Cassini-Huygens mission

The mission Cassini-Huygens is an automatic space mission carried out in collaboration by the Jet Propulsion Laboratory (JPL), the European space agency (ESA) and the Italian Space agency (ASI). Its objective is the study of the planet Saturn and several of its satellites, of which Titan. The Space probe Cassini-Huygens, made up of the orbitor Cassini and the module '' Huygens '' was placed in orbit around planet. Huygens landed on Titan.

The name of the mission is a homage to Jean-Dominique Cassini, astronomer French of Italian origin of at the origin of fundamental observations relating to Saturn, and to Christiaan Huygens, Dutch astronomer of the same century, which discovered Titan.

Presentation

The mission Cassini-Huygens is a joint mission of NASA, European space agency and Italian Space agency of which the principal goal is to explore Saturn and its satellite S, in particular Titan. The idea of this mission goes back to 1982. The total duration of the mission is estimated at 11 years, of launching the October 15th 1997 until in 2008.

The Cassini-Huygens probe is made up of the orbitor Cassini , equipped on the whole with 12 instruments, and the Atterrisseur Huygens , equipped with 6 instruments. Beginning 2004, the probe entered in orbit around Saturn and the January 14th 2005, Huygens was posed on Titan.

The Cassini-Huygens mission in particular already made it possible to have the first detailed images of Phœbé, to study in details the structure of the Saturn's rings, to study Titan thoroughly and to discover two news the moon S of Saturne.

Objectives

The objectives principaux mission are:

to determine the structure in 3D and the dynamics of the behavior of the Saturn's rings;
to determine the composition of the surface of the satellites and their geological history;
to determine the nature and the origin of the matter sinks present on Japet;
to measure the structure in 3D and the behavior of the Magnetosphere of Saturn;
to study the dynamics of the behavior of the atmosphere of Saturn on the level of its clouds;
to study the weather behavior of Titan;
to study the surface of Titan.

Origin of the project

The origin of the projet go back to 1982, when scientists of the European Fondation of science and National Academy off Sciences (the United States) had the idea to send towards Saturn a mission comprising at the same time a orbitor and an undercarriage. In 1983, the project was supported by the NASA which made the same recommendations. Of 1984 with 1985, NASA and the ESA undertook joint studies on the project. The ESA continued only the studies on the project in 1986, whereas in 1987, the astronaut Sally Ride defended the idea in a report/ratio.

In 1988, Len Fisk, an administrator of NASA adopted the idea of a common mission between NASA and the ESA. He wrote with his counterpart of the ESA, Roger Bonnet, while strongly recommending to him to choose the mission Cassini among the three choices which were offered to him, by saying that NASA would engage in the project as soon as the ESA would do it.

At that time, NASA became sensitive to the feeling growing within the ESA which NASA did not regard Europeans as the equal ones. The official ones of NASA and their advisers decided to be implied in the mission Cassini-Huygens to correct this feeling, ready to share the scientific and technological benefit of the mission. This initiative was partly influenced by the increasingly close co-operation that Europeans maintained with the Soviet Union space matter, closer than that which they maintained with NASA.

These considerations not only made it possible to improve collaboration between the ESA and NASA, but also to defend the project near the American Congress and, starting from 1994, it was on the rails.

The probe

Weighing 5.650 kg (weight with launching), it is made up of two modules:

Cassini itself, intended for the exploration of the Saturn planetary system;
Huygens , a module intended to penetrate in the atmosphere of Titan, the largest Saturn satellite, and to be posed there.

Modulate Cassini

Modulate Huygens

Chronology

See also: Chronology of the mission Cassini-Huygens

Launching and voyage

The probe was lancée the October 15th 1997 with 8:43 UTC of Cap Canaveral, the United States, on a rocket Titan-IVB/Centaur of the air force of the United States.

The voyage towards Saturn was accomplished by using with four recoveries the gravitational Assistance :

Venus the April 27th 1998
again Venus the June 24th 1999
the Ground the August 18th 1999
Jupiter the December 30th 2000

The probe arrived in orbit around Saturn, as envisaged, on July 1st 2004. As from this date, the nominal duration of the mission of the Cassini probe is four years, until July 1st 2008. But if all is well, this mission could last up to six years.

