Cassini - a spectacular success at Saturn
Sen—While NASA continues to consider the future of manned spaceflight, a robotic probe has been quietly continuing a spectacularly successful exploration of planet Saturn and its family of moons.
Cassini's survey of the ringed world, which has been going on since 2004, has been one of the greatest achievements of NASA, in conjunction with European partners.
It followed a similarly productive mission by the Galileo spaceprobe which orbited the giant of the Solar System, Jupiter, for eight years in the 1990s. Both built on early observations by Pioneer and Voyager flyby probes in the 1970s and 1980s.
The discoveries and other data flooding back from Cassini, nearly 1.5 billion km away across space, continue to mount up and to keep space scientists very busy.
Dr Carolyn Porco, who leads Cassini's imaging team, told Sen: "For one mission to have extended the mind's reach so much into so many different arenas has been breathtaking. Let's just say there's never been a mission quite like it." (Read Sen's exclusive interview with Dr Porco here)
Looking back, it seems strange that the Cassini project faced a fair amount of opposition when it was being designed and built nearly 20 years ago. There were not just the familiar worries about the budget from certain politicians. This time space scientists also had to confront a vocal protest from a campaign group over a nuclear power supply it needed to carry.
Previous spacecraft had relied on solar power to keep them alive, sprouting panel wings to soak up the Sun's energy as they went about their work. But Saturn was so far out that this energy supply would not have been efficient so the decision was made to carry 72 lbs of plutonium instead.
That was why there was a demonstration by hundreds of protestors outside the fence at Cape Canaveral, Florida, before Cassini's launch in October 1997. The compound was breached and police made dozens of arrests.
Galileo had been sent on its mission from Earth orbit after being hoisted by the space shuttle Atlantis in 1989. The much-heavier Cassini blasted off atop a Titan IVB/Centaur rocket to begin a journey of nearly seven years towards its target.
One reason that Cassini was heavier was that it was carrying a passenger—a robotic probe called Huygens. This was designed for a most daring role, an attempt to soft-land on the surface of Saturn's largest moon Titan.
The Cassini-Huygens mission, to give it its full name, was a partnership between NASA, the European Space Agency (ESA) and the Italian space agency (ASI). Huygens was Europe's major contribution to the mission which is managed by NASA's Jet Propulsion Laboratory (JPL).
As has become common in spaceprobe travel, Cassini took a roundabout journey to reach Saturn, first flying inwards towards Venus to being a series of swingbys of planets that would give it the gravitational boosts to speed it on its way.
After two flights past Venus and another by the Earth, the probe made a close approach to Jupiter, returning more useful data about that planet including the most detailed images ever taken of it.
Cassini went into orbit around Saturn on June 1, 2004, to begin an initial four-year mission. It immediately began making discoveries with three new moons detected that year. Another early observation was a close-up of a minor moon, Phoebe, which later helped identify it as an intruder from the Kuiper Belt at the edge of the Solar System.
But the mission was preparing for what was destined to be one of its greatest triumphs—landing on Titan, which at 5,150 km across is bigger than planet Mercury. And on Christmas Eve, 2004, Huygens separated from its mother ship to begin this descent.
What a challenge it was—to land a probe on a moving target orbiting another moving target. Many were holding their breath as the probe dropped into the orange petrochemical smog surounding Titan, the only moon to possess a dense atmosphere.
This methane-rich atmosphere allowed Huygens to descend by parachute, drifting and turning as it went and constantly recording the view of the the moon's dramatic landscape, plus the sounds with a microphone.
The pictures showed that Titan's orange surface features resembled those on Earth with shorelines and river deltas, though carved by running methane not water.
A programming error meant that Huygens transmitted its results home via Cassini on only one channel instead of two as had been planned. But it was still an astonishing success, returning 350 photos including its pebble-strewn landing site.
A Surface-Science Package designed by UK scientists under the leadership of Professor John Zarnecki collected physical data about the landing site. It was found to be spongey with a thin crust, leading the team to compare it to crème brûlée.
Huygens survived on the surface for around 90 minutes before its power died. But Cassini's radar has continued to penetrate the hazy atmosphere to reveal many seas and lakes of methane.
Cassini's suite of instruments, including radar, imagers and spectrometers, have told us lots more about Saturn and the dynamics of its atmosphere, including a peculiar hexagonal feature around the north pole. Scientists have also learned a lot about the make-up of the famous system of rings and the forces that created and shape them.
But the probe has also been directed in its orbit to make close passes to a number of Saturn's myriad of moons and moonlets. These are some of the important findings:
Rhea, the planet's second biggest moon with a diameter of 1538km, shows heavy crater scarring, and Cassini discovered it is encircled by its own faint rings.
Iapetus, 1,471 km across, had long been a mystery to planetary scientists because one side is bright and the other dark. Thanks to Cassini, the dark side is now believed to be a residue left when ice evaporated into space. The moon also displays a strange equatorial ridge.
Dione, a cratered moon 1,123 km wide, was discovered by Cassini to have great cliffs of ice up to hundreds of kilometres long where its crust has fractured.
Tethys, 1066 km wide, has a giant crater, Odysseus, and a huge valley, called Ithaca Chasma, that runs from the north to the south pole.
Enceladus, 512km in diameter, is remarkable for its pattern of tiger stripes, fissures from which great plumes of water ice are squirted into space adding material to one of Saturns's rings and affecting the planet's magnetic field. Cassini has flown through and "tasted" these jets which are believed to originate in a subterranean reservoir that might be home to life.
Mimas, with a diameter of 418km, has a heavily battered surface, including massive Herschel Crater a third the moon's diameter which gives it an uncanny resemblance to the Death Star In Star Wars. The moon was responsible for clearing enough material to create the famous Cassini Division, a gap in Saturn's rings which can be seen even with small telescopes.
Hyperion is irregularly shaped and 410 km wide at most. Its dramatic, bath sponge-like appearance was discovered by the Cassini probe.
Phoebe, though small at about 220km, is interesting because Cassini studies suggest it is a protoplanet that drifted in from the outer solar system to be captured by Saturn: see Saturn's oddball Phoebe was a wannabe planet.
Cassini's initial four-year mission was extended in 2008 and again in 2010. The latest extension, called the Cassini Solstice Mission, is due to run through until 2017 and is so named because that is the year when sumer begins in the ringled planet's northern hemisphere.
This remarkable spacecraft is still in good health and space scientists are looking forward to yet more exciting discoveries as Cassini continues to add to its treasure trove of valuable data and beautiful images from Saturn.