Space exploration: Cassini bows out in a blaze of glory
Last Monday, the veteran Cassini spacecraft made its final flyby of Saturn’s moon, Titan, as its fuel supplies dwindled.
For almost 13 years, the tiny flagship of space exploration zipped around the great, ringed planet and its moons, reaching speeds of 126,000km/h and sending data back 1.2-billion kilometres to Earth.
On Friday, the craft had its final encounter with Saturn, plunging into the planet’s atmosphere, where it was vaporised within minutes in a meteoric blaze.
"We’re a little frazzled," admits principal investigator of the Cassini Magnetometer, Prof Michele Dougherty, ahead of the suicide plunge.
The University of Natal-educated South African scientist has helped to lead the mission since Cassini’s launch in 1997. She was awarded the 2008 Hughes Medal by the Royal Society of London — in recognition of an original discovery in the physical sciences — for this work.
In a sense, Cassini and Dougherty, together with the other scientists on the project, formed a single exploratory being: the spacecraft was a far-flung limb collecting sensory information for the human team (the "brain") to interpret.
Some of the information the craft has unearthed during its 20-year stint in space has been astonishing. Dougherty says Cassini’s most exciting find is "the discovery we made of a strange atmosphere around one of Saturn’s moons, Enceladus".
In February 2005, the spacecraft’s magnetometer indicated that Saturn’s magnetic field was bent near to Enceladus. This was puzzling, as it seemed to suggest an atmosphere around the white, icy moon and yet Enceladus was tiny: it did not have sufficient gravitational force to maintain one.
Excited and somewhat apprehensive, Dougherty’s team decided to put its faith in the data. "We persuaded the project to take us much closer on a subsequent flyby."
What they discovered was not a uniform atmosphere, but plumes of water vapour erupting from great geysers in the south pole of the moon. Beneath its crust, Enceladus contains an ocean of liquid, hydrogen-rich, salty water. The plumes solved the mystery of the atmosphere and the presence of hydrogen raised the enticing possibility that Enceladus might support life.
Microbes could use the hydrogen in the underground ocean to obtain energy by combining it with carbon dioxide dissolved in the water, a strategy that was employed by Earth’s earliest life forms.
"There are essentially four ingredients you need for life," says Dougherty. "You need liquid water, which we now know we have in Enceladus. You need a heat source and we know there’s internal heating. And you need organic material, which we know is there.
"Those three ingredients need to exist over a long enough period so habitability can potentially form."
We essentially saw Titan as a place to go where we could almost see back in time and see what the Earth’s atmosphere had been likeProf Michele Dougherty
Dougherty says scientists do not know how long those ingredients have coexisted on Enceladus. Because the moon is losing water vapour, which feeds one of Saturn’s rings, the E-ring, it will run dry. But, she says, that could take thousands or millions of years.
Enceladus is not the only one of Saturn’s many moons — it has 62 — that might contain conditions for life. Titan, its second-largest moon with a radius of 2,575km, has a thick atmosphere believed to resemble that of early Earth’s, the reason for the Huygens probe that landed on the surface in 2005.
"We essentially saw Titan as a place to go where we could almost see back in time and see what the Earth’s atmosphere had been like," says Dougherty.
Again, Cassini revealed more than they expected. On that faraway world, wind is blowing, storms are brewing and rain is falling on mountains, deep gorges and gushing rivers.
Yet, the familiar features also serve to underscore the alienness of this world: Saturn fills more than a third of the hazy, orange-brown Titan sky, and temperatures average -179°C. The clouds and oceans contain liquid methane and ethane. Some scientists believe this presents the possibility of a fundamentally different basis for microbial life.
For six years, discovery of this surface liquid eluded scientists, despite the telltale canyons that snake across the planet’s surface.
"Then we finally saw glints of liquid on the surface," Dougherty recalls, "and we realised the reason we hadn’t seen any before was because it had been the dry season!"
But — and here’s the problem — on Saturn, there doesn’t seem to be a tilt. "Or if there is one," Dougherty says, "it’s incredibly small." And at the same time, the enigmatic planet radiates a healthy magnetic field, flying in the face of scientific understanding.
This mystery may yet be solved when Dougherty’s team analyses the final data from Cassini’s plunge into Saturn’s dense atmosphere, which may have been obscuring some aspect of the true magnetic field. In its last moments, Cassini collected data that will enable scientists to understand the composition of the planet’s atmosphere, adding to its impressive legacy.
Part of that legacy is the effect that Cassini had on Earth. The mission has fostered unique ties across teams and countries; between Nasa, the European Space Agency, the Italian space agency, Agenzia Spaziale Italiana, and scientists all over the world.
"I think one of the most positive outcomes of Cassini is that we showed that you can, at an international scale, work together as a cohesive team," says Dougherty.
It is partly because of this, she believes, that the Jupiter Icy Moons Explorer (JUICE) was chosen as the next — this time solar-powered — European mission to investigate Jupiter’s largest moons, Ganymede, Callisto and Europa, which are also thought to be potentially habitable.
The question of whether Titan and, particularly, Enceladus do contain life will remain unanswered. Although there are plans to send probes back, Dougherty says it’s not likely to be in her lifetime.
To guard against the slim possibility of any microbial hitchhikers on Cassini contaminating the potentially habitable moons, Cassini had to be destroyed. On Friday, Cassini entered Saturn’s atmosphere. Tumbling out of control, it lost signal with Earth about 1,510km above the cloud tops, before being vaporised by the immense pressure of the thick atmosphere.
"There will be sadness," Dougherty predicted of that final goodbye. "There will also be great pride because I think we all worked together incredibly well and produced some spectacular results."
And as Cassini died, so our eye on Saturn closed.
Farewell ringed planet. May the mysteries of your moons continue to stoke mankind’s hunger for exploration, and your elegant, 300,000km rings continue to spin, a great vinyl playing saturnine symphonies into space.