Cassini-Huygens - Finale 20 years of research

September 15 at 14:54 Moscow time (11:54 on UTC) ends without a month 20-year mission Cassini - orbital "half" automatic interplanetary Cassini-Huygens mission (mission landing probe station "Huygens" ended more January 14, 2005, after an hour after landing on Titan). It was only the 4th mission to Saturn after Pioneer 11 and two Voyagers, and the only one in which the device went into its orbit. The next mission to the Saturn system should take place no earlier than 2029 .

During his mission, the device made 293 revolutions around Saturn, among which he made 162 passes near his satellites and discovered 7 new ones, transferred 453,048 photos of 635 GB of scientific data to Earth and became the source for 3,948 scientific publications. They discovered the ocean on Enceladus, as well as the ocean, 3 seas and hundreds of small lakes on Titan. About 5 thousand people from 27 countries participated in this project, and its total cost amounted to $ 3.9 billion in which the initial shares were distributed as: $ 2.6 billion from the US agency NASA, $ 500 million from the European ESA and $ 160 million from Italian ASI.

Cassini design

Apparatus Cassini-Huygens in the process of testing. The round orange part in the foreground - Huygens, who landed on Titan, the white part - 4-meter antenna / radar Cassini


The probe, named after Giovanno Cassini (who opened Saturn’s 2nd to 5th satellites), is as much as 6.8 meters high and 4 meters wide with a dry weight of 2150 kg (this was the third interplanetary probe after the mass of Soviet "Phobos" ). Saturn reaches only 1.1% of the solar energy available to us in Earth orbit, so the probe is powered by 3 RTGs of the same enormous size as the apparatus itself - they have 32.7 kg of plutonium-238 (this is 3.6 times more than both Voyagers at the start, 6.8 times more than Curiosity has and probably the most plutonium available by NASA at the moment: 1 , 2). The device has 1630 separate electronic components and 22 thousand wired connections with a total cable length of 14 km, and is controlled by duplicated 16-bit 1750A computers (another one controlled the Titan IV launch vehicle that put the device into orbit). Scientific equipment includes 12 instruments grouped into three groups, which are intended for 27 separate scientific studies:

Optical range sensors:

1) Composite infrared spectrometer, including 3-band cameras ( CIRS ); 2) wide-angle and narrow-angle (33 cm in diameter) cameras of the visible range with a set of several filters for different colors and CCD matrices with a resolution of 1024x1024 pixels. (ISS ); 3) ultraviolet spectrometer, which includes 4 telescopes ( UVIS ); 4) the mapping spectrometer of the visible and infrared range, dividing the visible light into 352 spectral areas ( VIMS );

Sensors of magnetic fields and charged particles:

5) plasma spectrometer ( CAPS ); 6) cosmic dust analyzer fixing particles from microns to nanometers ( CDA ); 7) ion and neutral particle mass spectrometer ( INMS ); 8) a magnetometer placed on an 11-meter dielectric boom designed to reduce the influence of the apparatus’s instruments on this sensor ( MAG); 9) a tool for visualizing the magnetosphere, consisting of three ion and charged particle sensors located in different planes ( MIMI ); 10) a radio and plasma wave detector having three receivers of different frequencies ( RPWS );

Radio wave sensors:

11) 4-meter-in-diameter radar designed for the mapping of Saturn’s satellites ( Radar ); 12) scientific radio subsystem which consists in using the main 4-meter antenna for observing Saturn, its rings and satellites against the light by radio waves ( RSS ). The signal delay at Saturn is 68-84 minutes one way.

Note: the links are available 3D-models of each tool and the scheme of their position on the device.

Through thorns to Saturn

The weight of the orbital and landing probes was too large for them to be directly launched to Saturn (from 350 kg of Huygens, the total weight of the device was 2.5 tons) - even taking into account the fact that the Titan IV on which Cassini-Huygens flew was 40% more useful load than Titan IIIEon which the Voyagers flew. Therefore, the apparatus had to travel a lot around the Solar System, picking up speed with gravitational maneuvers to meet Saturn: after the launch on October 15, 1997, the 5.7 tons bundle of two vehicles filled with 2978 kg of fuel went to meet Venus. Having performed 2 gravitational maneuvers in her on April 26, 1998 and June 24, 1999 (in which they flew only 234 and 600 km from the planet, respectively), they returned briefly to Earth (flying 1171 km) on August 18, 1999 after which went already to Jupiter.


