Cassini Huygens

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The Cassini mission was the most sophisticated spaceprobe so far, designed to orbit among the moons and rings of Saturn for at least four years.

magnetometer boom

magnetometer boom

TITAN'S SURFACE

This composite of three infrared images from Cassini reveals details on the surface of Titan that are hidden in visible light by the opaque atmosphere.

Following the Voyager encounters with Saturn in the early 1980s, it was clear that the ringed planet and its moons still held enough secrets to justify an orbiter mission similar to the Galileo one already planned for Jupiter. The project became an international effort after a committee formed to look at future collaboration between NASA and the European Space Agency ESA recommended an orbiter-and-probe mission similar to what NASA had in mind. It was named Cassini after Gian Domenico Cassini, the 17th-century Italian-French astronomer who discovered the main division within Saturn's rings.

Cassini's original design was similar to Galileo - both were to be "Mariner Mark II" craft, as was a third mission, the Comet Rendezvous Asteroid Flyby (CRAF). However, when CRAF was cancelled the idea of developing a coherent second wave of Mariner probes was abandoned, and Cassini subsequently evolved into a much larger, heavier design.

While Galileo carried an atmospheric probe to descend into Jupiter's clouds, Cassini's cargo was even more ambitious - a lander that would parachute through the murky haze of Titan and send back data and pictures from the surface. This lander, Huygens, was ESA's main contribution to the project.

In order to power its array of scientific instruments, Cassini carried a large payload of plutonium in its radioisotope thermal -^MLS

generators. The possibility of an accident created much controversy before launch and during Cassini's flyby of Earth in 1999 (although smaller quantities of plutonium P

had already been used by previous \

spaceprobes). Fortunately, the launch, by a Titan IVB/Centaur m ¡M, rocket on 15 October 1997, went I jSmJ ^

flawlessly. With a total mass at launch of 5,655kg (12,4411b), the probe was so big that it needed a IPkJ

complex series of gravity assists to get up to a reasonable speed. For this reason, the flight to Saturn took more than six years, with flybys of Venus (twice), Earth, and Jupiter. Cassini reached

Jupiter in late 2000 and produced a unique scientific opportunity - the Saturn-bound probe was able to make long-range observations of the giant planet and its moons at the same time as Galileo.

Arrival at Saturn

Cassini's main engine executed a 96-minute burn to drop the spacecraft into orbit around Saturn on 1 July 2004. On its way into the Saturnian system, the probe had already provided the first close-up

BUILDING HUYGENS

Engineers fit the back cover to the Huygens probe. The gold tiles offered protection against the extreme temperatures encountered during entry into Titan's atmosphere.

BACKLIT RINGS

On 15 September 2006, Cassini passed into Saturn's shadow. Over three hours it took 165 imoges of the still-sunlit rings, resulting in this magnificent panorama. The box marks the distant glimmer of the Earth.

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high-gain antenna low-gain _Antenna radar bay radio/plasma wave subsystem antenna.

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Huygens Titan probe radioisotope thermoelectric generator

24 June 1999 Venus fly by

26 April 1998: Venus flyby

18 August 1999: Earth flyby

15 October 1997: launch mam engine

CASSINI'S MISSION

Cassini's long route to Saturn took it twice past Venus, once past the Earth, and then on to Jupiter and finally Saturn. Once there, the probe (left) was built to operate for at least four years and some 80 orbits of the planet.

3 December 1998: deep space manoeuvre to adjust orbit

1 July 2004: arrival at Saturn

130 December 2000: closest approach to Jupiter

GIANT PROBE

The enormous Cassini dwarfs technicians fitting instruments to it in Kennedy Space Center's Payload Hazardous Servicing Facility prior to launch.

GIANT PROBE

The enormous Cassini dwarfs technicians fitting instruments to it in Kennedy Space Center's Payload Hazardous Servicing Facility prior to launch.

photographs of the outer moon Phoebe. Now it could begin its work in earnest, with cameras, spectrometers, magnetometers, and a wide variety of other instruments recording every aspect of the planet and its rings and satellites. The lander separated from the main spacecraft II^HHH^j on Christmas Day, and Huygens dropped into Titan's atmosphere on 14 January 2005.

