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[14.0] Missions To Saturn (1)

v2.0.0 / chapter 14 of 20 / 01 jul 16 / greg goebel

* Saturn is the most distant of the planets known to the ancients, and indeed had almost certainly been known as long as people had been watching the skies. At its brightest, it outshines all stars except Sirius and Canopus.

The invention of the telescope showed Saturn to be surrounded by a neat system of concentric rings, and largely for this reason it has been a subject of particular interest for astronomy and space exploration. This chapter provides a history of the exploration of Saturn.

Saturn & moons montage


[14.1] THE DISCOVERY OF SATURN
[14.2] SATURN BEFORE THE SPACE AGE
[14.3] THE PIONEER, VOYAGER, & CASSINI MISSIONS TO SATURN

[14.1] THE DISCOVERY OF SATURN

* Nothing was known about Saturn in any detail until 1610, when Galileo Galilei pointed his crude telescope towards the planet. He was baffled by what he saw: a "triple planet", an object that seemed to consist of a large body, bracketed by two identical smaller bodies. Galileo was even more baffled when he observed the planet in 1612 and found that the two smaller bodies had disappeared. They later reappeared. Other observers were similarly baffled, and produced a wide range of bizarre theories, for example that the planet had "handles".

The Dutch astronomer Christiaan Huygens identified the first moon of Saturn in 1655, naming it "Titan". It would prove to be one of the biggest moons in the solar system. Huygens then went on to solve the puzzle of Saturn's mysterious appearance, publishing a book in 1659 that stated the planet was "surrounded by a thin flat ring, nowhere touching and inclined to the ecliptic." Further observations confirmed his theory, and by 1665 most astronomers had accepted that Saturn was surrounded by a ring. The simple explanation for the disappearance of the rings in 1612 was that the planet was then at a position in its orbit when the rings were edge-on to the Earth, or what would eventually be called a "ring-plane crossing". Nobody was sure what the rings consisted of, and many thought they were solid, though in 1660 a French astronomer named Jean Chapelain suggested, more or less correctly, that they were made up of a great number of small moons.

* A burst of new discoveries followed. In 1671, Jean-Dominique Cassini discovered the second moon of Saturn, which he named "Iapetus". Iapetus proved to be a very odd world, bright on one side and dark on the other, causing it to intermittently disappear as it orbited the planet. Cassini discovered a third moon of Saturn in 1672 and named it "Rhea".

In 1675, Cassini found a dark line in Saturn's ring. This would later prove to be a gap, which was named in his honor, subdividing the ring into an outer ring, which was named the "A Ring", and a brighter inner ring, which was named the "B Ring". Cassini ended this first "golden age" of Saturn exploration by discovering the planet's fourth and fifth moons in 1684, which were named "Tethys" and "Dione". No more moons of Saturn would be discovered for over a century.

* Indeed, during that long dry spell, nothing much new was learned about Saturn. The rings remained the planet's chief attraction, since observations of the disk of the planet lacked detail. Bands similar but less vivid than those on Jupiter were observed, but other "features" turned out to be illusions. One validated feature of the planet was its flattened appearance, which implied to observers that it wasn't very solid.

Major new discoveries about Saturn had to wait for the work of the Sir William Herschel. Following years of careful studies, in 1789 Herschel gave the ratio of equatorial to polar diameter as 11:10, which is roughly correct, and also discovered Saturn's sixth and seventh moons, which were named "Enceladus" and "Mimas". As it turned out, Dione orbited Saturn once for every two orbits of Enceladus, or in other words the two worlds had a "1:2 resonance" relationship. Mimas also proved to have a 1:2 resonance relationship with Tethys, with Tethys orbiting the planet once for every two orbits of Mimas.

New discoveries came steadily through the 19th century. Further observations of the ring system revealed another division in the A Ring, originally spotted by the German astronomer Johann F. Encke in 1837. He only saw it as a dark band and it was not confirmed as a gap until 1888, but it was still named "Encke's Division" after him. In 1848, William Lassell, a British brewer and exacting amateur astronomer, discovered an eighth moon, which was named "Hyperion". Hyperion performed three orbits of Saturn for every four of Titan's, or in other words had a "3:4 resonance".

However, the rings continued to attract the most attention, leading to new theoretical and observational insights:

In 1898, a ninth moon of Saturn was discovered by the American astronomer William Henry Pickering. The new moon became known as "Phoebe", and it had a very elliptical, high-inclination retrograde orbit. It was the first irregular moon of Saturn to be discovered.

* New discoveries about Saturn came slowly in the 20th century, at least up until the space age. Astronomers did observe a transient "Great White Spot (GWS)" the planet, apparently a cyclonic storm. Eventually, astronomers determined that such spots rose and fell on a cycle matching Saturn's year, cropping up in the planet's northern hemisphere when it had the maximum exposure to the Sun. There were major sightings of a GWS in 1903, 1960, and 1990. The first sighting was actually in 1876, and it remains a bit puzzling as to why no GWS was seen before then, since the spots could be big enough to be seen by the telescopes of a century earlier. In any case, brown and reddish spots were eventually discovered as well.

