The Cassini-Huygens probe's 20-year journey is about to end — it taught us to see Saturn all over again

Burning up in Saturn's atmosphere is its final fate.

27 countries took part. Launched on October 15, 1997, it flew 7.9 billion kilometers through space, carried out 2.5 million commands, fired its engine 360 times, completed 294 orbits, flew past Saturn's moons 162 times, collected 635GB of data, took 453,048 photographs, found 6 entirely new moons of Saturn, and yielded 3,948 pages of scientific papers …

That is how the Jet Propulsion Laboratory (JPL) at Caltech — the division that runs NASA's interplanetary exploration — sums up the Cassini-Huygens probe. In about 20 days, around September 15, this much-decorated spacecraft will plunge into Saturn's dense atmosphere of hydrogen, helium and ammonia and burn up, ending a 20-year journey of exploration.

Beyond that set of numbers, Cassini-Huygens was one of the most productive interplanetary probes the United States has ever launched. It flew close past several moons in the Saturn system, and the Huygens lander it carried touched down on Titan, Saturn's largest moon — the first human-made object to land on a body in the outer solar system. Nearly every high-definition photo of Saturn and its rings in the media today was taken by Cassini-Huygens.

Cassini-Huygens is named after two men who helped us first understand Saturn

The Cassini-Huygens probe was built jointly by three national space agencies — NASA, the European Space Agency and the Italian Space Agency — with people from 27 countries taking part. The two scientists in its name stand for the probe's two main parts: the Italian scientist Giovanni Domenico Cassini, who discovered the gap in Saturn's rings, gave his name to the main spacecraft; the Dutch scientist Christiaan Huygens, who discovered Titan, gave his name to the Titan lander, Huygens.

Everyone now knows that Saturn is the planet with the most conspicuous rings in the solar system. To the naked eye it takes on a distinctive sombrero shape, and on a clear night even a home telescope can pick out Saturn and its rings. When the famous scientist Galileo first saw it in 1610 through a hand-ground telescope, the resolution was so poor that for a while he thought Saturn had a companion star.

In 1655, Christiaan Huygens used a more powerful telescope built for astronomy to confirm that Saturn's rings exist, and described them precisely: Saturn is surrounded by a thin, flat ring that does not touch the planet and is tilted relative to the ecliptic. In 1675, Giovanni Cassini confirmed a gap in the rings 4,800 kilometers wide, a gap later named after him too. In 1859, James Clerk Maxwell argued that the rings could not be solid, because in theory a solid ring could not hold together and would inevitably break apart.

Later research confirmed that Saturn's rings are a vast mass of rock fragments, ice and dust, and that the ring system even has an atmosphere of its own. There is more than one gap: gravity has opened six that are now confirmed, and a moon, Mimas, runs through the Cassini Division.

Earth has a moon, and only the one. Saturn has dozens, traditionally named after the giants of Greek mythology. Most are really just big rocks: they are not large enough to reach hydrostatic equilibrium and pull themselves into spheres. Thirty are less than 10 kilometers across, and only seven are spherical.

The largest and most famous is Titan, the moon Huygens discovered. It is about twice the size of Earth's moon, and in the 1990s it was confirmed to have a dense atmosphere — the only moon in the solar system that does.

Before Cassini-Huygens, human probes had already been to Saturn's neighborhood

In 1973 the famous Pioneer 11 launched. What the public remembers best about it is the gold plaque bolted to the probe and carried out into the cosmos, showing any possible "aliens" what Earth and human beings look like. In 1979, Pioneer 11 passed about 21,000 kilometers above Saturn's highest cloud layer, catching a hurried glimpse of the planet. It was the first human probe to come close to Saturn.

In 1977, Voyager 1 launched, aimed at exploring beyond the solar system. To pick up speed from the gravity of the giant planets, it flew close past Saturn in 1980, took a mass of high-resolution photos, confirmed Saturn's huge family of moons, and found that visible light cannot get through Titan's dense atmosphere. In 1981, Voyager 2 also flew past Saturn and found more gaps in the rings.

Saturn is a giant gas planet with no solid surface, though it may have a solid core. It spins fast — one turn roughly every ten and a half hours — and at that speed it bulges into an oblate shape.

These probes showed more of Saturn's detail, and raised more questions. Why do gaps open in the rings? How did the rings form — a shattered moon, or gravitational capture? What is Saturn's atmosphere made of? Is there life on Titan? And so on.