The total budget of the mission is 3,26 billion dollars, of which:

1,4 billion dollar for the development of the project;
704 million dollars for the mission in itself;
422 million dollars for launching;
54 million dollars for the follow-up of the probe.

The the United States contributed to it for 2,6 billion dollars, ESA for 500 million and ASI for 160 million.

250 scientists are mobilized for the mission.

The probe is equipped to carry out 27 types of different investigations. Cassini is equipped with 12 instruments and Huygens of 6, the aforementioned instruments having often multiple functions.

Way

The trajectory of the sonde Cassini-Huygens uses the gravitational assistance while passing in the orbit of Venus, the Ground and Jupiter, in order to obtain a sufficient speed to reach Saturn. She traversed 3,5 billion kilometers to reach Saturne.

Here stages allowing to reconstitute the trajectory of the probe:

Launching: Crenel of the October 6th to the November 15th 1997. Launching is pushed back twice, the first time because of the cooling system and the second because of the system Informatique. Launching takes place finally on October 15th, 1997 with 04:43 local time (08h43 universal time).
Entered an orbit of transfer of Venus. Overflight of Venus the April 26th 1998 at a speed of 11,7 km/s and an altitude of only 300 km.
Correction of trajectory the December 3rd 1998.
Entered an orbit of transfer of Venus. Overflight of Venus on June 24th 1999 at a speed of 13,6 km/s and an altitude of 600 km.
Entered an orbit of transfer of the Earth. Overflight of the Earth the August 18th 1999 at 19,1 km/s and an altitude of 1200 km.
December 1st 1999: The antenna with high profit (HGA) is directed towards the Earth.

January 23rd 2000: approaches the asteroid (2685) Masursky to 1,5 million kilometers
February 2000: Correction of trajectory because of a defect of the radio operator system of Huygens. It was not taken account of the Doppler effect in the design of this system. The new trajectory makes it possible to correct this effect and the line capacities return almost to the normal.
August 18th 2000: Approaches Himalia, the moon of Jupiter, to 4,4 million kilometers.
October 1st 2000: Beginning of the Jupiter observation, to 84,4 million kilometers
December 30th 2000: Jupiter overflight to 9,7 million kilometers, at a speed of 11,6 km/s, which shortens the voyage towards 2 year old Saturn.
March 22nd 2001: End of the Jupiter observation to 84 million kilometers.
February 6th 2004: Beginning of the Saturn observation.
June 11th 2004: Overflight of Phœbé.
July 1st 2004: Capture by the attraction of Saturn and first crossing of the ring of planet.

Landing of Huygens

Times given correspond to times known as “earth received”, i.e. 67 minutes after the real events proceeded (time that it is necessary for a signal to traverse the distance separating the system saturnien from the Earth). Times are given in hours THIS (Paris time) .

the January 14th 2005 with 11:13 THIS: Huygens enters the red atmosphere orange of Titan, with 1.270 kilometers of altitude above its surface.
11:17 THIS: the pilot parachute (2,6 meters diameter) is deployed, whereas the probe, which is not any more that with 180 kilometers of surface, sinks to 400 m/s (either 1.440 km/h).

One of the functions of this parachute is to remove thermal protection postpones probe. In 2,5 seconds, this protection is removed and the pilot parachute is released. The principal parachute (8,3 meters diameter) is then deployed.

11:18 THIS: to approximately 160 km of surface, the front heatshield is released. It is important to eliminate these two shields because they could be a potential source of exo contamination on the surface of Titan.
: The openings of entry of the instruments GCMS and ACP are opened, 42 seconds after the deployment of the pilot parachute. Poles are deployed to expose HASI whereas DISR takes its first panorama. It will continue to take spectral images and data throughout the descent. SSP is also started, measuring properties of the atmosphere. Huygens starts to transmit data in distant direction of Cassini of 60.000 kilometers.

11:32 THIS: the principal parachute is released and another smaller parachute (3 meters diameter) takes over.

Indeed, on this level of the atmosphere (approximately 125 kilometers of altitude), the principal parachute would slow down so much Huygens which the batteries would not last long enough to be able to provide all energy necessary during the descent until the moment of the landing. Also, to collect a maximum of data, it is advisable not to slow down the descent too much and thus to use a smaller parachute at this time.