The schedule for this phase of the mission had been radically changed when Cassini was already on its way to Saturn. In early 2000, concerned ESA engineers had simulated the way that the lander would relay data via Cassini to Earth. They found a critical . p5 flaw - in the original flightplan, s ,/• | the relative motion of the two craft would distort Huygens's signals and 11 ) make them unintelligible to Cassini.

I By changing the direction of the 1 fiflj vehicles during this critical phase of the mission, disaster was averted.

As Huygens dropped towards Titan, aerial photographs revealed an eerily Earth-like eroded shoreline, and Huygens landed on what seemed to be a pebble-strewn river delta. This confirmed suspicions that Titan is a world where methane (which freezes at -182°C/-296°F) plays a similar role to water on Earth, occurring as ice, liquid, and vapour.

Discoveries in orbit

Although the Titan landing was an early highlight of the mission, Cassini has continued to deliver a wealth of data from its looping flight around Saturn. Its large engine and plentiful propellant have allowed it to make several course corrections, bringing it to within a few hundred kilometres of most of the major moons. One of its most spectacular discoveries was a huge plume of ice crystals erupting from the inner moon Enceladus - a surprising indication that there is liquid water just below its surface.

Cassini also went equipped to tackle Titan's hazy atmosphere, with a near-infrared camera that can see through the orange smog and map the terrain below. It has found hot spots, which might be active ice volcanoes on the surface, and A

reflective patches that are probably lakes of A liquid methane. Other moons are less active U but seem to have fascinating histories. Dione fl has towering ice cliffs, Hyperion appears to be the broken-up remnant of a much larger moon, and lapetus has a dark coating H of "soot" on one hemisphere and a bizarre ridge running around much of its equator. V

Saturn itself, though outwardly placid, has V revealed storms just as violent as those on 1

Jupiter, while its rings seem to be in a constant state of flux and change - composed of billions of individual ice boulders and pebbles, they are twisted and distorted by the gravity of the nearby moons.

ICY ENCELADUS

A colour-enhanced view of Saturn's inner moon Enceladus shows the coating of fresh "snow" that gives it the brightest surface in the Solar System. The moon's ice plumes erupt along the blue "tiger stripe" features.

CASSINI'S TOUR OF SATURN

All the giant planets have rings around them, but Saturn's are by far the most spectacular - vast, bright planes of orbiting ice chunks, arranged in countless ringlets. Cassini has kept well clear of them after an initial manoeuvre took it through the outer limits of the ringplane in order to get into orbit, but its spectacular images (right) have revealed fine structures and the presence of short-lived twists and knots of material where Saturn's family of moons and moon lets exert their gravitational influence. The moons themselves have also proved impressive - aside from Titan, they are mostly icy, airless worlds. Dione (top) is typical, though its surface is marked by ice cliffs and, like many Saturnian moons, it shows signs that it was not always so deeply frozen. Neve the I ess, it makes a beautiful picture against the shadow-striped bulk of Saturn. Enceladus (above) is far from typical, a world with unexpected ice fountains and a cracked surface that suggests water just below.

VENUS EXPRESS

Although it shared a basic design with Rosetta and Mors Express, Venus Express was re-engineered to cope with the hotter conditions close to the Sun.

VENUS EXPRESS

Although it shared a basic design with Rosetta and Mors Express, Venus Express was re-engineered to cope with the hotter conditions close to the Sun.

2 March 2004

ESA's Rosetta comet mission is launched.

3 August 2004

NASA launches MESSENGER, the first Mercury orbiter.

12 August 2005

Mars Reconnaissance Orbiter is launched from Cape Canaveral.

9 November 2005

Venus Express launches on a Soyuz-Fregat rocket from Baikonur.

10 March 2006

Mars Reconnaissance Orbiter arrives in an initial orbit around Mars and begins aerobraking.

11 April 2006

Venus Express arrives in its initial orbit.

7 May 2006

Venus Express reaches its final working orbit and begins science operations.

3 October 2006

Mars Reconnaissance Orbiter photographs the Mars Exploration Rover Opportunity at the edge of Victoria Crater.

6 November 2006

Mars Reconnaissance Orbiter begins its primary mission.

