Report From the Red Planet
The Sojourner rover starts its exploration of the surface of Mars after
the exciting landing just a few kilometres from the aim point on the 4th
of July 1997 in the Ares Vallis area, at 19.5° north. This is
about 850km from the Viking Lander 1 which landed in 1976
The Mars Pathfinder operations team is continuing its efforts to reestablish communications with the Pathfinder lander. Now renamed the Sagan Memorial Station. Although they are experiencing communications difficulties, the team is confident that the spacecraft is still operating on the surface of Mars, according to Mission Manager Richard Cook. The last time they were able to send a command to the Pathfinder lander instructing it to transmit a signal back to Earth was on Sol 93, which was Tuesday, October 7, at 7:21 a.m. Pacific Daylight Time.
Team members suspect that the spacecraft may not be receiving commands from Earth properly because the lander's hardware has become much colder than normal. In regular operations, when the lander's transmitter is turned on, spacecraft hardware warms up sufficiently to operate normally. Since the transmitter has not been on for several days, engineers suspect that temperatures within the lander are considerably colder than normal.
Predicted internal temperatures drop to as low as -50°C (-58°F) in the early morning and only rise to about -30°C (-22°F) in the late afternoon. These temperatures are about 20°C (38°F) colder than the coldest previous operational temperatures. The lower temperatures cause the spacecraft radio hardware to operate outside the range of radio frequencies that ground controllers have used in the past. During the past three weeks the operations team has been transmitting to the spacecraft at a lower frequency and sweeping through a wider frequency range, a technique that has been used on other missions to attempt to cause the spacecraft receiver to lock on to the transmitted signal. Once ground controllers finish this, they send commands instructing the lander to turn on its transmitter and send a signal back to Earth. To be certain that they investigate all possibilities, team members are also consulting with experts knowledgeable about the radio and other key elements of the spacecraft. They have identified some new scenarios that are being pursued to regain communications.
These recommendations include doing more testing of the engineering model hardware in the laboratory to better understand how the spacecraft might be behaving. Another recommendation has suggested shifting and increasing the range of frequencies being swept through much more than previously attempted.
According to Project Manager Brian Muirhead, the possibility exists that an unrecoverable problem may have occurred. Team members expected that, once the lander's onboard battery died, cold and thermal cycling could result in a failure of some other element of Pathfinder and thereby end the mission.
However, the Pathfinder project is funded to continue operations until August 1998, and the team will continue to do everything possible to reestablish communications until all options have been exhausted, Muirhead said.
The mission has already exceeded all of its goals in terms of spacecraft lifetime and data return. The science team, meanwhile, continues to process and analyze the large volume of data sent back by Pathfinder's lander and rover. Further science products are planned and new results will continue to be presented as they develop. The team will continue its daily uplink sessions with Mars Pathfinder.
21 October 1997, 8:30 a.m.
At this time, the status of the Mars Pathfinder mission remains the
same. We are attempting to regain communications with the lander, but have
not seen a signal since October 7th. We have high confidence, however,
that the spacecraft is still functioning, but the extreme cold temperatures
of the lander is preventing communications. We are pursuing several courses
of action which should allow us to recover and continue normal operations.
to commitments to the Galileo mission we moved our operations to the
34 meter antenna and commanded the spacecraft to turn on the SSPA transmitter. The SSPA is a backup transmitter in addition to our primary and auxiliary
transmitters. Unfortunately, we did not receive any signal during this
second attempt. We will continue with our efforts to reestablish
contact with Pathfinder and will promptly post any favorable developments.
Doesn't Time Fly
When You're Enjoyin Yourself
By Mike Frost
As with my previous story, "The Interplanetary Dangerous Sports Club", I have tried to stick to known physics - see Adrian Berry's column in the October '96 Astronomy Now, for example. But I leave most of the practical details as an exercise to the reader.
