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As we go to press, the 20-year-long mission of the Cassini-Huygens space probe is reaching its spectacular climax. Cassini is entering some of the last of its 22 weekly “dives” between Saturn and its rings, sending back to Earth new and unique scientific data. At the end of the final orbit, scheduled for 10:44am UTC on September 15th, Cassini will be intentionally steered into Saturn’s gas clouds, almost certainly burning up in a dramatic last hurrah. It is being destroyed for two main reasons: it’s running very low on fuel and NASA wants to ensure it cannot collide with (and possibly pollute) any of Saturn’s moons, thus affecting future exploration. Here we look at the remarkably successful Cassini-Huygens mission and what it has meant to scientists back on Earth. by ROSS TESTER cassin grand 16  Silicon iliconCChip hip siliconchip.com.au T he name “Cassini Grand Finale” was chosen from a public competition, reflecting its exciting journey to date, while acknowledging that it’s a big finish for what has been a truly great show. In fact, NASA invited applications from the public to join it at the Jet Propulsion Laboratory in Pasadena, California, for a Grand Finale party on September 15 (sorry, you’re too late to apply!). In a 3.2 billion dollar collaboration between NASA, the European Space Agency and Agenzia Spaziale Italiano – the Italian Space Agency. Cassini was launched on October 14th 1997 and entered into orbit around Saturn on 30th June 2014. The two spacecraft are named after astronomers Giovanni Cassini and Christiaan Huygens. Cassini/Huygens had several specific mission objectives: • Determine the three-dimensional structure and dynamic behavior of the rings of Saturn. • Determine the composition of the satellite surfaces and the geological history of each object. • Determine the nature and origin of the dark material on Iapetus’s leading hemisphere. • Measure the three-dimensional structure and dynamic behavior of the magnetosphere. • Study the dynamic behavior of Saturn’s atmosphere at cloud level. • Study the time variability of Titan’s clouds and hazes. • Characterise Titan’s surface on a regional scale. These objectives have not only been met – they’ve been massively over-achieved. It’s not the first time Saturn has been visited by a spacecraft from Earth. Pioneer 11 was the first, launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the Solar System and heliosphere. Last contact with the spacecraft was on September 30, 1995. Then in the early 1980s, NASA’s twin Voyager spacecraft had flown by and photographed Saturn and its largest moons but these were brief encounters and with mid-20th-century technology. Cassini was a whole new ball game, with 21st century technology, a mission measured in years, rather than hours and a huge array of instrumentation and data-gathering equipment on board. And while Voyager was able to send photographs back to Earth, Cassini (and Huygens) photography was in glorious, detailed, highdefinition. And colour! The launch and mission was previewed in SILICON CHIP September 1997, “The C assini Space Probe: unravelling Saturn’s Secrets” www. siliconchip. com.au/Article /4835 The launch vehicle was a Titan IV r o c k e t , which propelled the ni’s Finale Artist’s impression courtesy NASA siliconchip.com.au SSeptember eptember 2017  17 It’s not quite as simple as “aim, light the touch paper and stand back” (OK, you have to be old enough to remember skyrockets!). Cassini-Huygens travelled in an ever-increasing elliptical path using the gravity of Venus (twice), Earth and Jupiter to increase its speed and place it on a trajectory to intersect with Saturn, almost 93 months after its launch. (Courtesy NASA/JPL) 5.5 tonne probe into an Earth orbit in preparation for its journey to Saturn. Of the rocket’s 940,000kg launch weight, 840,000kg was fuel. Along the way, in January 2005 it successfully dropped a probe named Huygens (hence the mission name, CassiniHuygens) onto Saturn’s largest (and best known) moon, Titan. The Huygens craft was developed by the European Space Agency and “hitched” a ride on the side of Cassini. We covered this section of the mission in an article in May 2005: “Knocking on Titan’s Door”(www.siliconchip. com.au/Article/3056). Titan is huge: at 5150km in diameter, it’s about half the size of the Earth. Then again, Saturn itself dwarfs the blue planet – at 120km in diameter, you could fit 764 Earths inside Saturn! Even at its closest, Saturn is 1.2 billion (yes, B for