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By SAMMY ISREB Picture credit: NASA/JPL The Cassini space probe: unravelling Saturn’s secrets Following its spectacularly successful Mars landing, NASA is readying a spacecraft to probe Saturn and its moons. The Cassini probe, as it is known, should provide new insights into the solar system. 4  Silicon Chip Picture credit: NASA/JPL O N OCTOBER 6TH this year, NASA and JPL (Jet Propulsion Laboratories) will launch their latest space probe, the Cassini, using a Titan IV rocket. This launch will herald the start of an almost decade-long mission designed to explore Saturn and its moons. Many aspects of this mission are ground breaking, as we shall see. And as with other space probes, the Deep Space Network site at Tidbinbilla near Canberra will be involved in the mis­sion. Saturn Orbit Insertion: this is a computer-rendered image of Cassini during the Saturn Orbit Insertion (SOI) manoeuvre, just after the main engine has begun firing. The SOI manoeuvre, approximately 90 minutes long, will allow Cassini to be pulled by Saturn’s gravity into a 5-month orbit. Cassini’s close proximity to the planet after the manoeuvre will offer an opportunity to observe Saturn and its rings at high resolution. The launch The Titan IV rocket that will be used to launch the Cassini probe is immense, with a prelaunch weight of 940,000kg, of which 840,000kg is propellant. But despite the power of the Titan IV rocket, its launch energy is not enough to send the almost 5.5-tonne space probe directly on its way. To overcome this, the probe will first be sent towards Venus and will then use the gravitational field of this and other planets to accelerate it towards Saturn. Initially, the Cassini probe and Centaur upper stage of the rocket will be placed in an Earth orbit. This “stack” Cassini Interplanetary Trajectory: this graphic depicts the planned inter­ planetary flight path beginning with the launch from Earth on 6th October 1997, followed by gravity assisted flybys of Venus (21st April 1998 and 20th June 1999) and Jupiter (30th December 2000). The Saturn arrival is scheduled for 1st July 2004, which marks the beginning of a 4-year tour of the Saturn system. September 1997  5 Table1: Cassini Probe Mission Events Mission Event Date Launch on Titan IV launch vehicle 6th October, 1997 Aphelion 1 (furthest distance from the Sun 1st November, 1997 Perihelion 1 (closest approach to Sun) 23rd March, 1998 Venus 1 flyby 21st April, 1998 Deep space manoeuvre to target Venus 2 2nd December, 1998 Aphelion 2 (furthest distance from the Sun 4th December, 1998 Window for using high gain antenna begins 16th December 1998 Window for using high gain antenna ends 10th January, 1999 Venus 2 flyby 20th June, 1999 Perihelion 2 (closest approach to Sun) 27th June, 1999 Earth flyby 16th August, 1999 High gain antenna can be used from now on 29th January, 2000 Jupiter flyby 30th December, 2000 Science observations begin 1st January, 2004 Saturn orbit insertion manoeuvre 1st July, 2004 Manoeuvre to target probe on Titan 12th September, 2004 Huygens probe separates from Cassini to go to Titan 6th November, 2004 Manoeuvre to target for Titan flyby 8th November, 2004 Huygens probe mission at Titan (approx, 4 hours long) 27th November, 2004 First flyby of Titan, Saturn's largest moon 27th November, 2004 Nominal end of mission (after 11 years) 1st July, 2008 End of possible extended mission Unknown will orbit the Earth unpowered for about 15 minutes until it is in line with Venus, at which stage the powerful Centaur stage will be ignited to provide the final push towards Venus and to enable the probe to escape the Earth’s gravitational field. At the end of its 8-minute burn, the Centaur stage will separate from Cassini. However, before this occurs, the various subsystems in the spacecraft will be activated so that it can operate on its own. As well as this, before separation, the Centaur’s computer will point the Cassini’s high gain antenna towards the Sun. This is done so that the antenna shields the instruments and the avionics from the intense heat of the Sun as the spacecraft approaches Venus. Following separation, communication with the spacecraft will be made 6  Silicon Chip through the 34-metre antenna at the Deep Space Network at Tidbinbilla. This will enable ground controllers at the Jet Propulsion Laboratories to monitor the status of the probe and to send commands to prepare it for its long journey to Saturn. Gravity assist As already mentioned, the Cassini probe is not able to make it directly to Saturn. This problem is overcome by using a “gravity assist” technique four times during the flight: at Venus in April 1998 and again in June 1999; at Earth in August 1999; and at Jupiter in December 2000. During a brief period between the Venus encounters and shortly after the Earth flyby, the heat radiation from the Sun