The spacecraft Cassini will become a streak of ash when it tumbles into Saturn on Friday.
Less than a million miles away, the probe Huygens, which Cassini launched onto Saturn's moon Titan, will be the only evidence of its partner's journey. Huygens is probably in the very spot where Cassini delivered it 12 years ago, settled like a headstone amid moon dust and icy cobblestones.
Cassini and Huygens traveled together from Earth to Saturn's orbit. Their mission represented the symbiotic pairing of NASA and the European Space Agency. NASA's Cassini provided the propulsion. The ESA's Huygens spent eight years and 934 million miles stuck, barnacle-like, to Cassini's much larger belly. On Christmas Eve 2004, Cassini and Huygens split. Huygens landed on Saturn's moon Titan three weeks later.
A dozen years after the separation — and nine years beyond Cassini's expected lifetime — Cassini continued to orbit Saturn. The program was a massive success. Yet Huygens had the most ambitious goal of all: It is the only machine to land on a world beyond the asteroid belt. And, without Huygens, Cassini might never have left Earth's gravity.
In the early 1980s, scientists conceived of a Saturnian orbiter-lander, naming the proposal after Italian astronomer Giovanni Cassini, who discovered four of Saturn's moons. The ESA's Science Program Committee added the title Huygens in 1989. (Christiaan Huygens, a Dutch mathematician and stargazer, spotted Titan in 1655.)
Titan's surface was a mystery before the ESA and NASA sent the Huygens probe to investigate it. The moon, larger than Pluto or the planet Mercury, is a whopper of a satellite. Observed from space, Titan is a hazy orb. Methane transforms into hydrogen cyanide and other hydrocarbons in its nitrogen-rich atmosphere, coloring the sky orange.
“What lay beneath that smog?” wrote University of Hawaii astronomer Toby Owen, in a 2004 article about Huygens. “Speculation ranged from a global ocean of ethane to a rugged landscape carved out by precipitating hydrocarbons, to a nondescript, gooey surface resembling a refrozen chocolate ice cream dessert.”
Huygens's final design, a hockey puck set in a tea saucer, was a hedge against the unknown. It would withstand the friction of Titan's atmosphere. It was prepared to float, should it land on an ethane sea. But first the saucer had to fly.
The lander almost didn't make the journey offworld. Under the proposed fiscal budget for 1995, NASA would not have the funds for Cassini. The ESA, which needed Cassini to give Huygens its interplanetary ride, mounted a defense of the program.
“We were very seriously on the chopping block,” recalled Earl Maize, the deputy program manager of the Cassini-Huygens mission at NASA's Jet Propulsion Laboratory, to the Los Angeles Times.
Maize credited the ESA with keeping Cassini-Huygens alive. “I think without their intervention, Cassini might have met an entirely different fate,” he said.
Cassini and its metal limpet blasted into space in 1997. They had avoided financial doom — but disaster almost struck again. While the craft was flying toward Saturn, a Swedish engineer named Boris Smeds discovered a critical flaw in the plan to drop Huygens on Titan.
Huygens was chock-a-block with sensors: to listen to the winds of Titan, to taste the chemicals in its atmosphere and to record the 2.5-hour descent. But it was not powerful enough to send any of these signals back to Earth by itself. Huygens needed to reach Cassini, which would boost the information home.
The engineer realized that the exchange between the lander and Cassini's receiver would jumble the Titan data, as IEEE Spectrum reported in 2004. Under the initial plan, Cassini traveled at 3.4 miles per second relative to Huygens, a blistering speed that, thanks to the Doppler effect — the audio shift you can hear in a police siren — would have scrambled the signal.
Smeds's tests convinced the project leaders they needed to alter Cassini's trajectory. The new path successfully eliminated the Doppler scrambling. It also delayed Huygens's departure from Cassini until Dec. 24, 2004.
Huygens skidded onto Titan on Jan. 14. “Right on time,” Jean-Pierre Lebreton, the French physicist and ESA leader of the Cassini-Huygens mission, told Bloomberg Buisnessweek in 2005. “It was a very emotional moment. It was hard not to cry.”
On its way down, Huygens took the temperature of Titan's atmosphere. It was -150 degrees at 300 miles up, warming to -120 at an altitude of 150 miles, before dropping to -290 at the surface.
The probe also hinted at an answer to one of Titan's curiosities: where the moon got its methane supply. Sunlight destroys methane after a few million years. At 4.5 billion years old, Titan should be tapped out. But Huygens sensed a sharp spike in the amount of methane, jumping by 40 percent on the ground, per the ESA. This suggested that liquid methane exists on Titan, mostly trapped beneath the surface but leaking enough to replenish atmosphere.
The probe gave us “a new view of Titan, which appears to have an extraordinarily Earthlike meteorology, geology and fluvial activity,” wrote the ESA's Lebreton and his co-authors in the journal Nature in 2005.
On the moon, the familiar has a chemically alien twist. The scientists continued: “Instead of liquid water, Titan has liquid methane. Instead of silicate rocks, Titan has frozen water ice. Instead of dirt, Titan has hydrocarbon particles settling out of the atmosphere.” Titan's twist on Earthly features could in theory support microbial life, some chemists say, despite the frigid temperatures.
Then there is Titan's geography. If you stop a spinning Earth globe at random, the chances are good there would be ocean beneath your fingertip. Titan as seen by Huygens was far more intriguing than Earth, at least in the eyes of Erich Karkoschka, a scientist at the University of Arizona's Lunar and Planetary Laboratory.
“Where Huygens landed, there were riverbeds and mountains,” Karkoschka told The Washington Post on Tuesday. “If you had picked a random spot on Earth it wouldn’t be so interesting. It almost seems like a more lively planet than Earth.”
In a 2012 paper, Karkoschka and two other researchers analyzed Huygens's descent. It hit Titan, scooping a 4.5-inch divot in the surface, before bouncing up and landing on a flat flood plain. There it skidded about a foot and quivered for 10 seconds, before coming to rest. The overall effect was not unlike dropping something on snow capped by a frozen upper layer: soft, but topped by a harder crust.
And there's evidence that, below the surface of the flood plain, the region is still wet, Karkoschka said.
After 72 minutes, Cassini had flown past Titan's horizon, beyond the reaches of Huygens’s transmitter. About two hours after touching down, the lander's batteries died. But Karkoschka said that he had no reason to suspect that Huygens was not where Cassini left it.
“It could stay there a long time,” he said. “Maybe millions of years.”