The Cassini-Huygens Mission (1997-2017)

The Cassini-Huygens Mission (1997-2017)

The Cassini spacecraft was exploring Saturn and its moons for nearly 20 years.

Geography

Keywords

Saturn, space probe, Cassini, rings of Saturn, Solar System, Huygens, space research, planet, gas giant, outer planet, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, moon, astronomy, geography, gravitation

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Solar System

The Sun is one of the billions of stars in the Milky Way, located in the plane of our barred spiral galaxy, in the Orion spiral arm. The Sun, and the whole solar system orbit around 27,000 to 28,000 light years away from the centre of the disc, which measures 50,000 light years in radius. It takes roughly 240 million years for the Sun to complete one orbit. The environment of the solar system is sparse, the nearest stars – Proxima Centauri and the double system of Alpha Centauri – are 4.2–4.4 light years away from us, and there are only 11 stars within 10 light years.

By Solar System we mean the Sun and all the variously sized celestial bodies orbiting it. The solar system is the area where the gravitation of the Sun is dominant. This is a sphere of about 2 light years in radius; on its border the gravity of the Sun is equal to the gravity of the nearest stars. The solar system is completely filled with solar wind, a continuous flow of electrically charged particles emitted by the Sun.

The Solar System consists of the Sun, the planets, the moons of the planets, asteroids and comets, meteoroids and interplanetary matter i.e. dust and gas. Eight planets orbit the Sun; six of these have moons, the exceptions being Mercury and Venus.

In the order of their distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Planets can be divided into two separate groups: four terrestrial or Earth-type planets and four giant planets, also called Jovian planets. Terrestrial planets are closer to the Sun. They are smaller and denser, rotate more slowly, and have thinner atmospheres and weaker magnetic fields.

All of the planets orbit the Sun on nearly the same plane, in the same direction, their motion is direct, which means they move in an anticlockwise direction, as seen from the North Pole of the Earth. Except for Venus and Uranus, their rotation is direct too. The Sun also rotates in this direction.

Planets are kept in orbit by the gravity of the Sun. The mass of the Sun is 750 times larger than the total mass of the planets. There is also gravitational force between the planets; therefore, they influence each other’s motion. As a result, their orbits might undergo slow, minor changes.

Besides planets, there are billions of small objects in the solar system. Asteroids can be found just about everywhere. Many of them have orbits that cross that of the Earth. Most of the asteroids are located in two zones. The inner asteroid belt is between Mars and Jupiter (where there are at least 1 billion asteroids larger than 1 km in diameter); the outer belt, i.e. the Kuiper belt – is located beyond the orbit of Neptune (where several thousand Pluto-like, icy asteroids have been discovered).

Since 2006, Pluto has no longer been considered a planet. Pluto and a few other large asteroids are called dwarf planets. The orbits of the majority of the comets are entirely different from that of other objects: they have elongated elliptical orbits with different orbital planes. As the 5–20 km large icy core evaporates near the Sun, a rare, spectacular tail is formed. Due to solar wind, this tail points away from the Sun. Billions of comets orbit in the Oort cloud, the outer region of the Solar system, 0.5–2 light years from the Sun.

Since 1995, numerous exoplanets have been discovered around hundreds of stars. In many of these systems, giant planets orbit the stars, therefore we can safely assume they are not similar to our solar system.

Saturn's orbit

Saturn is the second largest planet in the Solar System, a spectacular outer planet. It is a gas giant (or Jovian planet). Saturn is the most oblate planet, because of its high speed of rotation and low density. It is the least dense planet in the Solar System, the only one with a density less than that of water (0.69 g/cm³).

Data:

Diameter: 120,536 km (9.45 Earths)

Mass: 5.6846×10²⁶ kg (95.2 Earths)

Average density: 0.69 g/cm³

Surface gravity: 1.065 g

Surface temperature: -180 °C

Number of moons: 62

Rotation period: 10 h 48 m

Axial tilt: 26.7°

Average distance from the Sun:
1,433,530,000 km = 9.58 AU = 79.7 light minutes

Orbital eccentricity: 0.054

Orbital period: 29.46 years

Saturn

Saturn is the sixth planet from the Sun, the second largest planet in the Solar System. It was named after one of the oldest of the Roman gods. Saturn was the god of sowing and seeds, the symbol of relentless time. He was identified with the titan Kronos in Greek mythology.

Saturn is the farthest planet that is visible to the naked eye.
The oval shape was first observed by Galileo Galilei with his primitive telescope, but he could not see the ring which caused it.


It was Christiaan Huygens, who first suggested that Saturn was surrounded by a ring. In 1675, Giovanni Domenico Cassini determined that this ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini division.

