Saturn rendezvous

Print edition : July 16, 2004

Saturn and its rings fill the field of view of Cassini's narrow-angle camera in this natural-colour image released by NASA on April 29. This is the last single `eyeful' of Saturn and its rings achievable with the narrow-angle camera on approach to the planet. - AFP/NASA

After seven years of a complex journey covering 3.5 billion kilometres, the Cassini-Huygens mission to Saturn enters a crucial phase as it readies to go into orbit around the solar system's second largest and most mysterious planet.

ON July 1, the Cassini-Huygens mission to Saturn, the most ambitious planetary exploration project ever undertaken, will enter its crucial phase. After seven years of complex journey over 3.5 billion kilometres, the most sophisticated robotic spacecraft ever to be flown will go into orbit around the solar system's second largest planet. It will mark the beginning of an extensive, four-year-long tour of Saturn, its intriguing rings and 31 moons - the largest being the planet-size Titan - and its powerful magnetic field.

Indeed, 13 of the moons - all small ones, with sizes varying from 3 km to 16 km in diameter - were discovered after the National Aeronautics and Space Administration (NASA) launched the Cassini-Huygens spacecraft from Cape Canaveral, Florida, on October 15, 1997. The Cassini-Huygens mission is expected to throw light on fundamental questions about the evolution of other outer planets, gas giants such as Jupiter, Uranus and Neptune. Nearly 25 years ago, data and images of Saturn beamed by the fly-by missions of Pioneer 11 (1979) and Voyager 1 and 2 (1980-81) had kindled the interest of scientists in Saturn (see box). Ever since then, the planetary science community has been eagerly awaiting the Cassini-Huygens mission.

More than the atmosphere of Saturn, which mainly comprises hydrogen and helium and smaller amounts of methane and ammonia, it is the dense orange-hued atmosphere of Titan (1.5 times denser than the earth's atmosphere), the planet's largest and the second largest moon in the solar system after Jupiter's Ganymede, that is specially fascinating to scientists because it could hold clues to how life evolved on the earth. Saturn's distinctive bright rings, which would barely fit the space between the earth and the moon, are made up of ice and rock particles that range from grains of sand to huge boulders. There is greater variety in Saturn's moons than in those of any other planet. In fact, the `Saturnian system' can be regarded as a miniature representation of the solar system. Titan is believed to contain organic compounds that may be important to the chemical chain responsible for the origin of life on the earth. Although too cold to support life now its surface temperature is 1790C - it may well be like a "frozen vault" preserving clues to what early earth might have been like. The entire mission has been designed from this perspective.

From the original vision to its completion, the Cassini-Huygens mission will span a period of nearly 30 years. In 1982, a joint working group of the European Space Agency (ESA) and the United States National Academy of Sciences began considering joint explorations of the solar system. With the success of the Voyager missions, a mission to Saturn, which is about 1.5 billion km from the sun (10 times the earth-sun distance), rose high on the agenda. While it was clear that an orbiter to Saturn would form a key component of such a mission, the intriguing findings about Titan's atmosphere by Voyager led the group to decide on dropping a probe into Titan rather than Saturn. By 1985, the ESA had come up with a novel design for a probe that could be manoeuvred into the dense and low-gravity atmosphere of Titan. Thus the spacecraft comprises a Saturn orbiter built by NASA called Cassini on which the probe Huygens built by the ESA rides piggy-back.

