When the seventeenth-century astronomer Giovanni Domenico Cassini studied the planet Saturn, he must not have imagined that a few hundred years later a research spacecraft named after him would make its long way to the beautiful planet. Everything you wanted to know about the mission
Yoram Ored, Galileo
Saturn is the second largest of the planets in the solar system (the first is known to be Jupiter) - its diameter in the equatorial region is 120,536 km (the diameter of the Earth, for comparison, is 12,800 km) - and it is the sixth in its distance from the Sun (1,429,400,000 km) m, i.e. 9.54 astronomical units); Its mass is about 95 times greater than that of the Earth.
It is the flattest planet in the solar system. Its compactness is so great that it can be felt even when viewed through a small telescope from Earth. Its polar diameter (the diameter of the planet, when measured from pole to pole) is about ten percent smaller than its equatorial diameter (the diameter measured through its equator) and more precisely 108,728 kilometers compared to 120,536 kilometers respectively.
Saturn was already known from prehistoric times, and Galileo Galilei was the first to observe it through a telescope. So far, the planet has been visited by three spacecraft - Pioneer 11 in 1979, Voyager 1 in 1980, and then Voyager 2 in 1981. For each of these spacecraft, Saturn was only one of several destinations. Cassini is the first spacecraft whose sole purpose is Saturn and its moons.
Saturn is a fascinating planet. It is the only planet in the solar system whose density is less than that of water, and therefore if we could place it on the surface of an imaginary ocean it would float on its surface. Saturn is also surrounded by a huge magnetosphere (the magnetosphere is the area around a planet where the strength of the magnetic field exceeds that of the magnetic field between the planets). On the surface of the planet, extremely strong winds blow, whose speed in the region of its equator reaches around 2000 kilometers per hour. One of the most interesting regions in its vicinity is the moon Titan, the only moon in its entire solar system with an atmosphere worthy of its name. Of the 31 moons of Saturn, this is the largest moon.
But Saturn's main attraction is its rings. They are made of billions of pieces of matter, most of which are pieces of ice that range in size from the size of a grain of sugar to that of a small house. The number of rings reaches thousands and according to the hypothesis their origin is in asteroids, moons and comets that broke into pieces before reaching the planet's environment.
Relative to today's unmanned spacecraft, Cassini is a large spacecraft. The builders of the spaceships of our time tend to make them smaller and smaller, which saves a lot of energy (required for their launch and ongoing operation) and therefore also a lot of money. In contrast, Cassini is huge in size. It weighs 5650 kilograms and is the size of a bus. The spacecraft consists of two main components.
One component is the spacecraft itself - Cassini, named after the seventeenth century Italian astronomer Giovanni Domenico Cassini. It will wander for four years around Saturn and explore it and its moons. The second component is the Huygens probe. It is named after the Dutch astronomer Christiaan Huygens, also from the seventeenth century. Huygens will dive into the atmosphere of the moon Titan, land on its surface and study it.
Controversial launch
Cassini's launch was accompanied by strong public opposition, which reached the point of demonstrations in front of the White House and in front of its launch base. The reason was the nuclear fuel, plutonium, that she carries on board. The plutonium is responsible for operating its systems. Like other nuclear fuels, it emits harmful radioactive radiation; The opposition stemmed from the fear that a malfunction would occur in the launch, for example an explosion during takeoff, a disaster that would have left a lot of fallout in the fall zone. Along with this, the use of nuclear fuel for the operation of the spacecraft is essential due to the distance of Saturn from the Sun: as we know, there is an inverse square relationship between the distance of an object from a light source such as a lamp or the sun and the intensity of the light hitting the object.
In the case of Saturn, which is ten times farther from the Sun than Earth, the intensity of illumination on its surface is about 100 times smaller than on ours. This makes it impractical to use solar cells to generate electrical energy in the vicinity of the planet. If they decided to supply a spaceship with energy through solar cells, the area of these would have to be the area of two tennis courts.