11:49 THIS: with 60 kilometers of altitude, Huygens determines itself its altitude, by using a pair of altimeters radar. The probe permanently monitors its own rotation and its altitude.
12:57 THIS: the Gas Chromatograph Farmhouse Spectrometer - the last of the instruments to being activated, is completely. The descent lasted 160 minutes on the whole. During this descent, the probe rotates, allowing the camera and the other instruments to view an integral panorama.
13:30 THIS: near surface, Huygens lights a lamp which will help to precisely determine the composition of the surface of Titan.
13:34 THIS: Huygens touches a flexible surface at 5 or 6 m/s (a score of km/h).

The nature of the ground is unknown but according to any probability, it could be ice. The Surface Science Package collects information during the minutes according to whether the probe is not posed.

15:44 THIS: Cassini, with 60.000 kilometers from there, continuous to collect the data which Huygens still sends to him during 10 minutes; although Huygens disappeared behind the horizon from Titan.
16:14 THIS: Cassini transmits its first data to the Earth.

Scientific results

Here a report of the principal discoveries of the Cassini-Huygens mission; :

Checking of the Theory of relativity

The October 10th 2003, the Astrophysicien Italian Bruno Bertotti of the Université of Pavia and its colleagues Luciano Iess of the Université of Rome “Sapienza” and Paolo Tortora of the Université of Bologna had the results of the test of the Theory of relativity of Einstein that the probe Cassini had carried out the previous year. During the summer 2002, the Ground, the Sun and probe it Cassini-Huygens were aligned exactly, the Sun being between the Earth and the probe. At the time of the communications with the probe and thanks to the antenna 4 meters in diameter of this one like at the new station on the ground of NASA Deep Space Network with Goldstone in California, the team of Italian astrophysicists could observe a slip of frequency in the waves radio received by and emitted since Cassini-Huygens , when those travelled near the Sun. According to the general theory of relativity, a massive object such as the Sun is supposed to curve the Espace-temps around him. Thus, a luminous ray or a radio wave which passes near star must traverse a larger distance because of this Courbure. This surplus of distance that the waves emitted by the probe had to traverse to reach the Earth delayed their reception and this delay could be measured and quantified and made it possible to check the theory with a precision 50 times higher than that of the preceding experiments carried out with the probes Viking.

Although deviations compared to general relativity are envisaged by certain cosmological models, no deviation was observed in this experiment and taken measurements agreed with the theory with a precision to the millième.

Jupiter

The probe Cassini-Huygens could observe Jupiter for almost 4 months of October 1st 2000 with the March 22nd 2001. It is approximate at a minimal distance from 9,7 million Kilomètre S the December 30th 2000 and could take several measurements. During this overflight approximately 26.000 stereotypes of the planet were taken among which one finds most precise ever realized (see illustration on the left). On certain photographs, more the visible small details measured approximately 60 km.

A major discovery was announced by NASA the March 6th 2003 and the nature of the atmospheric Jupiter circulation concerned. Certain stereotypes represented alternate dark bands with clearer zones in the atmosphere. The scientists considered these zones a long time, with their clear clouds, as being zones of ascending currents, on the basis of the fact that on Earth, the clouds are formed mainly in movements of ascending air. But the analysis of the stereotypes taken by Cassini gave another explanation. Individual cells of storm, comprising white clouds which go up, too small to be observed since the Earth practically everywhere, emergent, including in the dark zones. According to Anthony Del Genio of the Goddard Institute for Space Studies of NASA, We cuts has clear picture emerging that the belts must Be the areas off Net-rising atmospheric motion one Jupiter, with the implication that the Net motion in the zones has to Be sinking .

The other atmospheric observations revealed an oval structure dark and whirling in the upper atmosphere, of a size similar to the Grande red spot, close to the Jupiter north pole. The infra-red stereotypes as for them revealed certain aspects of atmospheric circulation close to the poles. They revealed a structure in form of bands girdling planet, broadsides of adjacent bands in which the winds blow in opposite directions.