While it was recognized early on that Saturn was largely gaseous, it was generally assumed that Saturn was a hot object, something like a small star, until the early 1920s, when Sir Harold Jeffreys showed it to be cold. Modern observations give the temperature at the cloudtops as about -176 degrees Celsius. This is actually warmer than would be expected, since Saturn radiates about 60% more heat than obtained from the Sun, mostly due to the slow release of heat due to its formation. Jeffreys believed that Saturn had a rocky core, with a surrounding layer of ice and a thick atmosphere, while others theorized that the core was made up of metallic hydrogen. Eventually, the consensus arose that Saturn does have a rocky core, surrounded a metallic liquid hydrogen layer, and covered by liquid hydrogen and helium.

Two more moons of Saturn were discovered in 1966. R. Walker discovered the tenth moon, "Epimetheus", which seemed to be a little over a hundred kilometers in diameter, and Audouin Dollfus of the French Pic du Midi observatory discovered the eleventh moon, "Janus", which was about twice as big as Epimetheus. However, there was confusion over this discovery at the time, since the orbits of the two moons and their sizes are similar, and nobody was quite sure which was which, or if there was one moon that was being reported as two. In 1977 Stephen Larson and John Fountain proved that there are two moons, with an unusual orbital relationship that led to all the confusion, as explained later. They shared credit for the discovery of Epimetheus with Walker.

Dollfus also determined that the rings were likely not more than 2.4 kilometers thick at most. A very faint ring encircling the others, named the "E Ring", was tentatively discovered by Walter Feibelman in 1967, and in 1969 Pierre Guerin spotted traces of what he believed was a "D Ring". In 1977, Stephen James O'Meara found something very unusual and unprecedented: hints of radial lines or "spokes" running through Saturn's rings.

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[14.2] SATURN BEFORE THE SPACE AGE

* By the late 1970s, despite all the observations, there was little detailed knowledge about Saturn, its rings, and its moons. The planet's gross properties were clear. Saturn is the 6th planet in order of distance to the Sun. Its mass is 95 times that of the Earth's, and second only to the mass of Jupiter among the planets. Saturn orbits the Sun at a mean distance of 9.538 astronomical units. Saturn's orbit is noticeably eccentric: at perihelion, or closest approach to the Sun, Saturn lies at a distance of 1,347 million kilometers, while at aphelion, the farthest distance from the Sun, it is at a distance of 1,507 million kilometers.

Saturn's orbital period is 10,759.2 Earth days (29.46 years). Like Jupiter, it rotates rapidly, the period being 10 hours, 39 minutes, 24 seconds. The fast spin is what flattens the planet; the equatorial diameter is 120,660 kilometers, while the polar diameter is only 108,000 kilometers.

Although Saturn is massive, it is by far the least dense of all the planets, with a mean density of only 0.69 grams per cubic centimeter, much less dense than water and only an eighth the density of the Earth. Saturn has a deep atmosphere of light gases and a relatively small rocky core. The atmosphere is about 88% hydrogen and 11% helium by mass, with trace amounts of ammonia and hydrocarbons such as methane, ethane, and acetylene.

Saturn's ring system still remained fascinating and mysterious. It is now known that all four gas giant planets in the Solar System have ring systems of one sort or another, but those of Saturn are in a class of their own. The rings clearly visible from Earth begin with the C Ring, with an inner radius of 67,000 kilometers from the planet's center, to the outer edge of the A Ring at 136,200 kilometers. The rings are no more than a few kilometers thick, and in some place even only a few meters thick. They are believed to consist of particles of rock and ices, ranging in size from dust to boulders about 10 meters across.

Not much was known about the eleven moons of Saturn discovered up to the 1970s. Their orbits had been determined, of course, but data on other details was uncertain. The list below gives the moons known at that time with rough estimates of their diameter in kilometers, arranged from farthest from Saturn to nearest:

   moon             diameter
   _________________________

   Phoebe                200
   Iapetus             1,500 
   Hyperion              300
   Titan               5,000
   Rhea                1,500
   Dione               1,000
   Tethys              1,000
   Enceladus             100
   Mimas                 400
   Janus                 200
   Epimetheus            100
   _________________________
   

Most of the moons were seen as airless bodies, apparently composed of water with quantities of rock. As mentioned, Phoebe had a retrograde orbit, Iapetus was bright on one side and dark on the other, and there were resonance relationships between the orbits of Hyperion and Titan; Dione and Enceladus; and Tethys and Mimas. Titan was observed to have an orange atmosphere, composed mostly of nitrogen with traces of light hydrocarbons.

By 1979, the first space probes were reaching Saturn, clearing up many of the planet's mysteries while raising new questions.

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[14.3] THE PIONEER, VOYAGER, & CASSINI MISSIONS TO SATURN

* Four spacecraft have been launched to Saturn. The first was the Pioneer 11 probe, which flew by Jupiter in December 1974 and then was sent on to Saturn. The probe made its closest approach to Saturn, 20,930 kilometers from the planet, on 1 September 1979. The data returned by Pioneer 11 was modest, though the probe did have the distinction of being the first spacecraft to visit Saturn. The information it returned helped in planning the missions of its successors, the NASA Voyager 1 and 2 space probes.

