On Wednesday, a school-bus sized spacecraft will dive out of the inky blackness of space more than one billion kilometers from Earth and zip through an icy plume that springs from the south pole of Enceladus.

Although it's just a tiny satellite of Saturn—less than one-sixth the size of Earth’s moon—Enceladus has become one of the most intriguing bodies in the Solar System. Earlier this year, NASA confirmed that in addition to the moon's geyser-like plumes, it has a global ocean beneath its icy crust. Where there is water and energy, scientists believe, there's the possibility of life.

A last flyby

At this point, the venerable Cassini spacecraft, which made these astounding discoveries during the last decade spent exploring the Saturn system, has expended most of its fuel. Before Cassini runs dry, however, NASA scientists say they will take one final, long look at Saturn’s mysterious moon. On Wednesday, the spacecraft will dip down to within 50km of Enceladus’ surface—closer than ever before—and fly through one of its plumes.

The spacecraft will not be able to determine whether anything lives in Enceladus’ global ocean, as its spectrometer can only detect molecules up to 100 atomic mass units. But the probe will be able to characterize the plume and help scientists devise a future orbiting mission to the tiny, icy world, one that will be equipped to find life.

“The joy of Enceladus is that you don’t need to land on it,” said Curt Niebur, Cassini program scientist during a news conference at NASA Headquarters in Washington, DC. “It is spewing samples into space all the time. We just have to fly by at the right time and the right trajectory.”

If only it were that easy. In reality, this is likely NASA’s last close look at Enceladus for decades. Even assuming funding for a new mission, which doesn’t exist and is unlikely to for some time, it takes years to plan forays to the Solar System’s outer worlds and years for those spacecraft to travel the more than one billion kilometers to reach Saturn.

Once there, unlocking the moon’s secrets will not be easy.

Future exploration

As NASA has extended its reach into the Solar System, the space agency has followed a three-pronged strategy to explore new worlds. First, it sends a spacecraft to fly by worlds, such as Mariner 4, which visited Mars briefly in 1965. Then, in 1971, Mariner 9 orbited the red planet. Finally, in 1976, Viking 1 landed on Mars. Now, with Cassini, Enceladus has had its first flyby mission. Up next is an orbiter.

But an orbiter may not do the trick, says Kevin Hand, the deputy chief scientist of Solar System exploration for NASA’s Jet Propulsion Laboratory in California. Hand, whose job it is to scour the Solar System for hints of life beyond Earth, has done the calculations of finding life in Enceladus’ plumes, and the results are grim. It may seem like a moon that literally spews its oceans into space would be the easiest of the outer Solar System's ice-encrusted water worlds to explore. But Hand does not believe so.

Hand noted that on Earth there are 1,000 to more than 100,000 microbes in a cubic centimeter of water. However, a more apt comparison to Enceladus’ ocean is probably Lake Vostok, Antarctica’s largest subglacial lake. Russian explorations of the lake have suggested it has about 100 to 1,000 microbes per cubic centimeter.

If life exists on Enceladus, it probably originated at the hot hydrothermal vents at the bottom of its roughly 10km deep ocean. On Earth, when scientists look at the concentration of bacteria and other compounds from hydrothermal vents, they find a lot near the seafloor. But as they sample closer and closer to the sea surface, concentrations of that life drop off quite significantly.

Scientists believe plumes are ejected from Enceladus when the ice shell periodically cracks open. As the water vaporizes, the pressure difference forces it up through these cracks. It’s possible that a few microbes traveling all the way from the bottom of the ocean will rise near enough to the surface to get blown into space.

But Hand believes most of the heaviest microbes will probably be too heavy to be entrained within the plume and escape the moon's gravity. They’ll likely just fall onto the surface. “I hope I’m wrong about that,” he told Ars. “Rarely am I that much of a pessimist.”

Here’s why Hand is a pessimist: factoring in all of the above, he has calculated that a probe would have to traverse through 12,000km of a plume to collect a single cell. By comparison, on Wednesday, Cassini will fly through a plume for a few seconds.

That’s not to say the probe won’t provide useful data this week. By flying so low, Cassini should be better able to sample the plume and understand the chemistry of the ocean and any hydrothermal vents below. More hydrothermal activity, scientists believe, would increase the odds of life existing there.

Nevertheless, if Hand is correct, even an orbiting mission in the coming decades wouldn’t answer the ultimate question of what lies beneath Enceladus’ icy surface. That mystery would require a lander to crack, and that seems unlikely for a long time. That’s because Jupiter’s ice-covered moon Europa, and its 100km deep ocean, rates as a higher priority for John Culberson, the Houston Republican who chairs the House subcommittee with jurisdiction over NASA’s budget. Culberson is working to get NASA to launch a Europa orbiter, with a lander, in the early 2020s. That mission would have to play out before the space agency can consider an orbiter or a lander for Enceladus.

And that’s fine with Hand and a lot of other scientists who believe Europa may be the better target.

“With the plumes on Enceladus, absolutely we should go and sample those,” he said. “But we have to be careful that we are not racing to the latest shiny object. Rewind the clock 15 years ago, when Galileo was in orbit around Jupiter. Everybody was on board with the idea that we had to get to Europa.”

Enceladus will undoubtedly get its turn one day. The little world is too intriguing to ignore. But its secrets will almost certainly remain safe for decades to come.