In 1982, two scientists — one from the European Science Foundation, one from the U.S. National Academy of Sciences — began planning a mission to Saturn. NASA rates such missions "flagship," meaning the most important, most expensive and most complex deep-space missions. Besides Cassini-Huygens, the flagships have included Galileo, which went to Jupiter; Viking, which went to Mars; and the Voyagers, which headed out of the solar system. Missions still running include the Chandra X-ray Observatory in Earth orbit, and the Mars Science Laboratory's Curiosity rover, quietly digging in the Martian soil.

Cassini-Huygens was a nuclear-powered spacecraft that cost $3.2 billion

Like other deep-space projects, Cassini-Huygens was designed to fly through the outer solar system, far from the sun, where solar cells cannot work properly. Like Galileo before it, Cassini-Huygens carried radioisotope thermoelectric generators.

In the film The Martian, the main character takes one of these apart on screen: a radioisotope thermoelectric generator is, at bottom, a big lump of plutonium-238, releasing neutrons and heat every moment. Through the Seebeck effect, the generator turns that heat straight into electricity for the spacecraft.

Cassini-Huygens carried three of them, holding 32.7 kilograms of plutonium-238 in all, enough to supply at least 900 watts to run the spacecraft's 18 instruments. This type of generator is NASA's standard design: the same model flew earlier on Galileo and later on New Horizons, which went to Pluto. They are very durable, with a life of decades; after 10 years of use they can still deliver at least 600 watts.

Fueled, Cassini-Huygens weighed more than 5 tons and measured 6 meters long and 4 meters wide — about the size of a van packed solid. The Huygens probe, a separate piece, was a disc the size of a gazebo, weighing 300 kilograms and fitted with three parachutes for landing on the surface of Titan.

The project's direct costs came to $3.26 billion: $1.4 billion to build, $420 million to launch, $700 million to operate. The total is not adjusted for inflation, and the United States paid 80% of it.

The mission's main tasks were these:

• Measure the three-dimensional structure and dynamics of Saturn's rings
• Measure the surface composition of the moons and the geological history of each
• Measure the three-dimensional structure and dynamics of the magnetic field
• Study the dynamics of Saturn's atmosphere at every level
• Study the nature and origin of the unusual dark material on Iapetus
• Study Titan's clouds, haze and surface features

Cassini-Huygens found that it rains liquefied gas on Titan

On October 15, 1997, Cassini-Huygens lifted off from Cape Canaveral on a Titan IV rocket. The similarly heavy Galileo probe had not gone up this way: in 1989 it rode into space aboard the Space Shuttle and was released once in orbit. Galileo's launch had slipped more than two years — its rocket was not ready and one delay followed another — before the Shuttle finally carried it up.

The U.S. Air Force's Titan IV rocket, in service since 1989, met the launch requirement and put Cassini-Huygens into orbit. But to save fuel the spacecraft did not fly straight to Saturn. It followed a spiral path, flying past several of the solar system's major planets in turn and using their gravity to accelerate before finally reaching Saturn.

On April 26, 1998, and June 4, 1999, Cassini-Huygens made two close approaches to Venus for gravity-assist adjustments and acceleration; it flew past Earth on August 18, 1999; passed asteroid 2685 on January 23, 2000; and swept by Jupiter on December 30, 2000, taking a set of sharp images of the planet with a resolution as fine as 60 kilometers — the mission's first scientific result.

On July 1, 2004, the spacecraft passed between Saturn's F ring and G ring and entered orbit around Saturn, the first human probe to circle the planet, on a planned four-year mission. At its closest, Cassini-Huygens came within 20,000 kilometers of Saturn's upper atmosphere — less than a third of Saturn's radius.

Its first big job on arriving in the Saturn system was to release the Huygens probe for the first close-up study of Titan.

Before the Cassini-Huygens mission, what we knew about Titan was very little: only that it had a dense atmosphere that visible light could not get through, and that it showed clear signs of geological activity. What the atmosphere was made of, and what the surface looked like and consisted of, were unknown.

On July 2, Cassini-Huygens approached Titan; on December 25, Huygens separated from its mother ship, Cassini, and flew toward the moon. On January 14, 2005, Huygens entered Titan's atmosphere at more than 20,000 kilometers an hour, slowed by atmospheric friction, then opened its parachutes. A few hundred meters above the ground the probe powered up and switched on its floodlight, and Earth received its first image of Titan.

Titan is an orange world. Its atmosphere is nitrogen, methane, ethane and assorted hydrocarbons; under the solar wind, methane breaks down into an orange haze, and the whole world turns orange. As on Earth, Titan's atmosphere has storms and clouds, with cloud layers over both the north and south poles.