A photograph of the Moon made by a narrow-angle camera of the device in the near ultraviolet, from a distance of about 377 thousand km and an exposure of 80 μs.

Flying through the asteroid belt, the unit met on January 23 with the asteroid Mazursky: unfortunately the distance was 1.6 million km, and the asteroid itself was only 15x20 km in size, so the photo was less than 10 by 10 pixels. On December 30, 2000, Cassini-Huygens met with Jupiter and his brother Galileo , whose mission was already approaching the final (he completed his mission almost 14 years ago with the same selfless feat that Cassini now intends to accomplish). This 4th gravitational maneuver finally gave the two vehicles sufficient speed to meet Saturn on July 1, 2004, by which time he had already traveled 3.4 billion kilometers.

In order not to waste time, the mission team used the device’s radio antennas to refine the Shapiro effect(slowing down the propagation of a radio signal when it moves in a gravitational field of a heavy object). The measurement accuracy was improved from previous results of 1/1000 in Vikings and Voyagers to 1/51000. Published on October 10, 2003, the results completely coincided with the predictions of the general theory of relativity.


The graph clearly shows the peaks of meetings with the planets (after which the apparatus increases speed), a long descent with a slight break in Jupiter (when the apparatus flew towards Saturn, gradually exchanging kinetic energy for potential, getting out of the "gravitational well" of the Sun), and a series waves at the end (when the unit went into orbit of Saturn, and began to rotate in its orbit).

Long-awaited meeting and main mission

On May 27, 2004, Cassini, for the first time since December 1998, turned on its main engine to give the device an impulse of 34.7 m / s, which was needed to correct the trajectory that held it on June 11, 2068 km from Phoebe , a very remote satellite of Saturn, which presumably formed in the Kuiper belt and was subsequently captured by the gravitational pull of Saturn. Because of the huge radius of the orbit of this satellite (averaging about 12.5 million km), this was the only meeting of Cassini with this satellite.

On July 1, the main engine of the apparatus was switched on again (already at 96 minutes), in order to reset 626 m / s of speed to enter Saturn orbit. On the same day, Mefon was discovered and Pallen reopened., which was discovered in one of the Voyager-2 images, but since it was not in the other images, the orbit of the celestial body could not be established and for 25 years it received the designation S / 1981 S 14. The very next day Cassini made the first flight past Titan, October 24, another satellite was opened ( Polidevk ), and on December 24, the landing probe Huygens was dropped.

On January 14, 2005, Cassini acted as a repeater for the landing probe (it will be discussed below), and the next day the device became as close as possible to Titan and, using its radar, discovered a 440-kilometer crater on its surface. On May 6, the Daphnis satellite was discovered , which lives on the edge of the Keeler slot :



At the edges of the 42-kilometer gap, waves were discovered caused by a very weak attraction of Daphnis (whose weight is only 77 billion tons, which creates a gravity 25-100 thousand times lower than the earth's):



Saturn’s equator and the plane of its rings are inclined 27 ° relative to the ecliptic, so that we can observe both poles of Saturn as well as observe its rings from the top and bottom. But since they are observed at a large angle and from huge distances (1.2-1.66 billion km depending on the relative position of the Earth and Saturn), it was simply impossible to see anything there, so let's say the Saturn hexagon was only detected flying past Voyagers.



Photo of Saturn in natural colors, consisting of 36 Cassini images taken on January 19, 2007 with three filters (red, green and blue). The shutter speed of the pictures was made with the expectation of the dark areas of the rings, so the surface of Saturn turned out to be strongly overexposed.

In 2005, it was found that about 250 kg of water vapor at a speed of up to 600 m / s leave him every second through the geysers of Enceladus. In 2006, scientists were able to establish that they are the source of material for the penultimate and the widest - ring E.



On July 22, 2006, the vehicle flew over the northern latitudes of Titan and for the first time dark areas were discovered on the radar map made by the vehicle, indicating that methane lakes are located on the surface in these places. During the 127 spans of the satellite, many areas of its surface were studied in detail, some of which showed dynamic changes. Among these was the Sea of ​​Liegei , measuring 420x350 km and an average depth of about 50 m with a maximum of more than 200 m (maximum depth recorded by the radar): The most likely cause of such measurements are waves, solids under or above the surface or bubbles in the thickness of the liquid (which affect the reflectivity of the surface).