The Cassini-Huygens mission is named after two 17th century European astronomers: the French-Italian astronomer Jean-Dominique Cassini (1625-1712) who discovered Saturn's four major moons, Iapetuss, Rhea, Tethys and Dione, and the Dutch astronomer Christiaan Huygens (1629-1695) who discovered Saturn's rings and Titan. Cassini also discovered that Saturn's rings are split largely into two parts by a narrow gap, known since as the Cassini Division. Interestingly, the spacecraft will make use of this gap to pass close to Saturn and thus be captured into a Saturnian orbit. The probe, however, is scheduled to be released into Titan's atmosphere only on January 15, 2005, when Cassini would have remained in orbit for at least six months. As Cassini has to travel such a long distance, the spacecraft required a much larger and more robust communications system than that used in Galileo (launched in 1989 on a mission that lasted 14 years), which travelled only half the distance to Jupiter. The high-gain antenna and the spacecraft radio system were provided by the Agenzia Spaziale Italiana (ASI, Italian Space Agency). Although the Jet Propulsion Laboratory (JPL) of NASA manages the overall programme, it is a joint endeavour of JPL/NASA, the ESA and the ASI, with the participation of over 260 scientists from the U.S. and 17 European nations. The operation of the Huygens probe will be controlled by the ESA from its control centre in Darmstadt, Germany. Both the ESA and the ASI have contributed scientific instruments on board Huygens and Cassini respectively. The total cost of the mission is $3.27 billion, of which the U.S. investment is $2.6 billion, ESA $500 million and ASI $160 m.

The communications sub-system consists of the high-gain antenna and two low-gain antennae. The primary function of the former is to support communication with the earth. It is also used for scientific experiments. The high-gain antenna was, and will be, put to some unusual use - as a shield to protect the spacecraft's instruments from the harmful rays of the sun. For most of the early part of the journey, the high-gain antenna was positioned toward the sun and functioned as an umbrella. It will also be used as a shield against very small particles that could damage the spacecraft as it crosses Saturn's ring plane to get into orbit around the planet. Cassini was the first planetary spacecraft to use solid-state recorders without moving parts instead of the good old tape recorder. The technology is since being used in all future spacecraft.

In designing Cassini, NASA conducted extensive studies to optimise the cost, mass, reliability, durability and availability of hardware. Like the recording medium, moving parts were eliminated from the spacecraft wherever the functions could be performed satisfactorily without them. The early designs involving moving science instruments, platforms or turntables were discarded in favour of fixed instruments. The pointing of any instrument would therefore require an appropriate rotation of the entire spacecraft. According to NASA, because of the care taken in its design, Cassini will return more scientific data about its targets than has been possible in any planetary mission so far. The Cassini-Huygens' payload consists of a carefully chosen retinue of sophisticated instruments - the orbiter for Saturn has 12 instruments whereas the Titan probe carries six more. Cassini will be the first orbiter around Saturn and will carry out various measurements until July 1, 2008.

The Huygens probe is designed to gather data during descent and a little after landing, if successful. It may fail to land if it impacts an ocean of methane or ethane, elements Titan is believed to have in significant quantities. The probe will provide the first sampling of Titan's nitrogen-rich atmosphere (87.97 per cent) and the first photographs of its surface, which has never been visible because of the thick orange clouds. The data will be beamed to Cassini, which will relay it to the earth. The descent will last two to three hours, and upon successful landing transmission will continue for about 30 minutes more, until the batteries run out. The orbiter will be in radio contact for up to four and a half hours, until it disappears over Titan's horizon after dropping the probe.

JUST as our understanding of Jupiter and its moons was radically changed during the eight years of Galileo's orbiting, scientists hope that Cassini-Huygens will revolutionise our understanding of not only Saturn but also the earth. A major difference between the two missions is that in the case of Galileo the probe was released into the Jovian atmosphere. Cassini-Huygens, including the orbiter and the probe, is one of the largest, heaviest and most complex interplanetary spacecraft ever built. Of all the interplanetary spacecraft, only the two Phobos spacecraft sent to Mars by the former Soviet Union were heavier. Fully fuelled, Cassini-Huygens weighed about 5.7 tonnes and stood about 6.7 m tall and 4 m wide. Fuel alone (3.2 tonnes) accounted for more than half the launch weight of the spacecraft, half of which will be needed for the spacecraft's approach to Saturn. The magnetometer instrument is mounted on an 11-metre-long boom that extends outward from the spacecraft. Most of the spacecraft and its instrument housings are covered with multiple-layered shiny amber-coloured or matte-black blanketing material for protection against radiation, extreme temperatures and the impact of dust particles and micrometeroids.