This same advertisement made it possible to call in question the nature of the Anneaux of Jupiter. The dispersion of the light by the particles of the rings revealed that these particles had very irregular forms and were likely to have for origin of the matter ejected following the impact of micrométéorite S on the Jupiter satellites, probably on Métis and Adrastée.

Saturn

Saturn and its environment

December 30th 2000: The probe passes to broad from Jupiter and takes images.
March 27th 2004: First Saturn image.
June 11th 2004: Overflight of Phœbé to 2.071 km of its surface and first image of this satellite which orbits to 13 million km of Saturn in the Sens retrogresses (direction of the needles of a watch).
July 1st 2004 (the June 30th for the the United States): First passage through the Saturn's rings. While slowing down using its retrorockets, the probe becomes captive orbit of planet. It is the first human object orbits about it around this planet. Its orbital insertion takes place at the time of sound périapse (at the point of passage nearest to Saturn), to 72.000 km of its surface. Thereafter, the probe, ordered since the Earth, will not cease modifying its orbit to approach the principal satellites saturniens. In addition, it will make go up little by little its périapse beyond 200.000 km to more slowly orbit in order to make sure of the overflights optimalement slow of its targets.
October 26th 2004: First passage close to Titan and first images, Titan remaining the target n°1. Since then, 44 overflights close to Titan are envisaged (including 80% already realized with the September 10th 2007). Among those: 41 with less than 4.000 km including 26 with less than 1.100 km.
December 25th 2004: Launching of Huygens starting from Cassini towards Titan.
December 31st 2004: First overflight of Japet to 123.000 km of its enlightened hemisphere.
January 14th 2005: Landing of Huygens in parachute on Titan after a long crossing of its atmosphere: first stereotypes of its ground.
January 15th 2005: New passage close to Titan. Cassini will on the whole pass 6 times close to Titan in 2005.
February 17th 2005: First passage close to Encelade. Cassini will on the whole fly over 4 Encelade times in 2005 pennies the 1.500 km, more exactly between 1.264 and 172 km! The stereotypes taken reveal in the southern hemisphere the existence of large Geyser S of water (liquid or vaporized) which is transformed into ice in the space and whose particles thus released supply the ring E, diffuse, of Saturn.
September 23rd 2005: Overflight of Téthys to 1.500 km.
September 25th 2005: Overflight of Hypérion to 514 km. It is the only overflight of Hypérion planned for the normal duration of the mission but this overflight at very low altitude made it possible nevertheless to reveal in a very detailed way the astonishing structure of its surface.
October 11th 2005: Overflights of Dioné to 500 km and, the same day, of the small satellite Télesto to 10.000 km.
November 26th 2005: Overflight of Rhéa to 500 km.
July 21st 2006: Overflight of Titan to the minimal altitude of 950 km.
August 30th 2007: Second overflight of Rhéa, to 5.737 km.
September 10th 2007: Second and last overflight of Japet to less than 1.650 km.
March 12th 2008: Overflight of Encelade in hedgehopping to 23 km above its geysers!

June 30th 2008: Theoretical end of the Cassini program, after 4 years and 76 revolutions around Saturn, the probe having then reached the end of its propellent reserves which allowed him until there modifying several times per month its orbit to fly over its various satellite targets. No decision was still made to insert the probe on a final orbit which would enable him to continue to photograph the system saturnien passively and to analyze it by means of its many measuring instruments which are in perfect operating condition for still of many years grace, in particular, with the electrical energy ensured by its radioisotope generator.

Study of the period of sidereal rotation of Saturn measured out of radio

The determination of the period of sidereal rotation of a planet is essential for the study of all the physical phenomena which are associated there since one bases oneself over this period of sidereal rotation for the establishment of the system of longitude of planet. In the case of the telluric planets, it is enough to observe the ground to obtain this period of rotation. In the case of the gas planets, there is not “ground” and the heart is hidden very deeply under the atmosphere of planet. The only observable one which is related to the rotation of the heart of these planets is their magnetic field. One thus studies the modulations induced by the rotation of the magnetic field of planet studied on his natural radio broadcasts to know his period of sidereal rotation.

