HELLO, there! How I Dived Through Saturn s Rings COVER STORY

Transcription

1 COVER STORY How I Dived Through Saturn s Rings On 15 September, 20-year old cassini crashed into Saturn. But here s an account of its life & times before it ended its glorious journey. Susheela Srinivas HELLO, there! I am Cassini, the spacecraft you know as the one that reached Saturn and performed some amazing feats recently. I travelled for seven long years, and on reaching the giant planet in 2004, I have been orbiting it ever since. In the course of my travels I have encountered many unique experiences, surprises, and path-breaking observations. Just as you would jot down important events from your life s journey in a personal diary, I too record observations and fi ndings during my travels, although in much more detail. Here are a few excerpts from my personal log that I cherish, and which I am eager to share with you. Hope you enjoy the events as much as I did! How I came to be Conceptualized in the 80s, I was designed for a mission: to reach the sixth planet in the solar system the gas giant Saturn. The task was a humungous one, integrating 5000 scientists and 17 countries, as this mysterious planet is situated in the outer solar system at 1.2 billion km from Earth (at the closest point). However, as you are aware, there is no straight-line path to travel in space. The actual distance travelled is many times more as we have to take gravity assists of nearby planets, moons and sometimes even the Sun, to catapult ourselves into faraway regions. I too had to follow this method to optimize on the fuel aspects and take gravity assists from Venus (twice), Earth and Jupiter. My older siblings Pioneer and Voyager had flown past Saturn in the 70s and 80s, taking a few images during the 14

2 All photographs courtesy NASA & JPL Cassini lifts off on 15 October 1997 Anxious scientists keep a watch on Cassini my circuitry! I took shape under the combined and ambitious venture of NASA, ESA and Italian Space Agency, designed to travel in deep space. At this juncture, I came to realize that I would be accompanied by Huygens, a lander probe, till we reach the gigantic moon of Saturn, Titan; whereupon, I was to offload Huygens, keep a track of its progress, and transmit the data to Earth. From there, I would be a lone voyager as an orbiter for the next many years. My trajectory incorporated a long travel (3.2 billion kilometres), spanning seven years to reach Saturn, and then remain in orbit around the planet till 2017, recording various occurrences on the planet. course. However, I was on an exclusive mission to reach the planet which has no solid surface, enter into its orbit and remain for a substantial time to observe its various aspects. My mission was fraught with many difficult manoeuvres, obstacles, enigmatic events, and mysteries. But I was game for all that! Being born for space travel, adventure runs in 15 October 1997 It is the launch day! Huygens is strapped on me to piggyback; we are spruced up and shielded. We are checked, re-checked. Experts take final measures around the various blinking lights on my system, as we are wheeled into the Cape Canaveral Air Force Station, Florida. From here the Titan IV-B/ Centaur rocket launcher will fire us off into space. Due to the long travel, I will be powered by three RTGs Radioisotope Thermoelectric Generators supplied by Lockheed Martin. In other words, 33 kg of naturally decaying plutonium oxide pellets will be my chief source of power generation for the mission. The entire mission will be managed by the Jet Propulsion Laboratory. 15

3 Saturn as seen from Cassini My Superhuman Senses I take pride in the special equipment I am given: they help me see wavelengths which no human eye can; I can feel magnetic fields to great depths and sense the presence of dust particles too. In total, I am loaded with 12 advanced instruments which survey, scrutinize and collect data of dust particles, electromagnetic spectrum, study the magnetosphere, observe the planet s aurorae, study the surface of its moons and at large, stare at the planet continuously to catch any unknown activity. My Competent Lieutenant My faithful companion the Huygens Probe has instruments contributed by different countries under the European Space Agency and assembled by Alcatel. This is a unique instrument developed to land on Titan s surface. Shaped like a shellfish, this 9-foot-wide, 318-kilo machine has an outer shell protecting the delicate inner equipment. The shielding was necessary during the probe s descent through Titan s harsh atmosphere, and also to survive for the 72-minute duration on the surface. Venus Flyby By 25 April 1998, I arrive at Venus for a gravity assist (use of the relative movement and gravity of a planet or other astronomical object to speed up a spacecraft, typically to save propellant, time, and expense). Gaining an acceleration of 7 km/sec, I forge ahead into space. By 24 June 1999, I complete a round trip around the Sun and again approach Venus for the second assist, accelerating further. Critical Earth-Moon Flyby I have clocked nearly two years of travel time now. I approach Earth for a speed boost and this will be my last glance at it. I am just 1100 km above the South Pacific Ocean. This phase is critical as I have to manoeuvre with precision. In the event of a failure, I will be jeopardizing thousands of lives on Earth, mainly due to the radioactive fuel I am carrying. But, this is a less than one-in-a-million chance. Haze on the horizon Phew! All goes off smoothly and I get a further speed boost. Now I am heading towards the Asteroid belt. I am the seventh spacecraft to enter this region, and it is considered to be pretty non-hazardous for us. However, since I am passing through, I whirr my Cosmic Dust Analyzer (CDA) to study this region too. I take five months to pass through here, and now it is April Glancing at Jupiter It is the end of 2000, and I am within 10 million km of Jupiter. I wave to Galileo who is exploring here. We team up to take some amazing images of Jupiter. That done, I move on towards my final destination. Approaching Saturn Two years have passed by and I am nearing Saturn. In 20 months I will be there. It is time for me to test my camera equipment. I capture fascinating images of the ringed planet. With mounting curiosity, I speed towards the destination. A Storm is Brewing! With just three months to reach, my cameras detect two huge storms on Saturn, gradually merging into one. A rare phenomenon observed only once earlier. Huge storm churning across the face of Saturn May-June 2004 Inching closer, I am on high alert and gazing into space. My powerful vision detects two new moons of Saturn! Named Methone and Pallene, though tiny, they did not escape my expert eye. From now, Saturn has 62 moons on record. 30 June 2004 The long and arduous journey has culminated and I now arrive at Saturn officially. There is jubilation in the tracking centre. But I cannot stop for a moment. I enter the orbit of the planet and begin the constant vigilance of its activities. I m now heading toward Titan. Goodbye, Dear Friend! It is Christmas Eve of I have successfully detached Huygens probe towards Titan. Huygens will be all alone for the next three weeks, exploring Titan and landing on it. I am only on standby mode to receive and beam back data. In a descent lasting nearly two-and-a-half hours, Huygens 16

