The Huygens Experience

Eight years ago, January 14, 2005, the European Space Agency’s Huygens probe plunged through a thick, mysterious, and planet-like atmosphere and landed, a bit roughly it seems, on the surface of Saturn’s largest moon –Titan. Huygens’ descent and landing gave mankind a glimpse of a world never before experience, not even by telescope, and yesterday marked the anniversary of this touchdown on the most remote alien surface ever visited by a landing probe.

The animation, showing the last part of the probe’s 2.5 hour descent through Titan’s athmosphere, was created by the European Space Agency (ESA) using data recorded by Huygen’s instruments and taking into account Titan’s atmospheric conditions, Sun and wind direction, the behavior of the parachute (with some artistic interpretation on the movement of the ropes after touchdown), and the dynamics of the landing itself. A photograph of the landing site returned from the probe is shown at the 1:25 mark in the video. Seen for the first time ever by human eyes, through the Huygens’ camera, the view is of ‘sand’ and ‘rocks’. The sand is composed of hydrocarbon aerosol particulates that have settled out of the atmosphere and the rocks are made of water ice.

Results published in October of last year revealed that on first contact with Titan’s surface the probe dug a hole 4.72 inches deep, then bounced and slid 10 to 15 inches across a flat surface. The probe then wobbled five times before coming to a standstill about 10 seconds after touchdown. A ‘fluffy’ dust-like material, most likely organic aerosols that are known to drizzle out of the Titan atmosphere, was thrown up around the probe following impact.

Cool Titan – Image credit: NASA

Titan is a cold world enclosed by a thick, hazy atmosphere impenetrable by telescopes and cameras. Having an equatorial radius of 1,600 miles, it is second only to Jupiter‘s moon, Ganymede, as to largest moons in our solar system. Titan is not only bigger than our own Moon, its also bigger than the planet Mercury. (To understand the size of Titan consider that it is only 62 miles in diameter lesser than Ganymede, and Ganymede is so large, roughly 3/4 the size of Mars, that according to NASA were it orbiting the Sun instead of orbiting Jupiter it would be easily classified as a planet.) The temperature at Titan’s surface is about -289 degrees Fahrenheit. Titan is of great interest to scientists because it’s the only moon in our solar system known to have clouds and a planet-like atmosphere. Titan’s atmospheric pressure is about 60 percent greater than the Earth’s, or roughly the same pressure found at the bottom of a swimming pool. The chemical composition of Titan’s atmosphere suggests to scientists that it may consist of compounds similar to those present in the primordial days of the Earth’s atmosphere. Titan’s atmosphere is mostly nitrogen, like Earth’s, but may contain much higher percentages of smog-like chemicals such as methane and ethane. The smog may be so thick that it actually rains ‘gasoline-like’ liquids. The organic nature of some of the chemicals found in Titan’s atmosphere might indicate that this very COOL moon could harbor some form of life.

Titan IVB rocket, with Cassini-Huygens on board, lifts off.

The Huygens Probe was named after Christiaan Huygens, a Dutch astronomer, mathematician, and physicist who discovered Titan in 1655. Huygens, the probe (not the astronomer!), hitched a 6.7 year interplanetary-cruise to Titan along with the NASA Cassini orbiter. On October 15, 1997, the Cassini-Huygens spacecraft was launched from Launch Complex 40 at Cape Canaveral in Florida on a Titan IV-B/Centaur launch vehicle. As tall as a 22-story building, the Titan/Centaur rocket lifted off perfectly with the 13,200-pound Cassini-Huygens spacecraft on board. Navigation to Saturn, and specifically to Titan, was a very complicated process coordinated by NASA’s Jet Propulsion Laboratory (NASA, JPL). Since it was such a massive spacecraft no existing launch vehicle could have sent the craft directly to Saturn, so a technique called ‘gravity assist‘ (or fly-bys) was used. Gravity assist maneuvers  work because of the mutual gravitational pull between a moving planet and a spacecraft. The planet pulls on the spacecraft as it is flying past, but the spacecraft’s own mass also pulls on the planet. This permits an exchange of energy. Mission designers planned multiple flybys of Venus, Earth, and Jupiter, using each planet’s gravity to boost Cassini’s sun-relative speed and send the spacecraft out to Saturn.

For most of the nearly 7 year journey the probe remained dormant, conserving power, except for bi-annual health checks. Because the distance from Earth is too great to provide signals and commands, this meant the programming of the probe had to be precise and work automatically so that valuable data can be communicated back to the Cassini orbiter for transmission back to Earth. A final health check was performed on December 25, 2004 prior to the probe’s separation from the orbiter. Huygens was then detached to coast in space for 22 days en-route to Titan on its own with no systems active except for its wake-up timer. Remaining on the Cassini orbiter was the probe support equipment (PSE), including the electronics necessary to track the probe and to recover the data gathered during its descent. Then, just 45 minutes before reaching the atmosphere of Titan, timers woke up the Huygens probe, and its main mission phase, a parachute descent through a hazy alien atmosphere to touchdown on the largest expanse of un-before seen terrain left in the Solar System, was ready to begin.

Huygens’ batteries and resources were sized for a mission duration of 153 minutes — a maximum descent time of 2.5 hours, plus at least 3 additional minutes (and possibly a half hour or more) on Titan’s surface. Early in the descent phase the probe’s radio link was activated allowing Cassini to ‘listen’ to the probe for the next 3 hours which included the descent phase and the first thirty minutes after touchdown. Soon after the end of this three-hour communication window, Cassini’s high-gain antenna (HGA) was turned away from Titan and towards Earth. Cassini remains in orbit around Saturn and will continue operations until 2017. The Cassini–Huygens mission is a cooperative project of NASA, ESA and The Italian Space Agency.

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