billion!) kilometres from Earth. To put that in perspective, the Sun is only 150 million kilometres away. But it wasn’t a straight A-to-B flight. Ignoring the fact that Saturn wouldn’t be in anywhere near the same position after more than a decade, the Cassini-Huygens spacecraft made close fly-bys of Venus (twice), Earth and Jupiter, using their gravity to “slingshot” the craft on its journey. In fact, Cassini orbited the Sun twice before setting out on the long path to Saturn. Without these gravity-assisted fly-bys, which used energy from the planets to increase Cassini’s velocity and change its direction relative to the Sun, there is simply no way that it could have carried enough fuel to make it to Saturn, let alone travel more than 2 billion kilometres around the planet once it arrived. 18  Silicon Chip Some anti-nuclear protestors on Earth claimed that having the radioactive-powered craft flying so close to Earth posed an unacceptable risk. NASA countered by showing that the closest Cassini would approach the Earth was more than one thousand kilometres. The also claimed that the chances of a collision were “less than one in a million”. So what’s it been doing? In a word, exploring! In many more words, conducting an amazing array of scientific and astronomic research not only on Saturn itself (even though it has never landed, and never will) but also on its many moons (many more than previously thought) and, of course, those rings which have fascinated man ever since he had the telescopes powerful enough to see them. The 22 “dives” Cassini is taking in the weeks up to its demise have actually been in and through those rings and between the rings and Saturn’s surface. Its speed is nearly 122,000 kilometers per hour relative to Saturn’s center and about 110,000 kilometers per hour relative to Saturn’s cloud-tops. At that speed you could travel coast-to-coast in Australia in less than three minutes, and it would take just over an hour to travel three times around the Earth at the equator. Scientists use the Doppler Shift in radio signals to measure its speed and the signal’s timing to determine its distance. The Cassini mission program was originally planned to end in 2008. That it has lasted another nine years is testament to the initial planning and design, the build quality and the “nursing” of the craft – and of course, it meant that siliconchip.com.au A somewhat stylised artist’s impression of Huygens parachuting to make a soft landing on Titan, which it did in January 2005. Titan was believed to be the only body (except for Earth) in the solar system with a liquid on its surface (a hydrocarbon, not water) but Cassini found clear evidence of water on another moon, Enceladus. (Courtesy NASA/JPL) an enormous increase in the amount of experimentation and sampling could occur. In April, Cassini started its dives through the gap between Saturn and its innnermost ring at nearly 122,000 kilometers per hour relative to Saturn’s centre, and about 110,000 kilometers per hour, relative to Saturn’s cloud-tops. At that speed you could travel from New York City to Los Angeles in less than three minutes and it would take just over an hour to travel three times around the Earth at the equator. On the way: Saturn’s moons Even before Cassini started orbiting Saturn itself, it had undertaken valuable research on the many moons and rings surrounding the planet itself. One of the defining features of Saturn is its number of moons. Excluding the trillions of tonnes of little rocks that make up its rings, as of September 2012, Saturn has 62 discovered moons. Perhaps Cassini’s most detailed look came after releasing the Huygens lander towards Titan, Saturn’s largest moon. Huygens descended through the mysterious haze surroundsiliconchip.com.au ing the moon and landed on January 14, 2005. It beamed information back to Earth for nearly 2.5 hours during its descent, and then continued to relay what it was seeing from the surface for 1 hour, 12 minutes. In that brief window of time, researchers saw pictures of a rock field and got information back about the moon’s wind and gases on the atmosphere and the surface. Cassini’s (and Huygen’s) discoveries and findings sent back to Earth revealed previously unknown data about their environments and appearances. Some of the achievements include: • Completed first detailed reconnaissance of Saturn’s family of moons and rings. • Delivered the Huygens probe to Titan for the first landing on another planet’s moon. • Discovered erupting geysers and a global subsurface