At the end of the week, the Huygens probe will reach Titan, a moon of Saturn. It will be one of the greatest achievements of the human race and the farthest a body sent from Earth has reached and landed. Yigal Fat-El reviews the star, its moons, the rings and all the special events in Israel planned for the landing *
Yigal Fat-El, Israeli Astronomical Society
On Friday, the Huygens probe will reach Saturn's moon Titan and land on its surface. It will be one of the greatest achievements of the human race and the farthest body that has reached and landed (if there are no malfunctions) a body sent from Earth. The planet Saturn is one of the most beautiful sights that can be seen by observing the sky through the lens of the telescope.
His rings are compared to the planet Hod Vader and thanks to them he is known to each of us. Huygens set out for Titan just over two weeks ago, when it detached from the mother spacecraft, Cassini, which is now orbiting Saturn and its moons. Cassini itself set out on the long journey about 7 years ago, to the planet that is about one and a half billion kilometers from the sun, a distance that is 10 times greater than the distance of the earth from the sun.
The observation of Saturn
The planet Saturn is now seen at its best - it is in a group of twins, a little south of the two stars Castor and Pollux, which give the group its name. These days Saturn and the two twins are seen at the zenith (overhead) near midnight. In the earlier evening hours, the three stars are visible to the east of the zenith, high above the western horizon with the Orion group to their right (south) seen in an elongated rectangle centered on the three stars of the hunting belt, also known in Israel as the North Arrow.
Saturn is the southernmost of the three stars - Pollux is in the center of the trio and Castor is the northern one. Castor itself is an interesting star to observe in a high-quality telescope: it appears to the eye as a single star, but through the telescope lens it will appear as two stars, similar in brightness, so close that only a high-quality telescope with high magnification can separate them.
Actually, Castor is a 6 star system. Each of the pairs described below is a system of two stars, so close that they cannot be separated with a telescope. Another pair, more distant, surrounds the main pair.
The most beautiful planet
Saturn is the most beautiful planet in the solar system and one of the most popular celestial bodies. Its rings, which are clearly visible even in the smallest amateur telescope, were already discovered by Huygens in the middle of the 17th century. It is possible that Galileo Galilei also observed the rings, but he did not understand their meaning and thought that they were two moons of Saturn. The same Huygens received the great honor that the probe that will land on Titan bears his name.
Saturn, like Jupiter, is a giant planet, most of which is composed entirely of hydrogen and helium. At the center of Saturn, there is probably a core of iron and rock, whose mass is estimated to be at least 10 Earth masses. Around the core is a region of metallic hydrogen. The region of metallic hydrogen, formed due to the great pressure, extends up to half the radius of Saturn.
Beyond it begins the atmosphere of the planet, whose upper edge is characterized by its orange color, on its horizontal bands of clouds, which, unlike the cloud systems on the surface of Jupiter, are difficult to distinguish. The diameter of Saturn is 9.5 times larger than the diameter of the Earth - 120,500 km.
A crowded planet
Despite the large difference in volume between Saturn and Earth, Saturn's mass is 'only' 95 times greater than Earth's. The reason - the low average density of Saturn is equal to 0.69 grams per cubic meter, less than the density of water! This means that if we found an ocean big enough, Saturn would float on its surface.
Saturn's distance from the Sun is 9.54 astronomical units and it completes one orbit every 29.46 years, while moving at a speed of 9.64 km per second. Therefore, the movement of Saturn relative to the constellations is quite slow - Saturn stays on average in each sign for about two years. The period of time between two oppositions of Saturn is 378 days.
rings of saturn
The most striking phenomenon in Saturn is, of course, the rings. The ring system has been observed since the invention of the telescope, but until the passage of the two Voyager spacecraft, Voyager 1 and Voyager 2, near Saturn in 1980 and 1981, it was thought that the rings consisted of 5 large rings, one inside the other. Saturn's ring system is tilted at an angle of 26.73, relative to the plane of Saturn's orbit around the Sun.
As a result, approximately once every 15 years (twice in every cycle of Saturn around the Sun), when the Earth passes through the plane of the rings, the rings turn their narrow side towards the Earth (the last time was in February 1996 and the next time will be on September 4 of 2009). Then, the rings disappear completely, a fact that indicates that Ovain is very small.
Indeed, from measurements made by the Voyager spacecraft, it was found that the thickness of the rings is 1 km at most, and they contain small blocks of ice, the size of which ranges from small stones to a few hundred meters at most. Another interesting fact discovered by the Voyager spacecraft is that the rings consist of thousands of rings and not just 5. The thousands of spaces between the rings are caused by the disturbances of Saturn's moons, chief among them the Moons of Mims.
A look at the moons
The second largest moon in the solar system (after Ganymede) and the only one, excluding Triton, a moon of Neptune, that has its own atmosphere. Titan was discovered by Huygens in 1655 and is the first moon of Saturn to be observed through a telescope.
Its diameter is 5,150 km (without the atmosphere) and it is distant from Saturn an average distance of one million and 221 thousand km. Titan's great distance from Saturn and its high brightness make it possible to observe it even with small field binoculars. Titan completes one revolution around Saturn once every 15.96 days.
Titan's composition is similar to that of Jupiter's two largest moons, Callisto and Ganymede. Its average density, 1.88 grams per cubic meter, indicates that most of Titan is ice and silicate rocks. Titan's atmosphere is also impressive on a global scale: the atmospheric pressure on Titan's surface is equal to 1600 millibars - 1.6 times the atmospheric pressure on Earth's surface.