"Bloody Students!" said the red-faced man beside me in the bar. "Coming in here noisy and drunk. Haven't they got their own bar?" "It's closed for refurbishment" I said by way of explanation, but he wasn't listening. "I thought they all were supposed to flat broke - how come they have the money to get drunk every night? It's my taxes they're spending, you know! And on what?" "I think they're mostly research students..." "Research!" he snorted, "Research!! Pah!.. Just wasting government money. See that guy over in the corner?" I half-looked but I wasn't paying attention. "He was in here the other night telling everyone he'd been bungee jumping into a black hole. Bungee jumping, I ask you, on my taxes. Makes you want to write to the newspapers..." But I wasn't listening. "Clive! Is that you?" The bungee jumper looked up from the table football. "Frosty! Good to see you.." He sauntered over, clutching an empty glass. I was expecting to have to buy him a refill but to my surprise he bought me a drink. He even offered the red-faced man a refill, but he shook his head. "You're looking in good shape," I said, "you don't look a day older than when we last met." "Don't exaggerate," he said, "I'm at least week and a half older." "What! It must have been three years ago!" "Well, it might have been for you - but remember, I've been bungee jumping on to an event horizon." I was beginning to figure him out. "Oh... gravitational time dilation..." "Precisely. Didn't tell us about that, did you?" "Well I can't think of everything... So what happened?" "Simple enough. We put the spaceship into a really tight hairpin orbit taking us as close as we dared go to the surface of M87's black hole. As we were falling in, we clipped our bungee ropes to the spaceship, then right at perihole, leapt out and hurtled down towards the event horizon. It was great! The distortion of light is amazing. You get multiple images of the whole sky! There are light beams orbiting the black hole. Pity we had to get out so fast!" "Yeah, I bet," I said admiringly, "but you were in for a surprise when your jump finished." "Exactly," he said, "we got back into the spaceship, which was already blasting out at maximum acceleration. And when we got back to the space port, we discovered three years had gone by!" He turned to old red-face. "Time runs more slowly close to a black hole you see. For us the whole trip was no more than twelve hours, but in the outside universe three years passed." "Three lost years," I said, trying to come to terms with his experience. "Well, look on the bright side," Clive said, "I had three years worth of grant waiting for me when I got back." The red-faced man exploded, "WHAT!! Three years grant for doing nothing! Why don't they stop it if you don't produce any work?" "Oh, is that one of the conditions?" Clive said absently, "I never read the small print." Then he grinned goofily. "Only kidding! I got a bonus, actually. I had the foresight to cover myself with instruments when I jumped. I looked like a christmas tree! We got the first ever readings from the vicinity of an event horizon, worth their weight in gold. Watch out for the paper in Nature!" I reckoned that if Clive carried on he stood a good chance of being thumped. I tried to change the subject. "So what's your next adventure going to be?" Clive took a long sip and wiped his mouth on his sleeve. "Supernova surfing". It was new to me too. "You'll have to explain, Clive." Clive was pleased he'd picked on something I didn't know about. He tried to look knowledgeable. "Supernovas happen when an ordinary star reaches the end of it's life, right? All its hydrogen and helium and carbon and diesel and stuff has been burned up, except for the bits in the outer atmosphere that haven't had the chance to ignite. But inside the star there's nothing left to burn, so there's no radiation pressure to hold up the star. Only gravity, causing the whole of the star to collapse in on itself. Now, for little stars, when this happens, you end up with a white dwarf star, which just fizzles out. And for really big stars, if you're really lucky, you get complete collapse into a black hole. But if you get just the right size of star, just as the star is about to collapse down to nothing, the unburnt stuff which has just fallen in from the atmosphere ignites. BOOM! A supernova." In his attempts to demonstrate a supernova explosion he showered several people with beer. I motioned to him to calm down. "And you're going to try and surf this?" He nodded vigorously. "Hang on in there in the outer atmosphere, pick the right moment, catch the shockwave, and ..... wheeeeee! Off we go!" Despite himself, the red-faced man was intrigued. "What sort of speed will you reach?" Clive tried to look cool, "Ninety nine point nine nine nine nine nine nine nine nine nine nine nine nine nine nine nine nine nine nine nine nine nine five percent of the speed of light. Fast - eh! We'd like to go that extra 0.0000000000000000000005 percent faster, but unfortunately only photons can do that - apparently we're not light enough." I was trying to get my mind round 99.9999999999999999999995 percent of the speed of light, but I couldn't. "How long do you intend surfing for at that speed, then?" "Oh, only forty days or so" I was surprised. "You won't get very far in forty days". But Clive was looking pleased with himself, so I figured I had a surprise coming. "Forty light days out from a star, you've hardly left the stellar system - I'm surprised you'd give up so early. Hardly worth getting up for!" But I was right. Clive was as pleased as punch. "Oh Frosty, you forgot time dilation again. It's getting to be a habit." "Ohhhhhh ..... so for you, forty days takes you to ...." Clive failed completely to look cool. "...twice the distance to the edge of the universe." Old red-face threw up his arms in disgust. "Oh, I'm going! How can you go past the edge of the universe?" "The observable universe, sorry." Clive looked cocky. "Of course, we don't know what's waiting for us there - well, if we did it wouldn't be past the edge of the observable universe. We don't even know if the laws of physics will be the same wherever we turn up." I remembered something from my cosmology lectures. "I think you might be in luck there. There's something called cosmological