will be low enough to allow the antenna to be pointed towards Earth. This will improve communications with the spacecraft and assist in its navigation. As well as using the gravity assists, the Cassini space probe will also use two types of fuels to get to its desti­nation. The first of these fuels is known as “bipropellant” and is used for large course alterations. Bipropellant is made up of two chemicals, mono-methyl-hydrazine and nitrogen tetra­oxide, which ignite when combined in the engine nozzles. These two chemicals are easy to store and, importantly, they do not freeze at the low temperatures that will be experienced on the mission. The second fuel used is hydrazine. This powers the “Reaction Control Thrusters” and will be used for very brief burns to alter the rotational position of the Cassini. The hydra­zine will only be used in small amounts and engineers are confid­ent that about half the original quantity will remain at the end of the planned mission. The main objective of the navigators at JPL is to keep the spacecraft to the planned trajectory for the entire mission. The navigation team provides the project with predictions of the trajectory of the Cassini probe, the various planets, and Sat­urn’s satellites. Based on this information, the team then deter­mines the trajectory correction manoeuvres (TCMs) that are re­ quired to maintain the preplanned trajectory. Without these many small corrections, the spacecraft would miss Saturn by many millions of kilometres. Tracking techniques In order to plan for TCMs, the navigators use a number of different techniques to track the spacecraft’s trajectory and determine its position. The three methods used are: (1) Doppler, (2) ranging, and (3) optical navigation. The Doppler technique is used to measure the speed that the Cassini is approaching or receding from the Earth and is similar to the Doppler technique used in radar speed guns. Basically, the Deep Space Network antenna sends a signal to the spacecraft which is then directly returned. If the spacecraft is approaching or receding from the tracking station, the fre­quency of the return signal will by slightly higher or lower, respectively. This frequency difference allows the spacecraft’s velocity to be determined ABOVE: Huygens Probe Release – artist’s conception of Cassini orbiter with the Huygens probe separating to enter Titan’s atmosphere. After separation, the probe will drift for about three weeks until it reaches its destination. Equipped with a variety of scientific sensors, the ESA Huygens probe will spend 2-2.5 hours descending through Titan’s dense murky atmosphere of nitrogen and carbon-based molecules, beaming its findings to the distant Cassini orbiter as it flies overhead. Picture credit: NASA/JPL Picture credit: NASA/JPL RIGHT: Huygens Probe Exploded View – the probe has a diameter of 2.7 metres and a mass of nearly 350kg. It contains a heat shield, parachute package, engineering equipment including batteries, and several scientific sensors to measure the properties of Titan’s atmosphere and surface. September 1997  7 succes­sive encounters. The first takes place just after the encounter and is designed to correct any errors in the trajectory. The second and third TCMs are essentially course corrections on the way to the next encounter. In addition to these manoeuvres, there is a large deep space manoeuvre between the two Venus encounters. An additional propul­sive correction manoeuvre is also needed before and after the Jupiter encounters. During the Saturn approach, the optical cameras will be calibrated so that images of Saturn’s satellites can be obtained. A flypast of Phoebe, Saturn’s most distant satellite, will occur some 19 days prior to the spacecraft’s arrival at Saturn itself. Communications Picture credit: NASA/JPL Huygens Descent Profile: this picture illustrates the Huygens probe descent profile, beginning with the initial encounter with the Titan atmosphere and subsequent deceleration. As the probe slows, a small parachute is released which deploys the main probe parachute. Once the parachute is fully open, the deceleration shield is jettisoned and the probe drifts towards Titan’s surface. About 40km above the surface the main parachute is jettisoned and a smaller drogue chute carries the probe the remaining distance. and therefore indicates where the probe is headed. Ranging operates on the principle that radio waves travel at the speed of light. Knowing this, navigators can “fire” radio waves at the Cassini probe and measure the time it takes for them to return. The distance of the probe from Earth can then be calculated. When combined with the Doppler method, this allows the spacecraft’s position and speed to be determined very accurately. The optical data consists of pictures of celestial bodies against a star background, as taken with the spacecraft’s 8  Silicon Chip cam­eras. The