The launch, in any case, ended successfully and without a problem. On the fifteenth of October 1997, Cassini lifted off from the space base in Florida on the back of the Titan 4b Centaur launch vehicle on its long journey to Saturn.
The winding road is many
Cassini's large mass requires enormous energy to achieve the necessary speed to bring it to its destination - Saturn. No rocket propulsion could provide this energy in a direct launch towards the planet. Therefore, to provide the necessary energy, its designers used a technology known as gravity assist. This technology uses the gravitational force of planets to accelerate the speed of the spacecraft, and has already been used in Voyager 1 and Voyager 2. At the same time, the price of using this technology is a long and winding path that the spacecraft must take through the vastness of the solar system.
For Cassini, the gravity of three planets was used to accelerate its speed - Venus, Earth and Jupiter. The use of their gravity allowed the spacecraft to reach the necessary speed.
The spacecraft was launched in the opposite direction to its destination. Instead of sending it in the opposite direction to that of the Sun, the direction in which Saturn's orbit is located, it was sent in the direction of the Sun, towards the planet Venus, which is closer to the Sun than the Earth. The spacecraft approached Venus and passed close to this planet. Due to the speed of the spacecraft, it did not enter the orbit around Venus, but was only diverted from its orbit by the gravity of this planet and its speed increased.
Venus' gravity was used twice: on April 26, 1998 and on January 24, 1999. Then, on August 18, 1999, Cassini passed by Earth and used its gravity to accelerate its speed. The next (and last) to accelerate the spacecraft was the giant planet Jupiter, the largest and most massive of the planets of the solar system. This happened on December 30, 2000.
At the end of its journey, when it approaches Saturn in July 2004, Cassini's main engine will be fired for ninety-seven minutes to slow its motion. Doing so will cause it to enter orbit around Saturn, where it will be trapped forever, as it continually orbits the planet.
Visit Peva on the way
Cassini's exciting exploration journey will actually begin on July 1st. On that day, the central engine of the spacecraft will be activated to slow it down and enter orbit around Saturn, but this event will be preceded by another fascinating event - the passage by the most distant moon of the planet - Phoebe, which will occur a few weeks before entering orbit around Saturn. Phoebe is one of Saturn's strange moons. One of its peculiarities is the direction of its rotation around the planet, a direction opposite to that of most of the planet's moons.
Another oddity is that it is one of the only two large moons of Saturn that do not orbit it in its equatorial plane; More precisely, its orbital plane is inclined at an angle of 30 degrees relative to Saturn's equator. Another peculiarity of it is its very low light reflection rate (albedo), which makes it a very dark body, unlike most of Saturn's moons.
Peva is a small moon discovered in 1898 by an American astronomer named William Pickering. Its diameter is about two hundred and twenty kilometers (about one fifteenth of the diameter of our moon) and its average distance from Saturn is about thirteen million kilometers. It rotates around its axis once every nine hours. The time it takes to orbit Saturn is eighteen months.
Peva has already been visited before. It was during the legendary voyage of the Voyager 2 spacecraft in the 2s. Voyager 2.2 passed within 2000 million kilometers of it and photographed it from that distance. Cassini, on the other hand, will photograph it from a distance of only 2 kilometers! Cassini will also use much more sophisticated cameras than those of Voyager XNUMX.
Cassini intends to study the internal structure of Phoebe, its composition, its geology, its surface structure, and to try to explain the reason for its strange orbit around Saturn. The transition near Peva will take place on June 11, 2004.
The trip around Saturn
On July 2004, XNUMX, as Cassini passes close to Saturn, its main engine will be activated. The engine will run against the direction of Cassini's movement so that its speed will gradually decrease. The purpose of this move is to cause the spacecraft to be trapped in the planet's gravitational field and enter a perpetual orbit around it. On that day, Cassini will begin an exploration tour around Saturn, which will last four years. During this tour, she will explore the planet itself, its rings, the magnetic field that surrounds it and its moons. A special place in this research tour is reserved for its largest and strangest moon - Titan.
