Fullscreen HTML5Med Res (??MB)HTML4

KNOCKIN T I TA N ’ S 8  Silicon Chip siliconchip.com.au A seven-year space mission came to a rousing conclusion in mid-January this year when a spacecraft named Huygens made a soft landing on the moon. So what’s unusual about that? Things like that happen all the time. But this mission was rather special. The moon in question was Titan, which belongs to Saturn, not Earth. by Tom Moffat H NG ON DOOR Courtesy NASA/JPL siliconchip.com.au uygens arrived on Titan with a soft plop onto a sea of slushy half-frozen methane. Many scientists believe this unpleasant goo also existed on Earth around the time life was formed, so anything learned from Titan could be applicable to Earth as well. The Huygens probe spent most of its journey attached to a mother ship named Cassini. Cassini wasn’t designed to land on Saturn or Titan. Instead, it went into orbit around Saturn, taking eye-popping pictures of Saturn and its familiar rings. Many of these photos are now on the JPL website for public download. Cassini will continue to fly for the next four years, making 74 orbits of Saturn and 44 fly-bys of Titan, swooping as low as 1200 kilometers. The Cassini-Huygens mission is a powerful example of cooperation between the scientific bodies of several countries. The Jet Propulsion Lab in the USA is responsible for the design, construction, and management of the Cassini orbiter. The European Space Agency was responsible for Huygens and the Italian Space Agency designed the spacecraft antennas. Creating the antennas was no mean feat, given the tasks they were asked to May 2005  9 the film “The Dish”. Because of the rotation of the earth only a few antennas around the world could see Huygens at any one time. In the Hobart installation, two small probe antennas are placed at the focus of the dish, one for left-hand and one for right-hand polarization. Signals from the probes are amplified and then sent to a very stable maser which locks the receiver’s local oscillator to a 5MHz pilot tone. The resulting intermediate frequency feeds an IF-to-video converter which produces baseband signals ready for recording. Back in the early days, a 2-inch videotape recorder was used for this but nowadays the job is done by a special Mark5 computer containing eight hard drives, each of 200 GigaBytes, for a total of 1.6 TeraBytes. During Huygens tracking, Hobart was using two Mark 5’s for a total of 3.2 TeraBytes. The whole multiple hard drive unit can be lifted out and sent elsewhere for data analysis. VLBI The Cassini orbiter and Huygens probe sitting aboard their Titan IV launch vehicle, prior to lift-off on October 15, 1997. The probe flew past both Venus and Jupiter after launch, their gravity giving the spacecraft a “pull”. perform. Huygens collected data as it dropped down toward Titan’s surface and during and after landing. Data signals were then transmitted to the orbiting Cassini, which re-transmitted them to Earth-based receivers. Given Huygens’ small size, it wasn’t exactly a powerhouse of radio energy. The transmitter power was similar to one bicycle headlight. The distance from Titan to Earth is about 1.2 billion kilometres. The path loss would be astronomical. To overcome excessive path loss it is necessary to add some gain somewhere 10  Silicon Chip in the system. In the case of Huygens, bumping up the transmitter power was not an option. So Cassini received, transmitted and added some much needed gain. But the most useful source of gain was the parabolic dish of a radio telescope. Cassini-Huygens graduated from a multi-nation into a world-wide mission when 17 radio telescopes joined in the tracking of Huygens. Australian participants included the University of Tasmania at Hobart (26m dish), Ceduna (30m), Mopra (22m) and Parkes which, at 64m, was the star of Most news coverage about CassiniHuygens concerns the wonderful pictures the spacecraft have been sending back and the data suggesting that Titan may resemble the cradle of life on Earth. But most interesting from a radio astronomy point of view are VLBI studies (Very Long Baseline Interferometry). This technology allows extremely accurate determination of the position of a radio emission source. A comparison: if someone could organize a table-tennis match on the surface of Earth’s moon, VLBI would allow continuous tracking of the position of the ball as it bounces back and forth between the players. VBLI requires several radio telescopes with a few common features. First, they must be widely separated (the Very Long Baseline part) and their clocks must be synchronised to within the accuracy of a maser oscillator. The output of each radiotelescope receiver must be sent to a correlator. This technique assumes that a radio source is being observed by several widely separated radiotelescopes. Because of their different viewpoints, each sees the object within a background of noise. Somewhere in there is the desired signal. Throw in another radiotelescope, siliconchip.com.au While cruising around Saturn in early October 2004, Cassini captured a series of images that have been composed into the largest, most detailed, global natural color view of Saturn and its rings ever made. and another, widely separated, and each will see what looks like noise. But one spike will appear in the same position from all the telescopes. So radio energy at one common place will keep building upon itself, and eventually, there is a correlated, real signal. The Macquarie Dictionary says: “Correlation = a mutual relation of two or more things”. You won’t find a correlator at every radio telescope site. The correlator used for Huygens lives in Holland, under the watchful eye of JIVE, the Joint Institute for VLBI in Europe. It’s a dedicated 60 TeraOps supercomputer spread over several equipment racks. So you can’t just pick up the correlator and take it to a raw data source. You must bring the data to the correlator. That’s what all those removable hard drive packs are for. The JIVE correlator can process data from up to sixteen radiotelescopes at up to 512 megasamples/second per station. Now they accept that the dish over there on the horizon is in fact a worldclass research instrument. Being asked to participate in Cassini-Huygens has brought it even more prestige. So it was that upon the night of January 14, 2005, radio telescope staff marched through the door carrying three large pizzas. It is traditional, at least within the space program, that the workers must be properly fuelled, just like the rockets, before undertaking a large and complex operation. It appears that radio astronomy is similarly affected. After one final cup of coffee, the staff moved into the radio telescope control room. Leading the