The next generation

Spaceprobes in the new millennium have continued to innovate and push back the boundaries of our knowledge, whether by transforming familiar worlds or by testing new technologies for the future.

Some ambitious spaceprobes have already left Earth but are following flightpaths that mean they will take many years to reach their destinations. These include the European Rosetta comet probe (see p.273) and NASA's MESSENGER to Mercury and New Horizons mission to the outer Solar System (see p.306). But some other probes are delivering faster results. ESA's Venus Express, launched in November 2005, began its mission around Venus in May 2006. It is the first probe to visit the planet since Magellan in the early 1990s (see p.266) and is based on the same spacecraft design originally devised for the Rosetta mission and successfully adapted for Mars Express. Many of its instruments are also modified spares made as backups for the Mars mission or Rosetta.

Venus Express does not carry an SAR radar like that on Magellan, but it has a wide array of instruments to learn about other aspects of Earth's inhospitable twin. These include a camera for photographing Venusian weather conditions, spectrometers for analyzing the atmospheric chemistry, and a magnetometer for studying the weak magnetic field. Perhaps the most important instrument is VIRUS, the Visible and Infrared Thermal Imaging Spectrometer - a thermal imager capable of identifying distinct layers in the atmosphere and compiling temperature maps of the surface that could reveal sites of active volcanism.

TECHNOLOGY

THE ION DRIVE

MARS

RECONNAISSANCE ORBITER

NASA's latest Mars orbiter is fitted with cameras to photograph the planet in detail, spectrometers to study the chemical makeup of the rocks, and a radar to look beneath the Martian surface.

DEEP SPACE 1

An artist's visualization of Deep Space 1 in flight shows the distinct blue exhaust glow that is a hallmork of ion engines.

DEEP SPACE 1

An artist's visualization of Deep Space 1 in flight shows the distinct blue exhaust glow that is a hallmork of ion engines.

Solar electric propulsion (often known as the ion drive) is a highly efficient alternative to chemical rockets - though unfortunately it is not capable of generating the huge thrusts needed to launch spacecraft out of a strong gravitational field. Electricity from solar panels or another power source is used to create a high voltage across an ionization chamber. When atoms of an inert gas such as xenon are fed into the chamber, they are ionized, breaking apart into electrically - ProPellant supply charged particles called ionization ions. The xenon ions are ^ , chamber repelled by a charged plate in the ionization chamber and pushed out of the back of the engine, generating thrust. Ion engines can propel spacecraft to very high speeds, but run for months not minutes.

A spy above Mars

Meanwhile, 2006 also saw the arrival of NASA's latest Mars probe, the Mars Reconnaissance Orbiter (MRO). As successor to Mars Global Surveyor (MGS), MRO arrived with immaculate timing, entering its final orbit two months before contact with the elderly MGS was lost. The orbiter carries a camera called HiRISE that, at maximum resolution, can take recognizable photographs of objects as small as 1m (3ft) across. MRO's main role will be to take detailed images of craters, canyon walls, and sediment layers, looking for more evidence of water and trying to estimate just when it disappeared from the surface. However, it will also act as an advanced communications relay for current and future surface missions.

Testbed for the future

One of the most influential spaceprobes of all may prove to be a small vehicle launched in 1998. Deep Space 1's primary mission, as part of NASA's New Millennium Program, was to test a variety of advanced technologies, including an ion engine, in space (see panel, left). This efficient engine can generate small amounts of thrust for many months - in this case 0.01 kgf (0.3ozf) for 678 days - gradually accelerating a spacecraft to very high speeds. Other tests on Deep Space 1 included a system for concentrating sunlight onto solar cells, artificial intelligence systems for onboard computers, and small, low-mass scientific instruments.

The probe was only supposed to operate for 11 months, but when it showed no sign of deterioration its mission was extended for a further two years, with the ion drive used for changing the spacecraft's orbit to encounter a nearby asteroid. The spectacular finale was a flyby of Comet Borrelly in September 2001.

DECEMBER 2019: MOON BOUND

In this artist's impression, NASA's Orion Crew Exploration Vehicle (CEV) waits in orbit as the first astronauts in four decades set foot on the Moon below. If oil goes well, the first lunar landing of the new century will come in the 50th anniversary year of Apollo 11.

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