Voyager 1 performed a flyby of Jupiter in March 1979 and was sent on to Saturn, performing its closest approach of 126,000 kilometers on 12 November 1980. Voyager 2 performed a flyby of Jupiter in July 1979, to then be sent on to Saturn, Uranus, and Neptune in turn. Voyager 2 made its closest approach to Saturn, 101,000 kilometers, on 25 August 1981.

The only spacecraft dedicated to Saturn exploration flown to date is the NASA "Cassini" space probe. Cassini was the biggest and most expensive interplanetary probe ever built. The 6.7 by 3.95 meter spacecraft weighed 5.7 tonnes and cost $3.3 billion USD. The probe was to make at least 74 orbits of Saturn, with 45 flybys of Saturn's moon Titan and flybys of the smaller moons.

Cassini-Huygens

Cassini carried the "Huygens" probe, the first planetary lander built by the European Space Agency. The 2.75-meter diameter, 320-kilogram Huygens was designed to parachute to the surface of Titan, taking pictures as it descended and after it landed, while measuring winds and temperatures as well as performing chemical analysis.

The Cassini probe was more modern than its predecessor, the Galileo Jupiter orbiter, and was accordingly more sophisticated. Cassini itself carried 12 scientific experiments, while Huygens carried six, giving the mission about half again as much science capability as Galileo. The Cassini orbiter used radar to obtain maps of the surface of Titan, with the radar using the probe's high-gain antenna to perform observations. Cassini's nearly two dozen microprocessors gave the spacecraft one to two orders of magnitude more computing power than Galileo. Cassini also used two highly reliable, 2-gigabit solid-state recorders, in contrast to the tape drives carried by Galileo. Finally, Cassini's three RTGs gave the probe 50% more power than Galileo, and allowed Cassini to return data at the same rate that Galileo would have attained had its main antenna not become stuck, from twice as far away.

Following a lengthy development -- though one whose troubles blessedly did not rival those of Galileo -- Cassini was launched from Cape Canaveral by a Titan 4B-Centaur booster before dawn on 15 October 1997. Cassini then performed a gravity assist trajectory that sent it past Venus twice, then by Earth in August 1999, casting it off towards Jupiter. After passing through the asteroid belt and imaging the asteroid 2685 Masursky, it performed its final gravity assist by flying past Jupiter in late 2000, returning some spectacular images in passing.

That same year, the mission faced a crisis. Engineers determined that there had been a design flaw in the receiver on board Cassini that would obtain data from the Huygens probe as it fell into Titan's atmosphere. The Doppler shift due to the probe's velocity would shift the frequency of Huygens' transmission out of the band that the receiver could pick up. Fortunately, the flaw was discovered with plenty of advance warning, and JPL engineers were able to come up with a modified trajectory for Cassini that put the frequencies transmitted by Huygens into the band that the receiver could pick up. Mission scientists were delighted with the elegant solution.

* On 22 June 2004, Cassini performed a main engine burn to begin its insertion into Saturn orbit. It entered into orbit on 30 June, crossing through the gap between Saturn's F and G rings (twice!) and obtaining spectacular closeups of ring details. There were worries about collisions with ring debris and the spacecraft's high-gain antenna was pointed forward to act as a shield during the ring-plane crossing; fortunately, although the spacecraft was hit by an estimated 200,000 particles ranging in size from smoke particles to fine sand grains, it suffered no real damage.

It was performing infrared observations of Titan on 2 July, and performed the first of roughly 450 radar observations in early October, imaging a strip of the moon. The imagery revealed a geologically diverse, icy surface. The radar system also performed radiometric measurements that showed Titan was warmer than it should be if it were simply a big ball of ice, suggesting it has a rockier composition than previously assumed. Release of the Huygens took place on 25 December, on the third orbit around Saturn.

Huygens on Titan

Huygens reached Titan on 14 January 2005. The descent lasted two and a half hours and the probe landed intact, though due to what appeared to be a command error only 350 images were returned during the descent, half of that expected. ESA officials were quick to state that the glitch was an ESA problem and not the responsibility of NASA, though NASA still planned to investigate. In any case, 350 pictures were plenty, and all other data were returned without difficulty. The probe returned a panoramic image of its landing zone, revealing a frozen landscape littered with small icy blocks in a drizzle of methane rain. The surface temperature was a cool 93.8 degrees Kelvin and the pressure was about half a bar.

Despite the glitch, not only had the ESA performed Europe's first successful planetary landing, it was also the very first planetary landing in the outer solar system. Capitalizing on the publicity surrounding the landing, NASA released a image of Iapetus taken by Cassini on a flyby at the beginning of the month. It provided unprecedented detail of the oddly-patterned moon, in particular revealing an extended ridge along the moon's equator.

Further operations followed, with Cassini returning torrents of information about the Saturn system:

Although the mission was scheduled to end in July 2008, the spacecraft was still in good condition by that time; the mission was extended, the expectation being that it will end in 2017, with Cassini commanded to fall into Saturn. Titan fly-bys have been a particular focus for the extended mission.

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