operation was Brett Reid, the station manager, Jamie McCallum, a PHD student, and Eric Baynes, technical officer. They went through a formal checklist, then did it again and again. Saturn wasn’t to come over the horizon until several hours later and nobody wanted to waste any of that extra time. Touch this, look at that – call it out – Check! . . . just like the captain of a jumbo jet doing his pre-flight. The operation was carefully scheduled: on Christmas day last year, Huygens was freed from its mother ship Cassini. That night, at 1013 UTC, Huygens would enter Titan’s atmosphere. At 1018 a parachute would pop out, hopefully slowing Huygens to some reasonable speed. A minute later the S-band radio link would begin transmitting. The Hobart operation The Hobart dish and the electronics to drive it were donated to the University of Tasmania by NASA about 20 years ago. It was rescued from the junkyard, having been deemed “surplus to requirements”. The dish was re-erected upon a small hill called Mt. Pleasant, from which it could be seen for many kilometres in every direction. Residents of peaceful towns such as Richmond and Cambridge were concerned that they would be exposed to high power radiation. It took a lot of PR work to convince the populace that the dish was for receiving only – it had no transmit capability at all. siliconchip.com.au The recorded data from many widely-separated radio telescopes are correlated to produce the VLBI image. Brent Carlson, National Research Council of Canada. May 2005  11 The University of Tasmania’s “hand-me-down” 26m Mt Pleasant radiotelescope was one of many stations around the world used to receive the unbelievably weak radio signals from the Cassini/Huygens probe. The object of this part of the operation was to measure the wind speed in Titan’s atmosphere using VLBI. Speeds over 400km/h were expected. Our local encounter with Huygens couldn’t occur until Saturn and its attending moons, rings, and orbiting spacecraft came over the horizon. With the dish’s elevation angle set to the contour limit of about 4°, we lay in wait. At 1019, Huygens’ transmitter switched on, cranking out 3.5W, just before Saturn made its appearance from Hobart. It’s not all head scratching – though there is a lot of that! Here Eric, Brett, and Jamie again go through the checklist. 12  Silicon Chip The hard drive data recorder had already been running for several minutes, having first disgraced itself by crashing while we were all outside admiring Saturn, slowly rising in the Tasmanian sky. By the time the signal reached Earth it was so weak that most radio astronomers felt it would be impossible to detect without the use of VLBI. And there would be no VLBI data until all the removable hard-drive packs had made their journey to JIVE’s correlator in Holland. This, of course, results in a challenge irresistible to radio astronomers: Try to resolve the S-band signal on your own. No VLBI, no help from Cassini. Who would be first? We go into a routine monitoring mode. Is the recorder playing up again? No, looks OK. The right lights are flashing. Eric rolls out a cart-mounted spectrum analyzer and patches it into the system. Its screen is showing lots of green “grass” (random noise). Then, oh-so-slowly, the grass develops a gentle hill. We stare at it until someone says “I think there’s something there”. The hump seems to breathe up and down, like the chest of someone sleeping. With some use of imagination, it is starting to look like an indistinct something or other centred right on Huygens transmit frequency. According to the experts, this shouldn’t be happening. Eric starts tweaking the spectrum analyzer’s gain and bandwidth, and the hump gets bigger. Then it deflates again, like letting the air out of a football. Brett comparing notes with other stations. siliconchip.com.au This is frustrating, and thrilling, all at the same time. Is it or is it not a signal from a billion kilometres away? It’s up again, then it fades away. All station personnel are staring at the screen, willing the signal to build up. Brett decides it’s time to compare notes with other radiotelescopes. Ceduna, Hobart’s sister station, has been off the air with a power failure. Between that and the Hobart computer crash, it looks like the gremlins are intent on spending the evening with us. The mystery continues for several hours as the signal builds up, disappears and builds up again. We phone more Australian radiotelescopes: Can you see anything on S-band? No, can you? It appears we’ve got it on our own, the first and only radiotelescope in the world to detect signals from Huygens. Again, tradition within the space program suggests that, whenever a significant success occurs, control room personnel should whoop and holler and jump around giving high-fives. And so it was with Huygens and its dinky little radio transmitter, heard first in faraway Tasmania. Congratulations all around. It wasn’t long before the European Space Agency mission control got wind of our “success” and Project Manager Leonid Gurvits phoned Hobart for more details. The ESA were in the middle of a press conference for the world-wide media. Are we SURE we’re hearing Huygens? Do we claim to be first? What if we’re not? So we say we’ll check further before making a formal claim. Artist’s impression of the final moments in Huygen’s descent from the Cassini “mothership”. Courtesy NASA/JPL. At 1231 GMT, it was expected that Huygens would land or impact on Titan, depending on the severity of its arrival. And sure enough, right on time, the S-band signal disappears rather suddenly. That’s it, Huygens has suffered a mighty prang, long live Huygens. Then comes a message from Parkes: Are you guys still tracking? It’s really nice and strong here now. . . Oh-oh. That couldn’t be right. . . unless we’ve done something wrong . . . Huygens is supposed to be dead! If it wasn’t Huygens, what was it? At time of writing, nothing was proved one way or the other. There is strong suspicion of an interfering Eric trying to coax a signal from the spectrum analyzer. . . siliconchip.com.au birdie, after a weak carrier on 5MHz revealed itself. This is the maser timing signal running all around the station. But, why did the S-band signal keep fading in an out, with the 5MHz signal remaining steady? Investigations are continuing. . . And for what it’s worth, the honour of being first to hear Huygens went to the Greenbank radiotelescope in the SC USA. Well done! Want to know more about Cassini and Huygens? Visit http://saturn.jpl.nasa.gov – or simply Google “Cassini probe” and you’ll find a treasure trove of information, pictures, video and more links . . . “I think there’s something there”. Is it from Huygens? May 2005  13