inflation; it happened immediately after the Big Bang, and it smoothed out the properties of the universe - that's why it all looks homogeneous for as far as we can see. But the point is that the smoothing, we think, carries on beyond the boundaries of the observable universe. We don't know for certain, of course, but our best guess is that if you don't go too far beyond where we can see now, physics will still function as it does in this neck of the woods." Clive pretended he already knew all this. "Like I said, we're only going to surf for forty days. Then we open the parachute, slam on the brakes, come to a halt and observe the new bit of universe we've ended up in." "..and as for getting back..." "Well, like you said, the universe should be much the same out there as it is here. So we find ourselves a supernova with the fuse burning, throw ourselves into it - and surf back! Forty days later we're back where we started. Of course," he looked shiftily, "we have to make sure we're blown out in the right direction. That might be a bit tricky." The red-faced man snorted. "This is ridiculous. You're telling me that you're going to hop into a supernova - and forty days later, you've left the observable universe. Then you find another supernova - and forty days later - you're back again! It's preposterous. You're going faster than light." "Ah, no, that's not quite correct." I said, "Time dilation again, you see; time goes slower as you approach the speed of light. It might only be forty days for Clive - but for us it's a little bit longer." Red-face snorted again. "Exactly how much longer." Clive muttered into his pint. "Twenty thousand million years." Even I was surprised at that one. "Twenty thousand million!!" Then I started mentally doing the sums... "Oh yes, you're right. It's twenty thousand million light years to the edge of the observable universe, give or take, so it's that sort of time if you make the journey, and ...." Clive perked up "...and imagine what twenty thousand million years of grant money looks like!" "Oh, I'm not listening to any more of this rubbish!" said the red-faced man and finished his G&T, "You're talking complete garbage!" "Well it might sound a bit weird," I said, "but it's all part of special relativity. Time slows down as you approach the speed of light. Just as general relativity predicts that time slows down close to a black hole." "Answer me this then," he said, putting his glass down, "everything's relative, right. So when your friend goes whizzing off at ninety nine point whatever the speed of light, as far as he's concerned, it's actually us who are going away from him." "Well...." "And when he comes back from his holidays, it's as though he's not moving at all, and we're speeding toward him..." "Yes, but ..." "So why isn't it Speedy Gonzales here who gets twenty thousand billion years older, and us who only age by forty seconds? It's all complete nonsense, do you hear, complete nonsense!" And with that he lurched off his bar stool and off across the floor. Clive looked at me with growing puzzlement. "He's right you know. How come it's me that time slows down for?..." I sighed. "If only he'd let me explain. It's called the twin paradox - one twin on Earth, one twin on a journey. The one on Earth ages most and the one in space least. But, you see, the situation isn't symmetric. The space traveller undergoes acceleration - twice, once at each end of the journey. If you like, it's the acceleration up to speed that slows down time." Clive looked dubious. "Well, I'll take your word for it. But there's something that worries me." "Well, you have every right to be worried about the acceleration you'll have to undergo. Nought to ninety nine percent of the speed of light in one second is going to squash you like a pancake." Clive shrugged. "No, that's OK, I've been on the Pepsi Max at Blackpool - I can handle acceleration! What was really worrying me was something you said earlier." "Like what?" "That inflation thing." "Oh, the cosmological inflation. Well that saves your bacon, tells you something about space even beyond where we can observe..." "Well, that's all very well, but..." "But what?!" He looked despondent. "The inflation. Won't it devalue my savings?"
The Cassini/Huygens Mission
by Ivor Clarke (adapted From an ESA Bulletin Article)
On October 16th 1997, an American Titan lV Iaunch vehicle carring the
robotic spacecraft called Cassini, lifted off from Cape Canaveral in Florida.
Weighing more than 5.5 tons at launch, it begin the long journey of many
years to the vast and exciting realm of Saturn. The spacecraft is too heavy
to be injected directly onto a trajectory to Saturn and so will require
the boosts of several planetary 'gravity assists'. The interplanetary
trajectory therefore includes fly-bys at Venus (in April 1998 as well as
June 1999), Earth (in August 1999) and Jupiter (in December 2000).
As a result, the October launch opportunity allows Saturn to be reached in about seven years. Cassini should reach Saturn with sufficient propellant to brake into orbit around the planet and accomplish its mission: to deliver the European Space Agency's (ESA) Probe Huygens to the large, hazy moon Titan, and then tour the Saturnian system for nearly four years. The Birth of the MissionVoyager-1 indeed confirmed the uniqueness of Titan in the Solar System. The chemical composition of Titan is very similar to that of the primordial Earth 4.5 billion years ago. The major difference between the two bodies is that Titan is much colder than the Earth; this cold temperature prevented life from appearing on Titan. Study of the organic chemistry on Titan will, however, provide important clues regarding the prebiotic chemistry that was at work on Earth after the birth of the Solar System.
Huygens is the ESA element of Cassini/Huygens, the joint NASA/ESA planetary mission to the Saturnian System. Titan, the largest of the moons of Saturn, is a major target of the mission. The Saturn Orbiter is provided by NASA, with a significant contribution from the Italian Space Agency.
The Cassini mission is named in honour of the French-italian astronomer Jean Dominique Cassini, who discovered the prominent gap in Saturn's main rings (now called the Cassini Division), as well as the icy moons Iapetus, Rhea, Tethys and Dione.
The Probe that Cassini will deliver to Titan is named after the Dutch scientist Christian Huygens. By using improved telescope optics, he found in 1659 that the strange 'arms' noted decades earlier by Galileo were actually a set of rings. Whilst observing Saturn, Huygens also discovered the moon Titan, and hence the choice of his name for the ESA Probe. The presence of an atmosphere around Titan was suggested by the Spanish Astronomer Jose Comas Sola in 1908, but was only confirmed in 1944, when an American astronomer of Dutch origin, Gerard Kuiper, discovered gaseous methane at Titan through spectroscopic observations. Titan's atmosphere was then thought to be mostly composed of methane, until Voyager-1 revealed that nitrogen is its major constituent, as on Earth. Voyager also confirmed the presence of gaseous methane, but in concentrations of only a few percent.
Voyager-1 flew within 4000km of Titan and radio soundings made during the flyby allowed the moon's atmosphere to be probed down to its surface. Among other things, this allowed the radius of the moon#s invisible surface to be measured precisely, while analysis of the spectroscopic and radio-sounding data provided the moon's main physical parameters.
Considering the size of the solid body itself, Titan is the second largest moon in the Solar System after Jupiter's Ganymede, and is larger than the planet Mercury. If, however, its thick atmosphere is included (the 1mbar pressure level is at 200 km), Titan can be considered the largest moon in the Solar System.
Unlike most of the other newly observed Solar System bodies, the wonders of which have been revealed from imagery, Titan, in the images taken by Voyager, showed a featureless orange face. Most of the excitement during the Voyager encounter was produced by the infrared spectroscopic observations, which revealed methane, ethane and several more complex organic molecules in Titan's atmosphere. Hydrogen was also detected in appreciable quantities (0.2%). The confirmation of the suspected presence of hydrogen cyanide (HCN), which is a critical building block for the more complex molecules of life, and therefore of great significance to exobiologists, confirmed the unique nature of Titan in the Solar System.
The Huygens Mission
Huygens' encounter with Titan is actually planned for 27th November 2004. Huygens will be released from the mother spacecraft, the Saturn Orbiter, after the seven-year-long interplanetary journey. The Orbiter will act as a relay station and the Huygens Probe's data will be received via the Orbiter's High-Gain Antenna, which will be pointed at the Probe throughout its mission. This configuration will preclude a simultaneous real-time link to Earth from the Orbiter and so the Probe data will be stored on the Orbiter's two solid-state recorders for later transmission to Earth.
Huygens main mission phase, which is designed to last 2 to 2.5 hours, is to plunge into Titan's atmosphere and parachute a fully-instrumented robotic laboratory down to its surface. During its descent, scientific measurements will also be performed during the entry phase, which will last just a few minutes. It is hoped the Probe resources will allow scientific measurements even after it has impacted on Titan's surface for at least a few minutes. The Probe's radio link will be activated early in the descent phase and the data will be relayed to the Cassini Orbiter for on-aboard storage and subsequent transmission to Earth. Once this valuable data have been safely received on Earth, the Orbiter will begin using its three dozen scientific sensors intensively to examine the vast Saturn system.
Scientists consider Titan as a deep-frozen Earth. In the wake of the excitement about the Voyager-1 observations (and those made by Voyager-2 six months later), several European scientists proposed a new mission to revisit Saturn and in particular Titan, in collaboration with the Americans. So the Cassini mission was subsequently born.
This vast joint project was started In June 1982, when the Space Science Committee of the European Science Foundation and the Space Science Board of the National Academy of Sciences of the USA set up a Joint Working Group (JWG) to study possible cooperation between Europe and the USA in the area of planetary science. One of the recommended missions was a Saturn Orbiter and Titan Probe mission. By September 1990, proposals for the Saturn Orbiter and the Huygens Probe were finalised. The technical and programmatic challenges facing the design team where formidable, for the Huygens Probe is ESA's first planetary atmospheric entry mission and some of the technologies required are very different from those needed for more traditional satellite missions. This presented a considerable challenge both to the Agency and to European Industry. Special systems such as the Thermal Protection System (TPS) and parachutes had to be developed specifically for the entry into Titan's atmosphere.
The Huygens Probe System and its mission
The Huygens Probe System consists of the Probe itself and the Probe Support Equipment (PSE). The Probe will descend onto Titan while the PSE remains attached to the orbiting Cassini spacecraft. The PSE consists of the electronics necessary to track and recover data from the Probe during its descent and to process this data for delivery to the Orbiter, from where it will ultimately be downlinked to the ground. The PSE also provides a command and data link to the Probe whilst the latter is attached to the Orbiter on route.
During launch and subsequent cruise phase, the Probe is attached to the Orbiter and will remain in a dormant state for the next seven years as it follows its cruise trajectory via Venus, Earth and Jupiter, before finally entering the Saturnian system in 2004. Dormant that is, except for its scheduled bi-annual health checks. These checkouts will follow the pre-programmed descent-scenario sequences as closely as possible and the results will be relayed to Earth for examination by Probe system and payload experts.
The Huygens system consists of two major parts, namely the outer 'shell', consisting of a front shield and back cover which protects the Probe from the high heat fluxes that it will encounter during Titan atmosphere entry, and the inner descent module which will carry the experiment complement through the cold Titan atmosphere. Since the Probe is carried to its destination by the Cassini Orbiter the interface between these two Spacecraft is also very important.
Since the Probe has to go from a very hot environment at the Venus flyby to a very cold one at the interplanetary distances. The mechanical system is dormant during this long cruise period.
On reaching Saturn, the spacecraft will fly through the rings of Saturn. The Cassini spacecraft will then enter a highly eccentric capture orbit around the planet. About 100 days after this event, a trajectory-correction manoeuvre will be initiated to raise the periapsis and target Cassini for its encounter with Titan. About 22 days before Titan encounter - and 60 days after the trajectory-correction manoeuvre - the Probe will be spun up and released from the Orbiter.
Two days later, the Orbiter will initiate a deflection manoeuvre to avoid Titan and optimise its position to set up the radio- communication geometry necessary for the acquisition of Probe data during its descent phase. This manoeuvre will also set up the initial conditions for the satellite tour after the completion of the Probe.s mission.
Into Titan's Atmosphere
Prior to the Probe's separation from the Orbiter, a final health check will be performed and the coast timer will be loaded with the precise time necessary to 'Wake up' the Probe systems 15 minutes prior to encountering Titan's atmosphere. For the next 22 days the Probe will simply coast to Titan with no changes imposed on the attitude parameters acquired at separation and with no systems active except for its Wake-up Coast timer.
At the end of its 22-day coast period, the Probe will be switched on via its triple-redundant coast-phase timer. After system initialisation, the payload instruments will be activated in a pre-programmed sequence; broadcast data will be distributed to the instruments, containing such information as spin rate, time, temperature, altitude and special-event flags. Whilst on the other side scientific data will be collected and packetised for relay back to the Orbiter.
The approach velocity of the Probe will be 6km/s and hitting the moon's atmosphere at this speed will produce the most dramatic environment of the entire mission for the mechanical system! The front shield is covered by tiles of a material that is essentially a Bow-density 'mat' of silica fibres. The tile thickness on the front shield is calculated to ensure that the structure will not exceed 150°C. This will ensure that Probe internal temperatures do not rise to high values during entry.
This entry phase will last about 3 minutes, during which the Probe's velocity will fall to around 400 m/s. The Probe's impact on Titan's atmosphere (mostly nitrogen, with some methane and argon) will cause a shock wave to form in front of the 2.7m diameter front shield. The plasma in the shock, just forward of the shield, will reach a temperature of around 12,000°C. Simultaneously, the deceleration force on the Probe will reach its maximum of around 16g.
At the end of this entry phase, a sequence of mechanical events will be triggered which are probably the most critical of the entire mission.
Three parachutes are to be used during the Probe's descent. The pilot parachute, of about 2rn diameter, will first be ejected when the on-board accelerometers detect a velocity of Mach 1.5 near the end of the deceleration phase, a command will be sent that causes the pilot chute to deploy. The pilot chute will then pull away the back cover, which is attached via a lanyard to immediately deploy the main parachute. The main chute, which is some 8m in diameter, will then deploy, rapidly slowing the Probe down and stabilising it through the transonic region.
When the front shield is released, the main parachute will pull the descent module away. This main chute is sized specifically to slow, during a period of about 30 seconds, the Probe's velocity from Mach 1.5 to Mach 0.6. The front shield will then be released and the descent module will descend slowly below the main parachute for about 15 minutes while initial scientific measurements are made. Thereafter, the main parachute will separate from the Probe and release a smaller stabiliser chute, causing the Probe to descend faster. During the last 20 km of the descent, the Probe's height above the Titan surface will be measured by a radar altimeter. The total descent time from deployment of the first parachute at some 170 km above the Titan surface will be 2.5 hours. Because of the thick atmosphere and low gravity conditions on Titan the Probe will sink slowly towards the surface.
One experiment, the Aerosol Collector, is designed to collect atmosphere gasses for chemical-composition analysis. The instrument is equipped with one deployable sampling device whIch will be operated twice in order to collect the aerosols in two layers; the first taken from the top of the atmosphere down to about 40km, and the second sample from about 23km down to 18km.
The Probe Transmitter will provide a very stable carrier frequency for the Probe-to-Orbiter radio link; the receiver on-board the Orbiter will provide an accurate clock for the doppler processing of the received carrier signal. The Probe's drift, caused by the winds in Titan's atmosphere will induce a measurable doppler shift in the carrier signal. The doppler signature will tell of any move made by the Probe as it descends.