On May 30, 2007, the 2-kilometer satellite Anfa was discovered , and on September 10, the device passed only 1600 km from Yapet , but already when transmitting the images a particle of cosmic rays hit the computer , which caused its transition to safe mode. Fortunately, no pictures were lost. Shortly before this event, a video greeting of Arthur Clark on this event came (according to one of his most famous novels - “2001: A Space Odyssey” - on the surface of Yapet was one of the monoliths).


Map Yapet with a resolution of 400 m per pixel (original 5 MB): Approximately 40% of the surface of this satellite is occupied by dark areas with an albedo 10 times less than bright areas. Now the source of such a big difference is the effect of separation of dust and ice, when ice evaporates from dark areas and precipitates on light ones, thereby making light areas even brighter and darker ones darker. The reason that the other satellites behave "normally" is that they have a shorter day, during which the surface does not have enough time to warm up.

Extension and Cassini's Equinox Mission

On July 1, 2008, the expanded 27-month mission of Cassini began, which included 21 additional spans of Titan, 8 Tethys, 7 Enceladus, 6 Mimas and one span each of Dione, Ray and Elena.

On August 15, 2008, Egeon was discovered , although it was named after a monster with 100 hands and 50 heads, but it was an almost harmless “pebble” 500 meters in diameter (it was so small that it had to be set in terms of its brightness , so that we do not know the shape of this satellite). And on October 9, Cassini completed his most dangerous maneuver — a span just 25 km from Enceladus (and this is at a speed of 17.7 km / s!). The mission team took such a risky step for the sake of a direct analysis of the water vapor composition of its geysers.

In the course of its 23 flights, Enceladus for the entire time of the mission (in 10 of which the device approached a distance of less than 100 km), it was found that the pH of the subsurface ocean is 11-12 units (which is of little use for terrestrial life forms), but nitrogen (4 ± 1%), carbon dioxide (3.2 ± 0.6%), methane (1.6 ± 0.6%) and traces of ammonia, acetylene, hydrocyanic acid, and propane ( which indicates the active formation of organic substances under the surface of Enceladus). Unfortunately, the device does not contain special instruments for registering complex organics (since they couldn’t even guess about finding such an apparatus during mission planning), so the answer to the question “Is it possible for life to exist under the surface of Enceladus?” Cassini left for his followers.

By July 26, 2009, the last of Cassini’s discovered satellites was discovered - the 300-meter S / 2009 S 1 , which was discovered due to the 36-kilometer-long shadow that it casts on the far edge of ring B along which its orbit lies:

Second Extension and Cassini Solstice Mission

In February 2010, a decision was made to further extend the mission, which began as early as September, and was to last until May 2017, when the final fate of the apparatus was to be decided. It included 54 more flights of Titan and 11 flights of Enceladus.

The efforts of Cassini and his team who managed to achieve an additional allocation of about $ 400 million for the next 7 years of the mission (bringing the cost of the program to almost $ 4 billion) were not in vain: in December 2010, during the flight of Enceladus, the device determined that it had an ocean under the North Pole (it was later established that the ocean is not limited to the polar region). That same year, the Great White Spot reappeared on the surface of Saturn.- a huge storm that appears in the atmosphere of Saturn approximately every 30 years (Cassini was very lucky with this, and he managed to register such storms twice - in 2006 and 2010). On October 25, 2012, the device recorded a powerful discharge inside it, which raised the temperature of the stratospheric layers of the atmosphere 83 ° C above normal. Thus, this vortex became the hottest among the storms in the solar system, beating even the Great Red Spot of Jupiter.

“The day when the Earth smiled” - a project organized on July 19, 2013 by Cassini’s visualization team, during which Cassini took a picturethe entire system of Saturn, which also hit Earth, Moon, Venus and Mars. A total of 323 photographs were taken, of which 141 were then used to compose the mosaic: the Earth is in the lower right corner, and the original without signatures is here (4.77 MB). Parallel to this, NASA launched the campaign “Pomashi Saturn” during which 1600 photographs were collected, of which a mosaic was collected on November 12, which was published on the cover of the New York Times on the same day (carefully, the original weighs 25.6 MB): From 2012 to 2016, the device recorded changes in the color of the Saturn hexagon (photo from 2013 and 2017, the original 6 MB):

"Huygens"

The landing probe, named after Christian Huygens (who discovered Titan in 1655, which the probe landed), is a 318-kilogram 2.7 meter device with 6 sets of instruments:

