TITAN—A FASCINATING STELLAR OBJECT

The last three decades have witnessed spectacular triumphs in the field of exploration of space. Some of our spacecraft have whizzed past planets send back photographs. Our spacecraft have landed on Venus and Mars; man has also actually walked on the Moon, that object of inspiration for poets as well as lovers since the dawn of time.

However, if one were to ask astronomers where they would like to send the next space probe, they might, in all probability, choose Titan, the largest of Saturn’s satellites. As a stellar body, Titan is in a class of its own.

Titan is considerably larger than our own Moon. It is also the only moon in our solar system that has what may be described as an atmosphere. The credit for its discovery (in 1944) should go to the renowned Astronomer Gerard Kuiper who also discovered the two satellites of the planets Uranus and Jupiter. Just when Scientists had begun to feel despondent that a Titan probe seemed a long way off due to economic constraints, a positive breakthrough was achieved 17 years ago.

In 2005, a robotic probe Huygens, named after Christian Huygens who discovered it in in 1655) landed successfully on Saturn’s moon. It was the first spacecraft to land on Titan, and the farthest landing a spacecraft has. Huygens along with its orbiter Cassini was launched from Earth on October 15, 1997. Huygens separated from the Orbiter on December 25, 2004, and landed on Titan on January 14, 2005 near the Adiri region. Huygens’ landing on Titan is also significant as it is so far the only one accomplished in the outer solar system and was also the first on a moon other than Earth’s.

Although it was expected that Huygens would touch down in an ocean, a factor taken into account in its design, it touched down on land. Although Huygens had been designed to gather data for a few hours in the atmosphere, it sent data for about 90 minutes after touchdown though it had been designed to gather data for a few hours in the atmosphere and possibly a short time on the surface When the mission was planned, it was not yet certain whether the landing site would be a mountain range, a flat plain an ocean of something else, and it was thought that analysis of data from Cassini would help to answer these questions.

The pictures taken by Cassini 1,200 km (750 mi) above Titan, suggested that the landing site appeared to be a shoreline., Huygens was designed to survive the impact, of a splash down on a liquid surface on Titan (assuming the landing site could be non-solid.), and send back data for several minutes under these conditions.

If that occurred it was expected to be the first time a human-made probe would land in an extraterrestrial ocean. The spacecraft had no more than three hours of battery life, most of which was planned to be used during the descent. Engineers expected to get at most only 30 minutes of data from the surface.

The probe had remained dormant throughout the 6.7-year interplanetary cruise, except for semiannual health checks. Prior to the probe’s separation from the orbiter on December 25, 2004 a final health check was performed, prior to the probe’s separation. The “coast” timer was loaded with the precise time necessary to turn on the probe systems (15 minutes before its encounter with Titan’s atmosphere), then the probe detached from the orbiter and coasted in free space to Titan in 22 days with no systems active except for its wake-up timer.

The main mission phase was a parachute descent through Titan’s atmosphere. The batteries and all other resources were sized for a Huygens mission duration of 153 minutes, corresponding to 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. The probe’s radio link was activated early in the descent phase, and the orbiter “listened” to the probe for the next three hours, including the descent phase, and the first thirty minutes after touchdown. Not long after the end of this three-hour communication window, Cassini’s high-gain antenna (HGA) was turned away from Titan and towards Earth.

Very large radio telescopes on Earth were also listening to Huygens’ 10-watt transmission using the technique of very long baseline interferometry and aperture synthesis mode. At 11:25 CET on January 14, the Robert C. Byrd Green Bank Telescope (GBT) in West Virginia detected the carrier signal from Huygens. The GBT continued to detect the carrier signal well after Cassini stopped listening to the incoming data stream. In addition to the GBT, eight other telescopes, also listened for the signal. from the Huygens.

The signal strength received on Earth from Huygens was too weak to detect in real time because of the signal modulation by the (then) unknown telemetry. Instead, wide-band recordings of the probe signal were made throughout the three-hour descent. After the probe telemetry was finished being relayed from Cassini to Earth, the now-known data modulation was stripped off the recorded signal, leaving a pure carrier that could be integrated over several seconds to determine the probe frequency. It was expected that through analysis of the Doppler shifting of Huygens’ signal as it descended through the atmosphere of Titan, wind speed and direction could be determined with some degree of accuracy. A position of Huygens’ landing site on Titan was found with precision (within one km – one km on Titan measures 1.3 arcminutes of latitude and longitude at the equator) using the Doppler data at a distance from Earth of about 1.2 billion kilometers. The probe landed on the surface of the moon at 10.573°S 192.335°W. A similar technique was used to determine the landing site of the Mars exploration rovers by listening to their telemetry alone. Titan’s atmospheric composition is nitrogen (97%), methane (2.5%), hydrogen ( .2%) and traces of other gases There are trace amounts of other hydrocarbons such as ethane acetylene and of other gases, such as hydrogen cyanide, carbon dioxide and helium. The hydrocarbons are thought to form in Titan’s upper atmosphere in reactions resulting from the breakup of methane by the Sun’s ultraviolet light, producing a thick orange smog. Titan spends 95% of its time within Saturn’s magnetosphere, which may help shield it from the solar wind.

Energy from the Sun should have converted all traces of methane in Titan’s atmosphere into more complex hydrocarbons within 50 million years—a short time compared to the age of the Solar System. This suggests that methane must be replenished by a reservoir on or within Titan itself. The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.

Truly Titan lives up to its name as the most fascinating moon in the Solar System.