In the case of Jupiter, the period of sidereal rotation was measured in this manner. The period obtained (9h 55m 29.68s) is thus determined with a very high degree of accuracy (the difference between each measurement does not exceed 0.08s, which makes a relative precision of 0.0001%). In the case of Saturn, the period of rotation was initially given thanks to the data of the probe Voyager. The sidereal period of Saturn was thus of 10:39 m and 24s (with a relative precision of 0.02%). In 2000, scientists (using the radio data of the probe Ulysses) observed that the period of modulation of the Saturn radio transmissions had changed since measurements To travel. New measurements give one period 1% longer than that measured by Voyager. Radios measurements obtained with the Cassini/RPWS/HFR instrument confirms the variation of the period of the modulation of the Saturn radio transmissions. Observations carried out over the first 2 years of orbits around Saturn (2004-2005) seem to show that the radio period varies slowly (on the scale of the year) from some fractions of percent.

As the number of sidereal revolutions of the Saturn heart cannot vary, it is the interpretation of the modulations of the radio broadcasts which probably should be re-examined. What does one know about these emissions? They are mainly emitted on the side day of the magnetosphere of Saturn and they are strongly correlated with the dynamic pressure of the solar wind. Various interpretations exist:

seasonal effect: the height of the Sun on the plan of the rings changes the free quantity of electrons on the lines of magnetic fields and thus changes the conditions of emission of the waves radios
effect of the solar cycle: the properties of the interplanetary Medium and the Solar wind strongly varies with the Solar activity. It was shown that the auroral Saturn radio broadcasts are very strongly correlated with the fluctuations of the parameters of the solar wind.
flutter effect: nonrandom fluctuation of the localization of the area activates out of radio in a sector of local time. Digital simulations showed that one can very easily obtain apparent periods of rotation different from the real period by flutter effect.
system of convection of the Saturn heart: theory inspired by what occurs in the Sun, but not very probable.

But none explains yet really variability observed, nor do not make it possible to obtain the period of sidereal Saturn rotation.

The problem of the definition of a system of longitude to Saturn thus remains whole. The problem is particularly thorny because if the period of Saturn rotation is indeed 1% slower than the period measured by Voyager, all the atmospheric Saturn system would be then in super-rotation (i.e. it would turn more quickly than the heart of planet) what is not easily explainable.

Saturn's rings

Collection of information on the spokes

The spokes are spots observed on the Saturn's rings by the probe Voyager in the years 1980. Cassini-Huygens made it possible to check the reality of this phenomenon and to invalidate certain theories which did not envisage their reappearance before 2007.

Radio screening of the Saturn's rings

In May 2005, Cassini began a series of experiments of screening, conceived to determine the distribution of the sizes of the particles of the rings and to take measurements of the atmosphere of Saturn. The probe carried out orbits specifically studied for this purpose.

To this end, the probe crosses the rings and emits radio waves in direction of the Earth. The variations of power, frequency and phase of these waves are then studied in order to determine the structure of the rings.

Titan

Other Saturn moons

Phœbé

The probe flew over Phœbé on June 11th, 2004. It is the only time that it will fly over it, because of the distance between Phœbé and Saturne. It was the first overflight of this moon since the mission Voyager 2 in 1981.

The first images were received on June 12th, 2004. The scientists noticed that the surface of Phœbé was quite different from that of the asteroids which the probe had approached. Certain parts of surface were very brilliant, which lets think that a great quantity of water (in the form of ice) must be on this surface.

Overflights of Encelade

During the first two overflights of Encelade in 2005, a deviation of the magnetic field of Encelade was noted. This type of variation of the magnetic field is characteristic of a mean but significant atmosphere. Other measurements seem to show that this atmosphere is primarily made up of ionized steam.

The Saturn new moons

The Cassini-Huygens mission right now made it possible to find about thirty the new moons around Saturn, thus doubling the number of known satellites saturniens. The last moon, discovered with semi-2007, constituting the 60e Saturn! While waiting to be definitively baptized, these satellites carry a provisional sequence number lasting sometimes more than one year: " S/2004 S 1" , renamed Méthone, " S/2004 S 2" , renamed Pallène and " S/2005 S 1" , renamed Daphnis. These provisional registrations mean: " Saturn/year of discovered - satellite 1 (2,3…)".