Saturn was first visited by Pioneer 11 in September 1979. In November 1980, the Voyager 1 spacecraft arrived at the Saturn system. It sent back the first high-resolution images of the planet, its rings and its moons. For the first time, we were able to study clear pictures of the surface features.
Almost a year later, in August 1981, Voyager 2 continued to study this system. On 1 July 2004, the Cassini space probe entered into orbit around Saturn, and provided a great deal of new information about the planet and its moons.
In early 2005, the Huygens probe was detached from Cassini and descended through the nitrogen atmosphere onto the surface of the moon Titan, where it found methane and ethane lakes.

Saturn is the second largest planet in the Solar System, a spectacular outer planet. It is a gas giant, that is, a Jovian, or Jupiter-like planet. It is also the most oblate or flattened planet, because of its high speed of rotation and low density. It is the least dense planet in the Solar System, the only one with a density less than that of water (0.69 g/cm³).

The internal structure of Saturn is similar to that of Jupiter, with a rocky core in the centre, a layer of liquid metallic hydrogen surrounding it and a layer of molecular hydrogen outside.
It has an atmosphere of mostly hydrogen, organised in fast flowing and whirling belts.
The winds on Saturn are among the fastest in the Solar System. According to data provided by Voyager, they can move as fast as 400 m/s. Saturn's atmosphere has a banded structure similar to Jupiter's, but Saturn's bands are much fainter and much wider near the equator.
The average temperature is -180°C. The internal temperature of Saturn at the core is 12,000 K. The planet radiates more energy into space than it receives from the Sun; the reason for this is not yet known.
Saturn's magnetic field is strong. The magnetic axis is aligned with the axis of rotation. Some photos from the Hubble space telescope show aurorae.

Saturn's moons

Saturn is mainly known for its ring system, one of the most spectacular objects in the Solar System. The rings can be observed with a small telescope. The rings are composed of rock and ice particles, ranging in size from speckles of dust to car-sized rocks.
The rings are so spectacular to the eye because of the high albedo of the ice in them. There are dozens of moons orbiting in the gaps between the hundreds of rings; it is the gravity of these moons that holds the rings together and thus they are called shepherd moons.

Saturn has 62 known moons. Only seven of these are large enough to be spherical (a spherical shape is only formed above a certain size and mass limit, due to gravity and internal heat).

The only large moon is Titan, discovered in 1655. Its orbital period is 16 days. The moons are made up of a great deal of water ice.

Cryovulcanism, water vapour erupting from below the surface, was observed on Enceladus, close to Saturn. Most of Saturn's moons only have a diameter of 4–8 km.

Cassini

The Cassini-Huygens spacecraft was designed and built as part of a major joint project between NASA (America's space agency), ESA (the European Space Agency) and ASI (the Italian Space Agency). A total of 27 countries were involved in this space programme.

The Cassini orbiter was named after the Italian-born French astronomer, Giovanni Domenico Cassini, who is best known for discovering four moons of Saturn. The other crucial part of the spacecraft, the Huygens probe, was named after the Dutch astronomer, Christiaan Huygens, who discovered Titan.

The Cassini-Huygens was one of the largest and most complex spacecraft ever to have been launched successfully in the history of space exploration. It measured 6.7 metres in height and 4 metres in width and weighed about 5,700 kilograms at launch.

The orbiter and the lander were designed to complete a total of 27 different scientific investigations, so they were equipped with a variety of special instruments.

Cassini and Huygens carried twelve and six scientific instruments, respectively. Most of these pieces of equipment were multifunctional. Several research teams assisted in the development of these unique scientific instruments.

Three aerials made the communication of the spacecraft possible. Cassini's 1,630 electronic components were linked with about 22,000 wire connections and about 14 kilometres of cabling.

The Cassini orbiter and its instruments were powered by three Radioisotope Thermoelectric Generators (RTGs) that transformed heat from the decay of radioactive isotopes into electric power. These used about 32 kilograms of plutonium. Since Saturn is too far from the Sun, it was not possible to use solar energy to power the spacecraft.

Cassini's thermal blankets made it look all the more striking. This finely sewn, durable, lightweight fabric protected the spacecraft from extreme heat and cold, and any damage caused by micrometeoroid impacts. The instruments could also be kept at optimum temperature. The temperature of the parts not covered with the thermal blankets varied between -220 and +250 degrees Celsius in space.

Path of Cassini

Cassini-Huygens was launched using a Titan IVB/Centaur rocket on 15 October 1997 from Cape Canaveral Air Force Station in the United States. Cassini executed four gravity assist manoeuvres to reach Saturn.

During a gravity assist manoeuvre, the gravity of a large celestial object is used to change the flight path and speed of the spacecraft. Thus, spacecraft can reach celestial objects far away in the Solar System in a shorter time using less energy. Cassini executed two Venus flybys, one Earth flyby, and one Jupiter flyby.

Interestingly, two years after its launch, the spacecraft was as close to Earth as it was shortly after it was launched. It was only 1,100 kilometres from the planet during the Earth-Moon flyby. This gravity assist manoeuvre gave the spacecraft a boost in speed of about 5.5 km/s.

