed several options for getting Juice

to Jupiter without the unrealistically large rocket that would

be needed to get it there without any gravity assists at all.

Using the Moon's gravity to change course allowed them to

catch Earth in front of its orbit around the Sun, which has the

effect of slowing down the probe, whereas passing behind the

planet would speed it up, Altobelli said.

That in turn allowed ESA's planners to target Venus and take

advantage of its exceptionally powerful slingshot effect.

"It's a very good configuration of the Moon's position

around the Earth ... So we are being opportunistic," Altobelli

told Reuters.

Following up on NASA's 1990s Galileo mission to Jupiter, the

ESA-led JUICE mission will orbit the solar system's largest

planet, perform fly-bys of its three large icy moons and finally

orbit Ganymede to study the potential to support life.

"It means studying the conditions and understanding whether

those moons could be a potential habitat, and could have

conditions favourable for life as we know it," Altobelli said.