4 A view of Saturn s Northern Hemisphere (left) and Southern Hemisphere (right) has successfully deployed its parachutes and landed on Titan. From here he will survive another 72 minutes before bidding farewell. Well done, my friend! You are the only probe to have travelled to the outer solar system and landed on a surface. The spectacular images of Titan captured by you will open new insights about this moon and scientists would be thrilled by the Earth-like meteorology and geology it holds I am diligently carrying out the mission. Observing the moons, I discover rare sights, the presence of water and surprising facts. I peer closely at the rings of Saturn, revealing previously unknown bands. The images I capture reveal many wonders and when scientists look at these insightful findings they will get clues to long-standing mysteries. I even perform the closest flyby of any of Saturn s moons just 25 km away! During this, my instruments expose directly to the plume, sampling the gases and dust particles. I catch one of Saturn s equinox: an event when the sun rays fall tangentially on the edges of the rings. It is then the rings seem to disappear altogether! I capture rare images of clumps in the rings and utilize that moment too to beam back valuable spectral and engineering data. Coloured clouds of Jupiter photographed by Cassini Mission Extended Due to my path-breaking achievements, my mission gets an additional seven-year extension! Now I will be looking at seasonal changes on Saturn till 2017, dubbed as the Cassini Solstice Mission I make a slight change in the orbital angle. This enables me to take a better look at the rings. In July 2013, this angling enables me to take a faraway glance at Earth from between the rings of Saturn. I capture the first interplanetary portrait and freeze it to posterity. By end of March in 2016, I return from the high angled plane and synchronize with the equatorial plane of Saturn. From here I begin a series of 22 highly inclined orbits which will take me close to the poles and help better understand the F-ring of Saturn. 26 April 2017 Historic Dive I am now on the brink of the first daring dive between Saturn and its innermost ring. This Grand Finale is a historic mission as no spacecraft has flown this close to the gas giant ever before. The allotted gap between Saturn s cloud top and its innermost ring is about 2400 km. I have to dive through this narrow gap without entering the planet s atmosphere. The region was estimated to contain smoky dust particles and free of larger particles which could be potentially hazardous to my sensitive instruments. Earl Maize, my project manager at JPL assures me, We expect the gap to be free of particles large enough to damage the spacecraft. However, the antenna will act as a protective shield. Based on this fist pass, we determine whether it is safe to expose the science instruments for future passes. I brace myself As a protective shield, I use my 13-foot-wide dish-shaped high-gain antenna, directing it towards the particle stream coming from the rings. The associated risk is that I would be losing all contact with Earth during the ring-plane crossing. It is inevitable, as I have to capture images of the planet s surface from up-close. I dive through the gap and come within the narrow window from the cloud tops and within 300 km of the innermost visible rings. Here I encounter a pressure comparable to atmospheric pressure on Earth at sea level. Throughout I have lost all contact with Earth. The tracking scientists at the mission control centre are on the edge of their seats all through this. This historic dive lasts 20 hours, at the end of which I emerge victoriously. Now I am ready to point the antenna towards Earth and re-establish contact successfully. I send spectacular close-up images of Saturn. Surprisingly the dust particles don t pose a threat to me. From now I begin a series of 21 dives till the end of the mission. 17