Small Daphnis (7 to 8 km in diameter) is, after Pan (26 km), the second " satellite of the anneaux" because it orbits inside the division of Keeler (42 km broad) which traverses the ring has Saturn, very close to its periphery, beyond the Division of Encke (the 325 km broad latter of the same ring and) which, as for it, contains the Pan satellite. These two satellites, by their passage, maintain open their respective divisions but their field of gravity generates waves inside the ring has on several hundred km of depth, as the stereotypes taken by Cassini show it!

Saturn magnetosphere

Analyzes

The landing itself raises some questions. The probe was to leave the fog at an altitude ranging between 50 and 70 km. In fact, Huygens started to emerge from the clouds to 30 kilometers only above surface. That could mean change in the direction of the winds at this altitude.

The recorded sounds when the probe was posed let think that it was posed on a more or less muddy surface, at least very flexible. “There was no problem with the impact. The landing much softer than was envisaged. ”

“Of the matter particles accumulated on the objective of the camera with high-resolution of the DISR which pointed downwards, which suggests that:

is the probe could be inserted in surface.
is the probe vaporized hydrocarbons on the surface and they gathered on the objective. ”

“The last parachute of the probe does not appear on the stereotypes after the landing, also the probe is probably not directed in the east, where we would have seen the parachute. ”

When the mission was conceived, it was decided that a lamp of landing of 20 Watts should ignite 700 meters above surface and illuminate the site at least 15 minutes after the landing. “In fact, not only the lamp of landing ignited with exactly 700 meters, but it continued to function more than one hour after, while Cassini disappeared beyond the horizon from Titan to continue its mission around Saturn” still indicated Tomasko.

The mass spectrometer embarked on board Huygens and which is used to analyze the molecules of the atmosphere detected the presence of a thick cloud of Méthane, high 18.000 to 20.000 meters above surface.

Other indications transmitted by DISR, fixed at before determining if Huygens had been inserted deeply in the ground, revealed what seems to be wet sand or loam. John Zarnecki, person in charge of “Gas Chromatograph and Farmhouse Spectrometer (GCMS)” which analyzes the surface of Titan, has déclaré : “ We are surprised but we can think that it is about a material covered with a fine film, under which a layer of a relatively uniform consistency like sand or boue. is; ”

Discusses on the energy source

As the probe evolves/moves very far from the Sun, it was not possible to use solar panels to provide energy necessary to the probe. This is why it embarks a thermoelectric Générateur with radioisotope which produces electricity directly starting from the heat produced by the natural disintegration of the Plutonium. This generator has one lifespan which exceeds by far the 11 years of the mission.

The Cassini-Huygens probe embarks 32,8 kg of plutonium (primarily ²³⁸Pu, very radioactive), which caused a controversy with ecologists, physicists and former members of NASA. Concerning the risks of contamination, the official estimates were the following ones: the chances of a plutonium escape during the first three minutes and half were of 1 over 1400, the chances of an escape during the rise of the rocket of one on 476, the chances of later terrestrial contamination lower than one on a million, with a risk of 120 died over 50 years if such a thing arrived. Many observers gave many other estimates. For example the physicist Michio Kaku envisaged 200.000 dead if plutonium contaminated a residential area, because of atmospheric dispersion, even if the trajectory of launching had been envisaged so as to remain far from the large metropolises and if the RTG is conceived so as to decrease the risks of dispersion of plutonium in the event of cancellation of the mission (explosion).

In the same way, an additional risk came from a second passage in the terrestrial orbit the August 18th 1999.

NASA published information wanting to be exhaustive and reassuring as for the risks related to generator RTG.

Rewards

The Marcel-Dassault price of the Academy of Science (France) was decreed in April 2007 with three scientists whose contributions were considered to be priceless to the résuissite of the Cassini-Huygens mission. It is about:

Jean-Pierre Lebreton, of the European space agency, mission leader of the European part of the probe and particularly person in charge of the Huygens undercarriage which succeeds in the exploit to be posed on the surface of Titan, on January 14th, 2005.
Daniel Gauthier, highly skilled research director at CNRS, the Observatory of Paris.
Tobias Owen, professor with the University of Hawaii, initiator with Daniel Gauthier of the idea to explore Titan. They hoped to find on this satellite an atmosphere comparable with that which existed on the Earth before the appearance of the life.

Random links: