Analysis Indicates Huygens Probe Had Rough Landing On Saturn’s Moon Titan In 2005
October 12, 2012

Analysis Indicates Huygens Probe Had Rough Landing On Saturn’s Moon Titan In 2005

[WATCH VIDEO: Touching Down On Titan]

Lawrence LeBlond for - Your Universe Online

The European Space Agency´s (ESA) Huygens space probe, which landed on Saturn´s largest moon Titan in 2005, had a bit of a shaky landing in the final seconds of its Titanian descent. After touching down on the distant landscape--the farthest away any spacecraft has ever landed--the probe “bounced, slid and wobbled” to its final resting place, according to a new analysis of the landing.

Experts conducting the analysis have found evidence the moon´s surface is more complex than previously believed. Through analysis of data from a variety of Huygens´ instruments that were active during the impact, the experts were able to reconstruct the chain of events of the landing. Instrument data were compared with results from computer simulations and a drop test model of Huygens designed to replicate the landing.

The team´s analysis reveals that, upon contact with Titan´s surface, Huygens dug a hole about 4.5 inches deep before bouncing out onto a flat spot on the ground. The team took into consideration a number of factors, including spacecraft weight (450 pounds by Earth standards) and force of impact (similar to dropping a ball on Earth from about 3.5 feet high). But due to Titan´s lower gravity, the probe only weighed 60 pounds at touchdown.

After making its initial contact, the probe tilted by about 10 degrees in the direction of motion and then slid 12 to 16 inches across the surface. Friction slowed the probe as it slid across the surface, forcing it into a wobble as it came to a stop. Huygens´ sensors continued to detect minute wobbling for a few more seconds after reaching its final destination.

“A spike in the acceleration data suggests that during the first wobble, the probe likely encountered a pebble protruding by around an inch [2 centimeters] from the surface of Titan, and may have even pushed it into the ground, suggesting that the surface had a consistency of soft, damp sand,” said Stefan Schröder of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.

Schröder is the lead author on a paper, published in the journal Planetary and Space Science, reporting the results of the landing.

The team´s work takes the evidence of a soft surface a step further by demonstrating that if something puts little pressure on a surface, the surface should be hard. In the case of an object putting more pressure on the surface, such as what Huygens did, it establishes that a softer surface exists, causing an object to sink in significantly.

“It is like snow that has been frozen on top,” said coauthor Erich Karkoschka, of the University of Arizona, Tucson. “If you walk carefully, you can walk as on a solid surface, but if you step on the snow a little too hard, you break in very deeply.”

The experts note the surface had to be dry as well. Had there been a wet, mud-like surface, Huygens´ instruments would have recorded a “splat” with no further indication of bouncing and/or sliding.

The analysis points to a surface that was soft enough to allow for a sizeable depression, but also hard enough to support a bounce, a slide and a wobble.

Karkoschka, a senior staff scientist at  UA's Lunar and Planetary Laboratory, said although dry and semi-soft, the landing zone appears to be a “big flood plain.” He also noted there had been  “a slight breeze in the lower atmosphere, causing the probe to parachute down at an angle. Because of the impulse, it bounced off sideways before it came to rest.”

Schröder added there was also “evidence of a ℠fluffy´ dust-like material - most likely organic aerosols that are known to drizzle out of the Titan atmosphere - being thrown up into the atmosphere and suspended” for a few seconds after Huygens made its impact.

Since the dust was easily lifted, it adds to the revelation the landing zone was most likely dry, suggesting there had been no rain of liquid methane for some time prior to the landing, Schröder suggested.

“You don´t get rain very often on Titan,” said Karkoschka, explaining that heavy downpours of liquid methane may occur decades or centuries apart. “When they do occur, they carve the channels we see in the pictures Huygens recorded as it approached the surface.”

He added that images of the landing site as Huygens approached showed a completely dry landscape, “suggesting it hadn´t rained in a long time.” When it did land, Karkoschka explained, “its downward-shining lamp warmed up the ground and caused methane to evaporate“¦that tells us that just below the surface, the ground probably was wet.”

“This study takes us back to the historical moment of Huygens touching down on the most remote alien world ever visited by a landing probe,” added ESA´s Cassini-Huygens project scientist, Nicolas Altobelli. “Huygens data, even years after mission completion, are providing us with a new dynamical 'feeling' for these crucial first seconds of landing.”

“Our analysis goes beyond Titan in that it gives us a better idea of how a probe lands, how we can measure surface consistencies and other parameters that are important for future missions sending spacecraft to touch down on moons and planets,” concluded Karkoschka.

The Cassini-Huygens mission is a joint project of NASA, ESA and the Italian Space Agency (ASI). The Jet Propulsion Laboratory in Pasadena, California manages the missions for NASA's Science Mission Directorate, Washington, D.C.

The Huygens space probe was named after 17th century Dutch astronomer Christiaan Huygens.