Even though the spacecraft was launched on the most powerful launcher then available - a Titan 4B rocket with a Centaur upper stage - its weight was too high for it to be sent directly to Saturn. Therefore, Cassini's trajectory (see schematic diagram) included four `gravity assists' to hurl it towards Saturn. In a `gravity assist', the spacecraft flies close enough to a planet to be accelerated by its gravity, creating a `slingshot' effect that accelerates the spacecraft. Cassini flew by Venus twice, in April 1998 and June 1999, and swung past the earth in August 1999. The three gravity assists took it to the outer solar system. The fourth and final slingshot was the result of a Jupiter fly-by in December 2000 that gave it enough acceleration to reach Saturn in July 2004.

Like Galileo, Cassini is powered by the heat generated by the natural radioactive decay of plutonium, which is then converted into electricity. During the December 2000 Jupiter fly-by Cassini was used to examine the planet's magnetosphere from far away while Galileo took data from closer proximity - its orbit around Jupiter. It was for the first time that a simultaneous observation of the kind was made. Cassini also produced some remarkable images of the planet's turbulent atmosphere in great detail. During this operation, most of the orbiter's scientific instruments were switched on and calibrated and data collected. The Cassini-Galileo joint study served as a practice ground for many of Cassini's instruments three and a half years before its Saturn arrival.

The long interplanetary journey had another unexpected advantage. In 2000, mission scientists detected a problem while testing the communications system that would enable Cassini to receive data from Huygens during the latter's descent on to Titan's surface. During a test that simulated the Doppler frequency shift that would occur during descent, it was found that the radio receiver on the orbiter was unable to receive data. (Doppler shift is the change in the frequency of waves from a source and is proportional to the relative velocity between the source and the receiver. This is what causes the change in the pitch of an approaching or receding train, for example.) The problem was studied for months and the collaborating space agencies decided to effect a change in trajectory to reduce the relative velocity between the orbiter and the probe and thus minimise the Doppler shift.

Cassini's first rendezvous with the Saturnian system occurred on June 11 when the spacecraft flew by Saturn's moon Phoebe, which is 13 million km from the planet and moves in an irregular elliptical orbit. Cassini passed within 2,000 km of this 220-km-wide satellite, which, scientists believe, may be a remnant of the primordial material that formed the rocky cores of the outer planets more than 4.5 billion years ago. Indeed, the hypothesis was confirmed on June 23, when scientists announced the results of their analysis of the data on Phoebe. The satellite is indeed made up of ice, rocks and carbon compounds that are similar to material found in Pluto and Neptune's moon Triton.

The most critical phase of the mission will be the orbit injection on July 1. Cassini will approach Saturn from below the ring plane, crossing through the Cassini Division between rings named F and G. To slow down, the spacecraft will fire its main engine for 96 minutes in reverse and allow itself to be captured as a satellite in orbit when it will consume half the amount of propellants it carried at launch. During the engine burn, the spacecraft will be within 18,000 km of Saturn. If successful, the process will put Cassini in a highly elliptical orbit that will be adjusted by subsequent engine firings. It will be the closest the spacecraft would have approached Saturn in its entire mission and will mark the beginning of 76 orbits to be completed during its four-year Saturn tour. The arrival will provide the unique opportunity to observe Saturn's rings and the planet itself.

Phoebe's true nature is revealed in startling clarity in this NASA mosaic of two images released on June 14, taken during Cassini's fly-by on June 11. The image shows evidence for the emerging view that Phoebe may be an ice-rich body coated with a thin layer of dark material.-AFP/NASA/JPL/SSI

During the next six months, before the probe descends on Titan, Cassini will fly by Titan twice to study its atmosphere and surface and prepare for the Titan probe mission. Huygens probe is bolted to Cassini and it is fed power through an umbilical cable. During its seven-year journey, Huygens has travelled in a "sleep mode", awakened once every six months for instrument check-outs. On December 25, Cassini will release Huygens, which, with umbilical cut and bolts removed, will coast towards Titan for about three weeks, spinning at the rate of 7 rpm (revolutions per minute) for stability. The probe carries two S-band transmitters and two antennae to establish radio contact with the orbiter.