1) a constant-frequency transmitter designed to measure wind speed Doppler effect (Doppler Wind Experiment - DWE);
2) sensors of the physical properties of the atmosphere measuring the density, pressure and electrical resistance of the atmosphere, as well as acceleration sensors along all three axes, which, together with the previous device, allow the determination of the density of the atmosphere (Huygens Atmospheric Structure Instrument - HASI);
3) cameras of the visible and infrared spectra, in parallel with obtaining images involved in measuring the spectrum and illumination at the current height of the device (Descent Imager / Spectral Radiometer - DISR);
4) aerosol particles pyrolyzer performing heating of samples taken from two different heights, and redirected them to the next device (Aerosol Collector and Pyrolyser - ACP);
5) gas chromatography-mass spectrometer measuring the composition and concentration of individual components of the atmosphere of Titan, and at the last stage also the evaporated top layer of soil (Gas Chromatograph Mass Spectrometer - GCMS);
6) a set of instruments for measuring surface properties, which include an acoustic sensor measuring the density / temperature of the atmosphere on the last 100 m of descent according to the properties of a sound reflected by a surface (Surface-Science Package - SSP).



Huygens separated from Cassini on December 24, 2004, and by January 14 reached the atmosphere of Titan. The descent in the atmosphere took 2 hours and 27 minutes, during which the thermal protection of the device and its three parachutes sequentially took effect, and after landing it transmitted data from the surface for another 72 minutes (until the Cassini probe, which served as a signal repeater, went beyond the horizon).


International cooperation of the Huygens probe The

ten largest discoveries of Huygens:

1) In the course of measuring the composition of the atmosphere from an altitude of 1,400 km to the surface itself, it was found that atmospheric layers above 500 km were warmer and denser than expected, and the average temperature here was -100 ° C with differences of 10-20 ° C, at an altitude of at 250 km, the temperature reached a maximum of -87 ° C (slightly above the minimum on Earth), and then dropped to -203 ° C at an altitude of 44 km. On the surface it was slightly warmer (-180 ° C) at a pressure of 1.47 atmospheres.

2) Western winds at an altitude of 120 km reached 430 km / h, at an altitude of 60 km, the device got into strong turbulence, after which the wind speed began to steadily decrease from 108 km / h to 55 km, to 36 km / h at an altitude of 30 km and 14 km / h for 20 km. At an altitude of 7 km, the wind direction changed and only a gentle breeze of 1-3.5 km / h acted on the probe. During the descent, the vehicle was demolished by 165.8 km from the initial point.

3) Sunlight was supposed to destroy atmospheric methane for tens of millions of years, and scientists were interested in the source of its replenishment. Measurements have shown that at an altitude above 40 km, the basis of the atmosphere is nitrogen with small methane inclusions, then the methane concentration begins to rise and reaches ≈5% at an altitude of 7 km. This was the first indirect evidence of cryovolcanism on Titan. On the surface of the planet, GCMS also found traces of more complex hydrocarbons, such as ethane, cyan and benzene.

4) Descending into the atmosphere, the device detected the presence of argon-36 and 38, as well as krypton and xenon in the atmosphere. Scientists have suggested that nitrogen and noble gases got into the atmosphere of Titan during its formation, but the ratio of argon-36 / nitrogen turned out to be a million times less than what is in the atmosphere of the Sun. This suggests that nitrogen did not enter Titan's atmosphere in pure form, but in the form of some nitrogen-containing compounds.

5) In the atmosphere of Titan, a 0.05% concentration of radioactive argon-40 was found, which also indirectly proved the existence of cryovolcanism (since its half-life was 1.3 million years, and during the existence of the atmosphere it should have almost all disintegrated).

6) Titan’s brown haze covering its surface turned out to be an aerosol of methane, ethane and hydrogen cyanide(highly poisonous substance). Haze was detected at all altitudes, with noticeable concentrations at altitudes of 80, 30 and 21 km, as well as methane clouds at altitudes of 16 and 8 km.

7) At altitudes of 130–35 km and 25–20 km, two samples of atmospheric aerosols were taken. It was found that their main components are carbon and nitrogen. Subsequent reproduction of the properties of aerosols on Earth allowed us to establish that at 80 km they are based on hydrogen cyanide, at an altitude of 44 km it is ethane, and at an altitude of 8 km methane forms its basis.