Atmospheric pressure
Such atmospheric pressure is greater relative to the mass of Titan, which is 30 times smaller than the mass of the Earth, and it results from the composition of the atmosphere which contains mainly molecular nitrogen and heavy molecules of carbon and hydrogen atoms. Also, the atmosphere contains small amounts of the noble gas argon. The low temperature of the atmosphere, which reaches only -176C, is also responsible for the high atmospheric pressure.
The height of Titan's atmosphere reaches 400 km; In its upper layers, at an altitude of 230 km above the ground, its color becomes blue: at this altitude and above it there is a layer of haze, which scatters the sun's rays, and its color is blue. The haze was well photographed by the Voyager 1 spacecraft when it was 434 km from Titan.
Back to Lord of the Rings NASA
Below this height the color of the atmosphere turns orange, a color that gives Titan its characteristic hue. Because of the thick atmosphere, we cannot tell what the topography of Titan is. It is possible that at an altitude of 20-30 km there are methane clouds that rain methane on the surface of the ground, which is probably covered in lakes of liquid methane.
The orbit of Titan is surrounded by a tube of hydrogen, which extends to the orbit of the inner moon - Rea. The tube is formed by hydrogen-containing molecules found in Titan's upper atmosphere, which are broken by solar radiation. The light hydrogen escapes from Titan, and continues to orbit Saturn in Titan's orbit. A similar phenomenon exists around the orbit of Io around the planet Jupiter.
* Yigal Fat-El is the administrator of the astronomy forum on Ynet. Most of the material is taken from his book: "A Guide to Knowing the Heavens" published by Cosmos Telescopes.
An unmanned spacecraft will land on another world's moon
by Tamara Traubman
"Huygens" will land on Titan, which is the largest moon of the planet Saturn; Titan may hold clues to explain the beginning of life on Earth
An unmanned spacecraft is expected to land tomorrow (Friday) on Titan, the largest moon of the planet Saturn. This will be the first time a spacecraft has landed on the moon of another world. The spacecraft is called "Huygens" and looks like a flying saucer the size of a car.
Titan, named after the giants in Greek mythology, has attracted the curiosity of scientists for a long time. It is larger than the planets Mercury and Pluto; It is the only moon in the solar system with an atmosphere; And it is possible that we will store clues in it to explain the beginning of life on Earth.
"Huygens" is a joint mission of the European Space Agency (ESA) and the American Space Agency (NASA), and its members are researchers from 20 different countries, including Israel. Prof. Akiva Bar-Nun, a chemist and space researcher from Tel Aviv University, is a member of the scientific team of the mission to Titan, and intends to try to decipher what Titan's atmosphere consists of. The extreme cold on Titan - minus 143 degrees Celsius - does not allow for life, but if it were warmer, its atmosphere might have crowned it as the first world, apart from Earth, that has the possibility of life.
Prof. Akiva Bar-Nun, from the Department of Geophysics and Planetary Sciences at Tel Aviv University, is a member of the scientific team of the NASA and ESA (European Space Agency) Cassini-Huygens spacecraft. This spacecraft is now orbiting the planet Saturn and has remotely inspected its moon - Titan - the only moon with an atmosphere in the solar system.
The research group of Prof. Bar-Non has been studying Titan's atmosphere for 30 years. She predicted the discovery of propane gas, which was indeed found in the atmosphere by NASA's Voyager spacecraft. Now she predicts the chemical composition of the haze that will be tested when Huygens passes through the atmosphere.
A new experimental result, measured by PhD student Ronen Jacobi, is the capture of the noble gases argon, krypton and xenon in the haze formed in Titan's atmosphere. This capture, together with the deposition of the haze from the atmosphere to the ground of Titan, explains the zero amounts of argon, krypton and xenon in the atmosphere of Titan, as found in the passage of the Cassini-Huygens spacecraft near Titan.
Prof. Bar-Nun will be present in Darmstadt, Germany, on January 14, 2005 when Huygens lands on the surface of Titan.
Even if "Huygens" survives for a few minutes - it's "a reason to party"
Seven years ago, the journey of "Huygens" began. It set off aboard the "Cassini" spacecraft, and arrived close to Saturn. On December 25, 2004, "Huygens" separated from "Cassini" and sailed towards the moon Titan. Tomorrow at 12:13 pm (Israel time) "Huygens" will try to penetrate Titan's atmosphere.
Her mission is not simple: she must land on a foreign world, without knowing what type of ground she will encounter (crater, cliff, ice surface, lake or sea), and collect data that will be sent to Earth. "We don't know what the spacecraft will land on," said Jonathan Lunin, a professor of planetary sciences at the University of Arizona and a member of the "Huygens" scientific team, in a statement published by NASA, "almost anything is possible."
Huygens will collect air samples from the atmosphere. Sensors will measure the temperature, the ground, the pressure, the winds and the electromagnetic fields - in the atmosphere and at the landing site. A microphone will listen for thunder and other sounds. And so, for the first time, we can know what another world sounds like. The information collected by "Huygens" will be transferred to the "Cassini" spacecraft, and will be transmitted, via the same radio, to scientists on Earth.
Mission leaders hope that "Huygens" will be able to survive at least 30 minutes from the moment of landing, before it is extinguished by the freezing cold and the unknown dangers of Titan. "Even a few minutes of data collection will be cause for a party," the mission leaders said.