5 This dive and the corresponding imagery will give the scientists valuable data to fine tune my further dives. Each week from now, I make similar passes to record valuable data. Au revoir! By September 2017, it will be 20 years since I left Earth on this mission. I can proudly look back and say I was successful in completing the mission requirements. No doubt, the images, data and information beamed back by me are valuable for scientists to solve many puzzles of our solar system. And who knows, what fascinating worlds lurk behind the images I photographed? Now I am in the final stages of the mission. As per protocol, the last plunge will be on 14 September 2017, and into the atmosphere of Saturn where I will burn and disintegrate. This is a necessary precaution and understood at the beginning of the mission that we self-destroy ourselves to prevent space contamination. Perhaps by the time you read this, I would have taken the final embrace. Farewell, then! Ms Susheela Srinivas is an Engineer, Entrepreneur and Writer. She contributes to publications in India and abroad. Address: #189, I F cross, 3rd Stage, 4th Block, Basaveshwaranagar, Bengaluru ; sushsri@gmail.com Cassini s Advanced Equipment 1. Optical Remote Sensing: Used to capture images of Saturn s electromagnetic spectrum, study its rings and the moons. Composite Infrared Spectrometer (CIRS) Imaging Science Subsystem (ISS) Ultraviolet Imaging Spectrograph (UVIS) Visible and Infrared Mapping Spectrometer (VIMS) 2. Graphical Instruments to Study Fields, Particles and Waves: These continuously monitor the environment around the spacecraft and measure the magnetic fields, mass, electric charges and densities of atomic particles. In addition, they also measure the composition of dust particles, plasma, and radio waves. Cassini Plasma Spectrometer (CAPS) Cosmic Dust Analyzer (CDA) Ion and Neutral Mass Spectrometer (INMS) Magnetometer (MAG) Magnetospheric Imaging Equipment (MIMI) Radio and Plasma Wave Science (RPWS) 3. Microwave Remote Sensing Radar Radio Science Subsystems (RSS) The Huygens probe The Huygens probe had the following modules: The Entry Assembly Module: contains equipment to guide and control Huygens after separation from Cassini. Along with it, a heat shield acts as a brake and thermal protection. The Descent Module: contains the scientific instruments and three parachutes (sequentially deployed) to control and guide the probe onto the surface of Titan. Huygens Atmospheric Structure Instrument (HASI): these were a group of sensors which measured the physical and electrical properties of Titan s atmosphere. 18

6 Doppler Wind Experiment (DWE): mainly installed to measure the wind speeds during the probe s descent through Titan s atmosphere. Descent Imager/Spectral Radiometer (DISR): this instrument was capable of imaging and spectral observations using a number of sensors in a wide field of view. Gas Chromatograph Mass Spectrometer (GCMS): A versatile instrument capable of measuring gaseous elements and also identifying new ones in Titan s atmosphere. Aerosol Collector and Pyrolyser (ACP): Collected aerosol samples from the atmosphere through a system of filters, pyrolysed them (a process of separating volatile compounds and heating in an oven) and decomposed organic compounds for analysis. Surface-Science Package (SSP): a set of sensors designed to analyse the surface of Titan at the point of impact or landing. Did you know? Saturn is called The Jewel of the Solar System. The ring-like bands around it give it the mesmerizing look and the fascinating name. Saturn is all gaseous substances, mainly helium. Hence it does not have a solid land-like surface. The beautiful rings of Saturn are not solid either; they are made up of ice, dust, and rock; the sizes varying as much as a grain of sand to a tall building. These rings are hardly a kilometre in thickness. Great winds and storms are part of Saturn s atmosphere. The wind speeds can be as high as 1800 km/hour. Storms last for months or years. One Saturn year is about 30 Earth years (and Cassini stayed near Saturn for nearly half a Saturn year). Due to fast spinning, Saturn days are around hrs. In addition, this causes an oblate (flattened at the poles and bulged at the equator) look to the planet. Saturn gives off twice as much as heat than it receives from the Sun. This is mainly because of the high core temperatures. Earlier it was thought that Saturn s rings formed later than the planet. However, information collected by Cassini indicates that the bands existed from the birth of the planet. Cassini-Huygens Records of Saturn s Moons Two new moons Methone and Pallene were discovered in this mission. Saturn has 60 moons till date, the sizes of which range from a small football field to as big as Mercury. Enceladus was found to have jets of water spewing from its interiors to the surface. The jets were found to contain water, heat, and methane all indicating the high probability of holding life forms. These are theorized as being similar to the hydrothermal vents found near our ocean beds, which are rich sources of life forms. Another natural satellite of Saturn Hyperion is the only known one, besides Earth s moon, to have a statically charged surface. Saturn and its moons have a strange inter-collaboration: the moons contribute to the rings and magnetosphere (in the form of matter from icy jets) and in turn collect material from this region. Cassini-Huygens gathered direct samples of Saturn s largest moon Titan. This is the only heavenly body besides Earth where a liquid form is found on the surface. Though Titan is outside the habitable zone in the solar system, this icy world has frozen water but essential compounds of methane and ethane are available in liquid form. Plumes of icy jets of water, dust, and methane spew into the Saturn s rings. These hydrothermal vents originate from Enceladus subsurface oceans, finding exits from surface fissures or cracks called tiger stripes. This mission has gathered and continues to obtain more and more data on these moons which source chemicals for life to exist. 19