Aboard are two imagers - one visible and the other infrared - looking downward and outward will observe the moon's surface as the Probe spins slowly. Mosaic panoramas can be assembled by panning several exposures. By taking several panoramas at different times during the last part of the descent, it may be possible to infer the Probe's drift, provided the surface is not featureless, thereby contributing to the wind speed measurements. The amount of light striking Titan's surface is about 350 times that striking the Earth's surface during a full Moon, about the same as at twilight. While this surface illumination is sufficient for imaging, a special lamp on the Probe will be activated a few hundred metres above the surface to provide enough light in the methane absorption bands to be able to make spectral-reflectance measurements. These will provide unique information for studying the composition of the surface material.
The Probe will impact the surface of Titan with a velocity of about 5 to 8 m/s. The probability of the Probe's survival cannot be estimated on purely engineering grounds, since there are too many unknowns. Assuming that the Probe survives the landing, science measurements will be programmed to go on for at least 30 min after impact.
The Surface-Science Package (SSP) is a suite of sensors designed to determine the physical properties of the surface at the impact site and to provide unique information about its composition. The package includes an accelerometer to measure the impact deceleration, and other sensors to measure the index of refraction, the temperature, the thermal conductivity, the heat capacity, the speed of sound and the dielectric constant of the (liquid) material at the impact site. It also includes an acoustic sounder, which will be activated a few hundred metres above Titan's surface. A tilt sensor is also included in the SSP to indicate the Probe's attitude after impact.
The Orbiter will 'listen' to the Probe for 3 hours in total, ie. for at least 30min after impact. The Probe's end-of-mission is set to occur at the end of this 8h-long communication window with the Orbiter, at which time the latter's high-gain antenna will be turned away from Titan. If the Probe should survive the impact, the depletion of its batteries or its cooling to about -180°C are thought to be the two most likely causes of mission termination.
The Radio/Data Link
All the data acquired by the Huygens Probe will be transmitted to the Orbiter in real time via a S-band link. The highest data rate required will be 8 kbit/s. The output power of each of the Probe transmitters is 10 W, which is the state of the art limit for solid-state amplifiers. The Probe antenna gain is limited by the need for a wide beam, capable of covering a Probe aspect angle as seen from the Orbiter ranging from about 30° to 50°.
On the Cassini Orbiter, the link will be received via a high-gain antenna pointed towards the Probe's nominal position. This antenna is also to be used for communication with the Earth, which means that real-time downlinking of the Probe data is not possible. It will therefore be stored on-board the Orbiter until after completion of the Probe's mission. At that point, the Orbiter will be slewed so that its high-gain antenna is Earth-pointing and transmission of the data gathered by the Probe can commence.
The Huygens Probe will have to operate in an autonomous and fault-tolerant manner during its Titan descent as neither ground control nor failure recovery will be feasible due to the extreme distance and signal propagation delay involved. Therefore, no telecommand access to the Probe is provided for after its separation from the Orbiter.
The Cassini mission
At the end of the Probe mission phase, the Saturn Orbiter equipped with 12 scientific instruments of its own, will start a four-year tour of the Saturnian system. This so-called 'satellite tour' involves more than 40 Saturn-centred orbits, connected by Titan gravity-assist fly-bys or propulsive manoeuvres. The size of these orbits, their orientation to the Sun-Saturn line, and their inclination to Saturn's equator are dictated by the various scientific requirements. These include: Titan ground-track coverage, icy-satellite fly-bys, Saturn, Titan or ring occultations, orbit inclinations and ring-plane crossings. The Saturn Orbiter will observe Titan during each of its fly-bys of the moon.
The Moon Titan
The detailed nature of Titan's surface will probably remain an unsolved puzzle until the arrival of Cassini and the Huygens Probe. However, prior to and particularly since the mission's selection, there has been a substantial programme of ground-based observations of this object. The Hubble Space Telescope has also contributed to the recent Titan observations.
Since 1988, annual ground-based radar observations using the Goldstone facility in California as the transmitter and the Very Large Array in New Mexico as the receiver, have revealed a highly reflective surface to Titan, which is inconsistent with a global ocean of light hydrocarbons. The large observed day-today variability in the radar reflectivity is also quite puzzling. Although the ever-improving optics of modern ground-based telescopes are making Titan's surface more accessible to today's astronomers, the new data is tending to complicate the puzzle about the nature of Titan's surface rather than resolve it!
Apart from the surface question, the hunt for new molecules in Titan's atmosphere is also the subject of many ground-based observations. Recently the acetonitrile molecule, CH~CN, was discovered in Titan's atmosphere by French astronomers. Its presence was predicted by photochemical models and it has significant implications for those who look at Titan as an "Earth before life". The presence of argon in the Saturnian moon's atmosphere is also the subject of continuing research, since it would provide important clues about the origin of Titan's atmosphere. Any argon concentration also has important consequences for the aerothermo-dynamic heat fluxes that Huygens will experience during the entry. Another topic of research that is actively being pursued is the question of the zonal circulation of Titan's atmosphere, which is super-rotating like that of Venus. Knowledge of these zonal winds was also important for the design of the Huygens Probe mission, since a large uncertainty in their magnitude would have impose severe constraints on the design of the link between the Probe and the Orbiter.