measurements extracted from these pictures can then be used to determine where the spacecraft is with respect to every­thing else in the field of view. In many cases, however, optical data is used to determine where the celestial body is rather than the position of the spacecraft. This will especially apply to some of the satellites of Saturn that have unknown orbits. During the early part of the cruise to Saturn, the focus of the navigational team will be on successful planetary flybys. The TCMs required typically involve three manoeuvres between The Cassini craft communicates via a 4-metre high gain antenna, along with two wide-beam low-gain antennas. The craft transmits to Earth at a frequency of about 8.4GHz, while the Earth base stations respond at about 7.2GHz. The radio link provides data transmission rates that vary from a low 40 bits per second, right up to 170,000 bits per second. The signals will take around an hour to reach the Earth from Saturn and vice versa! Back on Earth, the three stations that make up the Deep Space Network, will be used to communicate with the spacecraft. This network consists of three sites spaced around the world: (1) Tidbinbilla, Australia; (2) Goldstone, California (USA); and (3) Madrid, Spain. Before any important data is sent from Cassini, it is first placed into one of two solid state recorders carried aboard the craft. These solid state recorders each have a storage capacity of two gigabits. When enough data has been accumulated and the right conditions prevail, an inbuilt processor (called the Com­mand and Data Subsystem) will transmit the information to Earth. Releasing the probe An important part of the Cassini spacecraft is the Huygens probe, which was supplied by the European Space Agency. This probe carries a well-equipped robotic laboratory which will be used to scrutinize the clouds, atmosphere and surface of Saturn’s moon Titan. It will be released by Cassini in November 2004 and will LEFT: The Saturn System – this montage of images of the Saturnian system was prepared from an assemblage of images taken by the Voyager 1 spacecraft during its Saturn encounter in November 1980. This artist’s arrangement shows Dione in the forefront, Saturn rising behind, Tethys and Mimas fading in the distance to the right, Enceladus and Rhea of Saturn’s rings to the left and Titan in its distant orbit at the top. BELOW: Cassini Spacecraft (with Huygens Probe attached) – roughly two storeys tall and weighing more than 5.5 tonnes, Cassini is one of the largest interplanetary spacecraft ever launched. Three separate antennas – one high gain and two low gain – will enable the orbiter to communicate with Earth. Propulsion for large changes to the orbiter’s trajectory is provided by two powerful 445-N engines. Sixteen smaller thrusters will serve to control Cassini’s orientation in space and make small changes to the spacecraft’s flight path. Picture credit: NASA/JPL drop into Titan’s atmosphere several weeks later. After releasing the probe, the Cassini spacecraft will perform a manoeuvre so that it will be above the probe when it arrives at Titan. This will allow the spacecraft to monitor data transmissions from the probe as it approaches Titan’s surface. As before, the received data will be stored in the orbiter’s solid state recorder before being downloaded to one of the Earth sta­tions. As the probe enters Titan’s upper atmosphere it initially uses a heat­ shield to decelerate. Subsequently, at an altitude of about 175km, the probe deploys its main parachute, jettisons the heatshield and begins its experiments. Fifteen minutes later, it jettisons the main chute, deploys a smaller parachute, and de­scends the last 140km or so to the surface, collecting data all the way and transmitting it back to the spacecraft. As the Huygens probe breaks through the clouds of Titan, an onboard camera will capture pictures of the Titan panorama. Other instruments will directly measure the organic chemistry in Titan’s atmosphere and remotely measure the composition of the surface. Once the mission has been completed, the spacecraft will aim its Picture credit: NASA/JPL antenna towards Earth and transmit the recorded probe data. This data will actually be transmitted twice and will be verified on the ground before it is overwritten in the data recorders. After the Huygens probe has completed its mission, the space probe will set about tackling various other scien­tific missions. The spacecraft carries a number of instruments and the main units and their scientific aims are listed in the accompanying panel. The planned mission will finish in 2008, after spending about four years at Saturn and its moons. By then, the Cassini probe will have collected huge amounts of data over its 11-year mission lifetime and will have provided new insights into Saturn and other SC parts of the solar system. September 1997  9