The light of the Sun and the charged particles of Saturn lead to the decomposition of nitrogen and methane molecules, which due to their chemical activity join other molecules in the atmosphere, thereby forming more complex structures, including polyaromatic hydrocarbons(which managed to fix in the atmosphere and on the surface). Larger molecules have a greater density, and gradually concentrate in the lower layers of the atmosphere, thereby contributing to the formation of even larger molecules there.

8) Initially, the device should have received 700 photographs, but due to design errors - only half of them were obtained: already during tests during the flight it was found that a software error could lead to the fact that Cassini could lose the transmission frequency Huygens signal, which was supposed to move relative to him during the landing. The program code could no longer be rewritten, so Huygens’s landing path was recalculated so that during the landing process it moved perpendicular to Cassini (to minimize the relative speeds of the machines). To do this, Huygens had to reset a month later than previously planned.

But in fact, another attack was discovered: the probe had two communication systems, but since the amount of data associated with the photographs was quite large, they were transmitted simultaneously through both channels, without duplication. Due to a program error, Cassini did not listen to one of the channels, due to which half of the photos were simply lost.

However, the obtained 350 photographs (among which 3 cameras of the probe were made and several stereos) turned out to be enough: on them were found the beds of dry rivers 100 m deep with very steep slopes indicating the fast flows raging in these places. The landing site was a riverbed strewn with pebbles with a diameter of 10-15 cm.



9) Scientists wondered whether there are thunderstorms on Titan, and what is their character. Therefore, ultra-low frequency radio receivers were installed on the device to record the Schumann resonance . Although no lightning was detected by the device, the sensors recorded a signal at a frequency of 36 Hz, as well as an ionospheric conductive layer extending at altitudes from 140 to 40 km with a peak around 60 km. This indicated that the lower reflective layer did not coincide with the surface of the planet (as on Earth), but was located at a depth of 55-80 km below its surface. Model of the structure of Titan Dominic Fortres from University College London , made on the basis of Huygens and Cassini data. Ice VI



pictured here - although it melts at 81 ° C, it has nothing to do with Kurt Vonnegut’s ice-nine , and does not pose a threat to us, so you should not be afraid of the Titanian life forms, even if they are there).

10) Finding the landing site of the probe turned out to be quite a difficult task, since although the Huygens side-view camera could register surface details at a distance of up to 450 km, Cassini radars did not see the terrain features that were captured by Huygens cameras. This effect, dubbed the “ghost dunes”, turned out to be associated with surface sediments of hydrocarbons that do not reflect radio signals. Thus, Cassini actually looks “through” them, revealing only layers of dirty ice being below the surface of the planet and having less pronounced relief.

This allowed scientists to establish that the most likely candidate for building materials for the dunes of Titan are granules of hydrocarbons and / or nitriles with a low content of water ice and characteristic sizes of 0.1–0.3 mm (close in size to the earth’s sand ), the source of which is , in conditions of very slow wind at the surface - is saltation .

"Grand Finale"

In May 2017, the further fate of the apparatus was decided: by the end of the second extended mission, he had very little fuel left, and 19 possible options for completing the mission were considered, including a collision with Saturn, its main rings or ice satellites, orbiting Saturn to the heliocentric orbit or a stable orbit around Titan / Phoebe (and even a collision with Mercury). As a result, it was decided to send the device into the atmosphere of Saturn, in order to protect Saturn’s satellites from their possible biological pollution. To accomplish this task, on April 22, the device performed a maneuver near Titan, which redirected it to a 2,000-kilometer gap between Saturn and its nearest ring.

Since then, he has completed 21 orbits at a distance of only 1600-4000 km from Saturian clouds, all the while approaching the atmosphere of Saturn, and is currently at its last 22nd orbit. The device will take its last images for a couple of hours before entering the atmosphere, after which it will deploy its 4-meter antenna towards the Earth, and transmit data on the composition of the saturian atmosphere from its spectrometers until it can parry atmospheric disturbances. Soon after the loss of his connection with him, he will collapse, and burn in the dense layers of the atmosphere of Saturn - somewhere in the constellation Ophiuchus for 1.4 billion kilometers from us.

References:

Countdown to the “Big Finale” of the mission
Interactive 3D model of the device
Raw photos of the device (395,328 pieces)
Top 10 photos taken by the device by year (for the period 2011-2016)
Top 10 scientific discoveries by year (for the period 2005-2016)