During its exploration mission around the planet, Cassini will complete 74 orbits around it and pass very many times near its moons, of which 44 times near Titan. Cassini will move toward Saturn below the plane of its rings (that is, between the plane of the equator and the south pole of Saturn) and will rise above this plane through the gap between the F and G rings. Its central engine will be activated shortly after the spacecraft passes its rings: this action will place Cassini in a highly elliptical orbit around the planet.
During the operation, it will come as close as 18,000 kilometers from the face of Saturn, the smallest distance it will achieve during its entire journey. Its journey above the rings will last one hour and forty-four minutes before it returns below its rings again. The ability of the spacecraft's scientific instruments to function while entering orbit, when its central engine is running, is limited. Ironically, precisely this time, due to the incredible proximity of the spacecraft to the planet, is ideal for receiving data about it.
Descend into the mysteries of Titan
One of the most significant and exciting events during Cassini's research tour of the Saturn system will surely be the landing of the Huygens probe in Cassini's belly to the face of Saturn's largest moon - Titan. During its landing in the moon's atmosphere and after its landing on its surface, the probe will explore the large moon, which is the second largest in the solar system (the first is Ganymede, one of Jupiter's moons).
Titan orbits Saturn at a distance of 1,221,830 kilometers and its diameter is 5150 kilometers (larger than our Moon). It was discovered by the astronomer Huygens in 1655. It is surrounded by a thick atmosphere that reaches a height of 600 kilometers above its surface. The atmosphere is sealed and does not allow viewing the surface of the moon. It consists mainly of nitrogen, but it also has small percentages of argon and small percentages of methane. The atmospheric pressure on the surface of Titan is about 1.5 bar, which is to say - 1.5 times greater than that on the surface of the Earth.
The Huygens probe will detach from Maxini on the twenty-fifth of December 2004, with its equipment including six scientific instruments aimed at studying the composition of Titan's atmosphere, its dynamics and taking pictures of Titan. After detaching from Cassini, it will make its way slowly and carefully towards the moon. To stabilize itself, the probe will rotate around an axis at a speed of 7 revolutions per second. On the fourteenth of January 2005, it will break through the thick atmosphere covering Titan and land on its surface.
When Huygens encounters Titan's atmosphere, its speed will decrease at a high rate: in less than two minutes, it will slow down from a speed of about six kilometers per second to a speed of about four hundred meters per second, which is about one and a half Mach. This will occur between the heights of 350 and 220 kilometers above the surface of the moon. Only then will the probe begin to deploy the parachutes it is equipped with, whose role is to further slow down its speed.
According to the plan, Huygens will take over a thousand pictures during its descent to the soil of Titan and at the same time its other instruments will sample the composition of its atmosphere.
The data that will be recorded by Huygens will be transmitted to the Cassini spacecraft, which will pass over the big moon at that time. These will be picked up by Cassini's antenna which will be pointed at Titan. Shortly after receiving the data, Cassini will point its antenna at Earth and begin transmitting the data to us.
A variety of research objectives
Lexini has many and varied targets, including the study of Saturn itself, the study of its rings and the study of its moons, including Titan, the crown jewel of the planet's moons.
Among other things, Cassini is expected to help solve questions such as the origin and nature of the lightning in Saturn's atmosphere, whether the surface of the moon Titan is solid or liquid, and what is its internal structure.
About six and a half years of travel have already passed for Cassini on its way to Saturn. During this time, Gama has already covered a distance of billions of kilometers. So far her journey has been flawless. All that is left is to hope that the journey, which is nearing its end, will continue to run perfectly and that its sophisticated systems will also function perfectly, especially when entering the complex system of Saturn on its rings and moons and during the entire period that Cassini will explore it.
Good luck Cassini. Goodbye Saturn.