But the resemblance is one of form only. Titan and Earth are completely different worlds. Titan's surface temperature is about minus 179.2 degrees Celsius, and its atmosphere blocks sunlight that was already far weaker than Earth's, leaving the surface with a thousandth of the light Earth gets. At that temperature methane condenses into clouds, which high in the atmosphere turn into rain and snow and fall to the ground. What falls on Titan's surface, in other words, is rain and snow made of liquefied gas.

This "liquefied gas" snow fills Titan's hollows and craters, which makes the surface look smooth. Methane rain and other hydrocarbons form lakes and seas on the surface, and the flow of liquid has cut rivers, canyons, even alluvial fans. Titan's largest methane region is named Kraken Mare, after the sea monster. It covers more than 400,000 square kilometers, larger than Italy, and is more than 160 meters deep, though its waves and tides are very weak — a few centimeters to a dozen or so.

Picture it: under an orange-yellow sky, a dark liquid stretching to the horizon, creeping slowly under a methane wind. The cold is so deep that the surface looks viscous and does not reflect light. In colder places still, the liquid freezes into ice — but solid methane is denser than the liquid, so the ice sinks to the seabed. Orange methane slush piles up along the shores, and liquid methane runs down river mouths into the sea.

Huygens came down not on a sea but on yellow ground scattered with rounded rocks. What those rocks and that soil are made of is unknown, but it is expected to differ from Earth's; they are probably solid hydrocarbons, ice and broken rock.

Scientists guess that life quite unlike Earth's could exist on Titan's surface: it might breathe hydrogen, metabolize acetylene the way life on Earth metabolizes sugars, and exhale methane.

Data Cassini sent back in July 2017 showed that Titan's atmosphere appears to hold high-molecular-weight carbon compounds and acrylonitrile, substances that could serve as essential components of living cells.

More than sixty moons have been found around Saturn, and its north pole has a hexagonal storm that changes color

After releasing the Huygens lander, Cassini kept orbiting Saturn for another 12 years. Over that decade and more it flew past Saturn and its rings again and again, taking pictures and gathering piles of data.

Cassini found 7 new moons of Saturn. Together with the earlier finds, Saturn's confirmed moons now number more than 60, of which 52 have official names.

In the Keeler Gap in Saturn's rings, Cassini found a new moon, Daphnis — basically a big rock 7 kilometers across. But it interacts with the rings strangely: its gravity sets the ring material around it rippling like waves. This turned out to be common between Saturn's rings and its moons.

In 1981, flying past Saturn, Voyager 1 observed a huge, year-round storm at Saturn's north pole. Shaped by fluid dynamics and Saturn's rotation, the storm forms a remarkably regular hexagon. Cassini studied it again and found it more than 13,800 kilometers across — slightly wider than Earth — turning once every ten and a half hours.

Cassini also found that the hexagonal storm shifts between blue and gold with the seasons and the light. Scientists suspect that sunlight breaks down parts of Saturn's atmosphere and produces hazes of different colors.

A Cassini flyby of Enceladus produced an important discovery: under that moon's frozen surface there is very likely an ocean of liquid water reaching around the whole moon. NASA reckons this makes Enceladus the likeliest place in the entire solar system to hold life beyond Earth.

After two extensions, Cassini will plunge into Saturn's atmosphere next month

On April 16, 2008, NASA announced two more years of funding for the mission's work on the ground, and renamed it the Cassini Equinox Mission. On February 3, 2010, NASA announced a second extension, through May 2017 — far past the probe's design life, though it was in good shape and could keep working.

By late 2016, NASA had begun considering how to end the Cassini mission. It listed several possibilities: sending the probe farther out, to Neptune or out of the solar system; crashing it into the rings, into Saturn itself, or into one of its moons; or, more simply, leaving it in orbit around Saturn.

In the end, after weighing the fuel left, the orbits still open to it, and what the science required, NASA decided to let Cassini burn up in Saturn's atmosphere. One important reason: Enceladus and Titan may well hold life, or an environment that could support it, and Cassini — never sterilized before launch — very likely carries Earth microbes. Crashing it into one of Saturn's moons could contaminate them.

In April 2017, Cassini began its final mission. It dropped its orbit, passing between Saturn and its rings 3,100 kilometers from the planet, coming within 320 kilometers of the rings' inner edge.

NASA expects that around September 15, 2017, Cassini's orbit will fall low enough to reach Saturn's atmosphere, where the spacecraft will burn up, ending a 20-year scientific mission. It will keep sending back data until the moment it is destroyed.

NASA is already planning new missions to Saturn — flying into Titan's atmosphere again, or designing a submarine to go down into Enceladus's frozen ocean.

Cover image: NASA