The Cassini-Huygens spacecraft reached Saturn seven years after its launch in June 2004 and entered orbit a month later.

The Huygens lander on Titan

The Huygens probe, the other vital part of the mission, was a lander built by ESA. This disc-shaped spacecraft had a diameter of 2.7 metres and a weight of 318 kilograms. It was covered with a shield to protect the instruments inside it while it travelled through Titan's atmosphere.

On 25 December 2004, Huygens separated from the Cassini orbiter to which it was attached and reached Titan after three weeks. On 14 January 2005 it took 2 hours and 27 minutes for the Huygens probe to land on Saturn's largest moon.

The landing speed was reduced by the shield and three parachutes. The two most important tasks of the six scientific instruments of the probe were to study Titan's atmosphere and surface.

The collected data was sent to the Cassini orbiter before it was forwarded to Earth. Sadly, Huygens could send only 350 images, because one of Cassini's receivers did not work due to software failure. The information the probe sent, however, is invaluable.

Huygens worked for 72 minutes on the surface of the giant moon. It was the first and, so far, the only landing in the outer Solar System. Consequently, the Huygens probe also holds the record of landing the furthest from Earth.

Titan

Achievements

The Cassini-Huygens mission set many important scientific goals. Most of these goals focused on exploring Saturn along with its rings and moons. The large-scale mission was unique from every aspect in the history of space exploration.

The original mission was completed in 2008, eleven years after the launch of the spacecraft. However, due to the possibility of gathering additional invaluable data, it was extended for two more years (Cassini Equinox Mission) and then for another seven years in 2010 (Cassini Solstice Mission). In the Grand Finale, which began in April 2017, the spacecraft was directed to Saturn's atmosphere, where it was eventually destroyed on 15 September 2017.

The spacecraft of the approximately $3.26 billion mission spent nearly 20 years in space. During this time, it covered a distance of 7.9 billion kilometres, captured 453,048 images, and collected 635 gigabytes of scientific data.

It is almost impossible to list all the scientific results of the Cassini-Huygens mission, but here are some of the most important ones:

- discovering new rings surrounding Saturn;

- studying the structure of the rings and the processes in them;

- discovering new moons around Saturn;

- studying the new moons;

- observing geysers on the surface of the moon Enceladus;

- studying the surface and weather on Titan that are similar to those on Earth

- studying Titan's atmosphere;

- observing the evolution of a Saturnian storm;

- mapping Saturn's hexagon, a weather pattern at the northern pole of the planet;

- studying the icy moons;

- solving the mystery of the two-toned colouration of the moon Iapetus;

- discovering methane and ethane lakes on Titan's surface;

- mapping Saturn's magnetosphere.

During the 20 years the mission lasted, nearly 4,000 science papers were published. There is no doubt that after the analysis of all the collected data, more publications will be available regarding further questions and conclusions.

The highly successful Cassini-Huygens mission provided new perspectives for humanity on the Solar System and planet Earth as well. It also gave new importance to the theory of extraterrestrial life.

Animation

Narration

The Cassini-Huygens mission set many important scientific goals. Most of these goals focused on exploring Saturn along with its rings and moons.

Saturn, a spectacular outer planet, is the second-largest planet in the Solar System. It is mainly known for its spectacular ring system. Saturn has 62 known moons, the largest being Titan.

The Cassini-Huygens spacecraft was designed as part of a major joint project. The Cassini orbiter was named after the Italian-born French astronomer, Giovanni Domenico Cassini. The other crucial part of the spacecraft, the Huygens probe, was named after the Dutch astronomer, Christiaan Huygens, who discovered Titan.

The orbiter and the lander were designed to complete a total of 27 different scientific investigations, so they were equipped with a variety of special instruments. Cassini and Huygens carried twelve and six scientific instruments, respectively. Most of these pieces of equipment were multifunctional.

The spacecraft was launched on 15 October 1997 from Cape Canaveral Air Force Station in the United States. It took four flybys and seven years for the spacecraft to reach Saturn in June 2004, entering orbit a month later. On 25 December 2004, Huygens separated from the Cassini orbiter to which it was attached and reached Titan after three weeks. On 14 January 2005, it took 2 hours and 27 minutes for the Huygens probe to land on Saturn's largest moon.

The original mission was completed in 2008, eleven years after the launch of the spacecraft. However, due to the possibility of gathering additional invaluable data, it was extended for two more years (Cassini Equinox Mission) and then for another seven years in 2010 (Cassini Solstice Mission).

The spacecraft of the approximately $3.26 billion mission spent nearly 20 years in space. During this time, it covered a distance of 7.9 billion kilometres, captured 453,048 images, and collected 635 gigabytes of scientific data. The highly successful Cassini-Huygens mission provided new perspectives for humanity on the Solar System and planet Earth as well.

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