The different gasses, initiated by solar ultraviolet radiation and high-energy electron impact, induces a complex photochemistry in Titan's atmosphere. This produces a lot of complex organic chemistry which gives rise to a photochemical fog, the so-called "Titan haze", which shrouds the moon.
Analysis of the Voyager images and infrared spectroscopic measurements have provided a great deal of information about the physical properties of haze particles, called 'aerosols', which are probably organised into several layers. With the top layer as high as perhaps 500km, haze particles are composed of a wide variety of H, C, N compounds and opaque refractory materials, but very little is known about their chemical composition or the chemistry at work in the solid and liquid phases of those particles.
At first, the Voyager scientists were disappointed because the surface remained hidden from the spacecraft's cameras. However, the understanding of the atmosphere, and the use of simple thermodynamics laws, led to the hypothesis that Titan's surface was covered by a global ocean of ethane/methane and nitrogen, which is soluble in both. The presence of an ocean would have profound implications for the atmosphere and the CH ~ "Hydrological" cycle. The putative oceans themselves, for the complex organic chemistry that would be at work there, and the potential exobiological implications, are the subject of numerous studies.
Properties of Titan
Surface radius 2575 ± 0.5km
Mass 1.346 x 1023
Surface gravity 1.345 m/s2
Mean density 1.881 g x cm-3
Distance from Saturn 1.226 x 106
Orbital period 15.95 days
Rotation period 15.95 days
Surface temperature 94° K
Surface pressure 1496 ± 20 mbar
The scientific objectives
Broadly speaking, the scientific aims for the Cassini/Huygens mission
* determine the dynamical behaviour of Saturn's atmosphere
* determine the chemical composition and fine physical structure and
the energy balance of Titan's atmosphere
* observe the temporal and spatial
variability of Titan's clouds and hazes
* characterise the Titan surface
* determine the structure, composition and geological history of Saturn's
* study the structure of the rings and the composition of the ring
* study the structure, chemical
composition and global dynamics of Saturn's magneto-sphere.
An important aspect of the Cassini mission is the study of the interactions
and interrelations between the various parts of the system. The study
of the interrelation between the rings and the icy satellites, and that
between the satellites and the ionosphere of Titan with the magnetospheric
plasma,are key Cassini mission objectives.
More specifically, at Titan it is hoped to:
* determine the physical characteristics of the atmosphere, particularly
its density, pressure and temperature as a function of height
* determine the abundance of atmospheric constituents, including any
noble gas; establish isotope ratios for abundant elements; constrain scenarios
of formation and evolution of Titan and its atmosphere
* observe vertical and horizontal distributions of trace gases; search
for complex organic molecules; investigate energy sources for the atmospheric
chemistry; model the photochemistry of the stratosphere; study the formation
and composition of the aerosols
* measure winds and global temperature; investigate the cloud physics,
general circulation and seasonal effects in Titan's atmosphere; search
for lightning discharges
* determine the physical state, topography and the composition of the
surface; infer the internal structure
* investigate the upper atmosphere, its ionisation, and its role as
a source of neutral and ionised material for the magnetosphere of Saturn.
The Huygens Probe will address the first five of these objectives by
making local in-situ and remote-sensing measurements all along its trajectory.
The Cassini Orbiter; during its repeated flybys of Titan, will address
all six objectives in a complementary manner and on a global scale. We
can already look forward with confidence to receiving pictures from Titan's
hidden surface early in the next century.
by Mike Frost
Not many of us would think of going to Mongolia for a holiday in the
middle of their winter even if there was a solar eclipse due. But
our very own intrepid explorer Mike Frost set off earlier this year. .
This is his moving account of the trip.
According to Mongolian legend, the Rakh is a gigantic monster possessing nine heads, a snake's tail, and an enormous belly. A long time ago, Rakh managed to drink the eternal water held by Buddha, and the Buddha ordered Ochirvani, a lesser deity, to punish him for the theft. Ochirvani pursued the monster and was directed on his way by the Sun and Moon; on finding the beast, a vicious battle ensued. According to some legends, Ochirvani split the Rakh in two; in others, he left a hole in Rakh's belly the size of a ger (the Mongolian tent, or yurt, in which many of he population still live). Rakh, of course, had supped eternal water and could live forever, and vowed to take revenge on the Sun and Moon. Ever since then, he has attempted from time to time to eat both Sun and Moon — but always, just as he thinks he has finished the meal, it slips through the hole in his belly.
