Titan 469
provided through three radioisotope thermoelectric gener-
ators (or RTGs).
The 5650 kg (6 ton)Cassini–Huygensspacecraft was
launched successfully on October 15, 1997, from the
Kennedy Space Center at Cape Canaveral at 4:43a.m. EDT.
Because of its massive weight,Cassinicould not be sent di-
rectly to Saturn but used the “gravity assist” technique to
gain the energy required by looping twice around the Sun.
This allowed it to also perform flybys by Venus (April 26,
1998, and June 24, 1999), Earth (August 18, 1999), and
Jupiter (December 30, 2000).Cassini–Huygensreached
Saturn in July 2004 and performed a flawless Saturn Or-
bit Insertion (SOI), becoming trapped forever in orbit like
one of Saturn’s moons.
TheCassiniinstruments have since then returned a great
amount of data concerning the Saturnian system. During its
4 year nominal mission, theCassiniorbiter will make about
40 flybys of Titan, some as close as 1000 km (Voyager 1
flew by at 4400 km) from the surface.Cassiniwill perform
direct measurements with the visible, infrared, and radar
instruments designed to perform in situ (on-site) studies
of elements of Saturn, its atmosphere, moons, rings, and
magnetosphere. One set of instruments studies the tem-
peratures in various locations, the plasma levels, the neutral
and charged particles, the surface composition, the atmo-
spheres and rings, the solar wind, and even the dust grains
in the Saturn system, while another performs spectral map-
ping for high-quality images of the ringed planet, its moons,
and its rings.
Additionally, the mission saw the deployment of the
European-built Huygensprobe. After release from the
Cassiniorbiter, on December 25, 2004, this 300 kg probe
plunged into Titan’s atmosphere on January 14, 2005, at
11:04 UTC and descended through it by means of several
parachute brakes (Fig. 2), which slowed the probe from
supersonic speeds of 6 km/s during entry and down to
5 m/s at impact. The five batteries onboard the probe lasted
much longer than expected, allowingHuygensto collect de-
scent data for 2 hours and 27 minutes and surface data for
1 hour and 12 minutes. During its descent,Huygens’ camera
returned more than 750 images, while the probe’s other in-
struments sampled Titan’s atmosphere to help determine
its composition and structure. The telemetry data from
Huygens was stored onboard Cassini’s Solid State
Recorders (SSRs) at a rate of 8 kbits/s, while the space-
craft was at an altitude of 60,000 km from Titan. Although
some data from Huygens was lost during its transmission to
Cassinithrough a stream called Channel A, in the end all
of the measurements were recovered because Titan’s weak
signal was captured by Earth-based radio telescopes!
As well as measuring the atmosphere and surface prop-
erties, the probe took samples of the haze and gases. These
in situ measurements complement the remote-sensing data
recorded from the orbiter.
TheCassini–Huygensmission has already provided a
wealth of data. The analysis is in the first stages, and the
Cassiniorbiter promises to unveil yet more of Titan’s se-
crets in the years to come. What follows is an attempt to
provide the reader with a precise account of current infor-
mation on Titan’s environment from all available means of
investigation.2. The Atmosphere of Titan
The most interesting feature of Titan, as has been argued
previously, is its amazing atmosphere, a close analog to the
Earth’s primitive gas envelope according to some theories,
but it is located almost ten times further away from the
Sun.FIGURE 2 Huygensdescent profile in Titan’s
atmosphere. The three parachutes that helped brake
the descent and reduce the speed to about 5 m/s on
the surface are shown. The total descent lasted 2 h
28 mn. The probe spent 1 h 12 mn on the surface. The
signal from Huygens received on Earth via
radiotelescopes was a total of5h42mnincluding 3 h
14 mn at the surface.