Ever since the Galileo spacecraft flew past Jupiter's icy moons in 1989, scientists interested in life beyond our planet have been desperate to return.
Europa Clipper which fired from the Space Center Kennedy Center in Cape Canaveral does just that.
Galileo found clear evidence that although Ganymede, Callisto, and Europa have barren, frozen surfaces, there are likely vast oceans of water beneath them.
And as every astrobiologist knows, where there is water, there is a chance for life.
Held in a liquid state by Jupiter's enormous tidal forces, Europa's ocean could be the largest in the solar system.
At 100 miles deep and containing twice as much water as all of Earth's oceans, this ocean makes it a prime candidate for exploration.
After a six-year, 2.8 billion-mile journey, Europa Clipper – the largest planetary science mission ever launched by NASA – will spend four years orbiting Jupiter, flying past its icy moon.
It will use nine instruments to image the lunar surface, study its atmosphere, measure the thickness of its icy crust, confirm the presence of an ocean and attempt to measure its depth and salinity.
But before we get too excited, if there is life on Europa, the Europa Clipper won't “see” it writhing under the ice.
First, the crust is thought to be at least 16 km thick, which is too deep for Jupiter's weak Sun to penetrate it.
The Europa Clipper spacecraft will be tested in April. Photo: Reuters
Without photosynthesis, if life exists, it is expected to be more like the bacteria lurking in the dark around hydrothermal vents deep on the seafloor here on Earth.
On Europa it can draw energy from geothermal heat, or even from Jupiter's radiation fields, and from simple organic molecules as food.
However, we go beyond Europa Clipper's remit to confirm whether the lunar environment is consistent with these theories.
The main advantage would be whether Europa Clipper would see a plume of water emerging from the lunar surface, which happens on other icy moons. This would mean that chemicals present in the water below could be directly analyzed.
“If conditions exist that support life, we expect them to be deep in the dark,” said Dr. Adam Masters, a space scientist at Imperial College London.
Europa Clipper took off under sunny skies
“So when the water gets to you, it saves you a lot of hassle,” he said.
However, your chances of getting an answer are doubled. Dr. Masters is working on another mission, the European Space Agency's Jupiter Icy Moons Explorer (JUICE).
It will reach Jupiter shortly after Europa Clipper and will study Europa as well as another leading candidate for life, Jupiter's moon Ganymede.
But even if these missions find tantalizing chemical evidence of life, it will take decades to confirm its existence, let alone understand its alien biology.
For this reason, one of the probes' other goals is to search for potential landing sites on one of these icy moons.
If Europa Clipper and JUICE find evidence that Jupiter's moons have suitable conditions for life, the challenge for future space explorers will be to find a way to get through kilometers of ice to see it.