On January 15, 2005, the battery-powered probe will enter Titan's atmosphere at a speed of about 20,000 km an hour, which extends about 1,000 km above the moon surface (10 times the height of the earth's atmosphere). The probe is designed to withstand a huge temperature range: the extreme cold of space (about 2000C) and the intense heat it will encounter during atmospheric entry (>12,0000C). A saucer-shaped heat shield will protect it from the high temperature. At about 300 km above Titan's surface, the probe will release a decelerator to slow down its descent. At an altitude of about 170 km, when the speed has come down to 1,400 km an hour, the main parachute, 8.3 m in diameter, will be deployed (see diagram) to slow down and stabilise the descent. It will also allow the heat shield and the decelerator to be jettisoned. To limit the duration of the descent to two and a half hours, the main parachute will be jettisoned 15 minutes after the probe enters the top of the atmosphere. A smaller drogue chute, 3 m in diameter, will be deployed to support the probe for the remainder of the descent. Throughout its descent through the orange haze, Huygens' atmospheric structure instrument will measure the physical properties of the atmosphere. The vertical profile of Titan's atmosphere is quite similar to the earth's (see graph). The probe's Gas Chromatograph and Mass Spectrometer (GCMS) will determine the chemical composition of the atmosphere. Another instrument will collect and vaporise solid particles so that they are also analysed by the GCMS. The Descent Imager and Spectral Radiometer (DISR) will take pictures of Titan's methane clouds. As the probe reaches a height of 50 km, the DISR will capture panoramic views of the surface. In the last few hundred metres of descent a bright white lamp will illuminate the surface, which is normally muddy red because of the absorption of blue frequencies by the atmosphere, to enable the DISR to carry out a spectral analysis of the surface composition.

The Doppler shift in the probe's radio signal will constantly be monitored to deduce the strength of Titan's winds. The Huygens Atmospheric Structure Instrument (HASI) will measure temperature, pressure and electric fields that could indicate the presence of lightning. As the probe nears impact, its surface science package will activate a number of sensors to measure surface properties to determine whether the surface is hard, snowy or liquid. Huygens will impact the surface at about 25 km/hr. The chief uncertainty, however, is whether its landing will be a thud or a splash. If Huygens lands in a liquid - a hydrocarbon lake or ocean - the on-board instruments will measure the liquid properties while the probe floats for a few minutes. The probe is designed to float even though liquid hydrocarbons are significantly less dense than water. If it lands with a splash, it will not be able to return data for very long because the extreme low temperature of the liquid (180C) will prevent the batteries from operating. Also, ethane will damage the science package instruments.

After Huygens' descent, Cassini will continue its orbital tour of Saturn, with at least 76 orbits, including 52 close encounters with seven of Saturn's moons. Gravity-assisted fly-bys of Titan will govern the trajectory of Cassini's orbits. Cassini is expected to execute as many as 45 targeted close fly-bys of Titan, getting as close as about 950 km above Titan's surface. Accurate navigation and targeting of the point at which Cassini flies by Titan will be used to shape Cassini's four-year orbital tour and enable it to get a close-up view of Saturn's satellites and rings and different parts of its magnetosphere. The arrival on Saturn is bound to give some spectacular images of the rings and the planet itself. Later, the first fly-by of Titan and the Huygens descent will result in enormous amount of data and as the orbiter continues its encounters with the giant moon it will continually yield new findings about this strangely beautiful world. Of course, Titan harbours too many mysteries to be unravelled by one single mission. But the findings may prove revealing enough for scientists to undertake follow-up missions to Saturn, undoubtedly the most mysterious planetary system.

This article is closed for comments.
Please Email the Editor