ocean on Enceladus (In 2015, Cassini did a series of flypasts of Enceladus to get more information about the gas and dust in the plumes). • Found clear evidence of present-day hydrothermal activity on Enceladus – the first detection of hydrothermal activity beyond Earth. September 2017  19 Saturn’s largest moon, Titan, passes in front of the planet and its rings in this true colour snapshot from NASA’s Cassini spacecraft. This view looks toward the northern, sunlit side of the rings from just above the ring plane. It was taken on May 21, 2011, when Cassini was about 2.3 million kilometers from Titan. Credit: NASA/JPL-Caltech/Space Science Institute Cassini Mission Quick Facts Cassini Orbiter Dimensions: 6.7m high; 4m wide Weight: 5,712kg with fuel, Huygens probe, adapter etc; (unfueled orbiter alone 2,125kg) Orbiter science instruments: composite infrared spectrometer, imaging system, ultraviolet imaging spectrograph, visual and infrared mapping spectrometer, imaging radar, radio science, plasma spectrometer, cosmic dust analyzer, ion and neutral mass spectrometer, magnetometer, magnetospheric imaging instrument, radio and plasma wave science Power: 885W (633W at end of mission) from radioisotope thermoelectric generators Huygens Probe Dimensions: 2.7m in diameter Weight: 320kg Probe science instruments: aerosol collector pyrolyser, descent imager and spectral radiometer, Doppler wind experiment, gas chromatograph and mass spectrometer, atmospheric structure instrument, surface science package Huygens Probe Titan Release: December 24, 2004 Huygens Probe Titan Descent: January 14, 2005 Huygens’ Entry Speed into Titan’s Atmosphere: about 20,000km/h Mission Launch vehicle: Titan IVB/Centaur Weight: One million kilograms Launch: Oct. 15, 1997, from Cape Canaveral Air Force Station, Florida USA. Earth-Saturn distance at arrival: 1.5 billion km (10 times Earth to Sun distance) Distance traveled to reach Saturn: 3.5 billion km Saturn’s average distance from Earth: 1.43 billion km One-way Speed-of-Light Time from Saturn to Earth at Cassini Arrival: 84 minutes One-way Speed-of-Light Time from Saturn to Earth During Orbital Tour: 67 to 85 minutes Venus Fybys: April 26, 1998 at 234km; June 24, 1999 at 600km Earth Flyby: August 18, 1999 at 1,171km Jupiter flyby: December 30, 2000 at 10 million km (closest approach 5:12am EST) Saturn Arrival Date: July 1, 2004, UTC Primary Mission: 4 years Two Extended Missions: Equinox (2008-2010) and Solstice (2010-2017) Cost of Mission: about $3.27 billion (U.S. contribution is $2.6 billion and European partners’ contribution $660 million) 20  Silicon Chip siliconchip.com.au • Revealed Titan as a world with rain, rivers, lakes and seas. • Revealed Saturn’s rings as active and dynamic – a laboratory for how planets form. • Discovered and then pinned down details about a giant methane lake on Titan. • Discovered 80km-wide landslides on Iapetus. • Took a close-up view of Rhea, revealing a pockmarked surface. • Discovered a huge ring, 8 million miles away from Saturn, probably made up of debris from Phoebe. Cassini reaches Saturn Cassini went into orbit around Saturn on July 1, 2004. On September 27, the spacecraft then moved on to siliconchip.com.au the next, primary, stage of its mission, called the Cassini Equinox Mission. This phase allowed scientists to study seasons and other long-term weather phenomena on the ringed planet and its moons and to continue observations of the magnetic bubble around the planet, known as the magnetosphere. Originally planned to end on July 30, 2008 the mission was extended to June 2010. This studied the Saturn system in detail during the planet’s equinox, which happened in August 2009. The spacecraft’s life was further extended in 2010, with the Cassini Solstice Mission, which concludes with Cassini making its final dive into Saturn’s atmosphere on September 15 this year. The extension enabled another 155 revolutions around the planet, 54 flypasts of Titan and 11 flypasts of Enceladus. Earlier this year, an encounter with Titan changed its orbit in such a way that, at closest approach to Saturn, it will be only 3,000km above the planet’s cloudtops, below the inner edge of the D ring. This sequence of “proximal orbits” will end when another encounter with Titan sends the probe into Saturn’s atmosphere. To say that scientists around the world have been enthusiastic about Cassini (and Huygens) is a massive understatement. While it has been 20 years since launch, they will spend that long again analysing the data! SC September 2017  21