On the morning of March 9th 1997, 3000 astronomers from the world over gathered in northern Mongolia to witness Rakh's revenge — a total eclipse of the Sun. The path of the eclipse began at sunrise in the extreme east of Kazakhstan, sweeping east across Mongolia and clipping the far north of China, then curving northwards into the Siberian wilderness. Maximum duration of totality was 2 minutes 50 seconds from Siberia, and about twenty seconds less in Mongolia. Unlike many other recent total eclipses, the track did not cross any major cities. Ulaan Bataar, the capital of Mongolia, witnessed 99.6% of totality, but the only towns of any significant size beneath the Moon's shadow were provincial centres such as Moron and Darhan in Mongolia and Chita in Siberia. So although the track of totality was wide — over two hundred miles across — it crossed a barren wilderness populated mostly by nomadic herds-people.
Many British eclipse watchers joined the expedition organised by Explorers Tours. Most of the 200 plus party arrived by air from Beijing on the Friday beforehand, with the exception of an intrepid band who arrived on the trans'siberian express early the following morning. Ulaan Bataar gave a good indication of the conditions expected from Mongolia — crisp wintry air, sparsely vegetated ground, low mountainous scenery, and a vicious overnight frost. Best of all, Mongolia was living up to it's self appointed title "The Land of Blue Sky" (for land-locked Mongolians, the vast sky, covering the never ending steppes, occupies much the same place in the national psyche as the sea used to for the British). Weather conditions had been clear for several days and the frontal system seemed to be holding fair.
In Ulaan Bataar we gained some idea of how the Mongolians felt about their eclipse. At the monastery of Gandan Tengchinlen, where Mongolians were now free to worship after decades of communist suppression, crowds of worshippers gathered to spin the prayer wheels and pay homage to the Buddha and his entourage. Our guide explained that there was still a superstitious fear of the eclipse — especially with the presence of a comet.
The more secular Mongolians were exercising their newly granted entrepreneurial talents and were selling T-shirts, badges and even eclipse watercolours. The government showed their commitment to free-market principles by selling a commemorative stamp and medallion. For twenty five dollars, you could even purchase a commemorative certificate signed by the president. The national airline, MIAT, offered eclipse flights - $2500 for half an hour in a Boeing 727, and, intriguingly, only $800 for thirty minutes in a Russian built AN24. A less welcome throwback to totalitarian days was the sudden imposition of a ten dollar tax on foreigners who wanted to travel north to view the eclipse.
Early in the morning of the ninth, a convoy of eight buses left Ulaan Bataar and headed northwards toward the eclipse zone. There was some cloud overhead but mostly the skies were clear and the air bitterly crisp. At 4am, in the middle of nowhere, the road train halted for a "Hale-Bopp stop". This was a wonderful opportunity to view the comet far from the light pollution which plagues most British observers. Hale-Bopp was of course magnificent, low in the northern sky, the tail stretching through ten or more degrees. But the skies still seemed less than clear. And, although we failed to realise the significance, almost all the observers commented that the temperature was not as cripplingly cold as we had been led to expect — prior bulletins had warned of the dangers of frostbite and hypothermia. The exact nature of the change in weather became horribly clear as we continued to speed northwards. It began to snow.
At 7am the convoy rolled into Mongolia's second largest "city"; Darhan, a bleak industrial concrete complex of some ninety thousand souls. Still the snow drifted down from gloomy skies. We were due to eat breakfast at the Russian club, watched over by stern babushkas. But Brian McGee, leading the expedition, gave us a stark choice. To see the eclipse, in his opinion, the best option was to move north as fast as possible in an attempt to pass through the weather front — and that meant forgoing breakfast and leaving immediately. Some people chose to stay behind and take their chances in the relative comfort of Darhan. Most returned to the buses, which sped northward and upwards on the gravel track leading out of Darhan toward the Russian border, a hundred miles away.
The thrill of expectation which normally greets first contact took on an edge of uncertainty — still the Sun remained hidden by cloud, but at least the snow had stopped, and the skies showed distinct signs of clearing. The track through the snowy wastes (reminiscent of the Pennines or Peak District in midwinter), which would normally see only a few trucks a day, was filled with screaming coaches, minibuses, jeeps and hire cars, all hurtling hell-for-leather northwards, watched over by a circling army helicopter. If only WE had the ability to climb above the clouds!
The time for totality crept closer. Twenty minutes to go — and the Sun showed itself for the first time, still gauzily, to loud cheers. Now we faced an agonizing dilemma. Ahead the road would crest a ridge and start to drop. Should we stop and maintain the height we had gained, or plough on northwards hoping to clear the clouds altogether? Fifteen minutes to go, and our coach stopped behind another in the party. Almost by default the decision had been made — here, if anywhere was where were going to view the eclipse. A mile to the north east was a range of hills over which our eclipse would take place. Behind us was a slight ridge on which earlier Japanese arrivals had assembled their equipment. We rapidly did the same — at least the temperature, still well below freezing, was not the flesh threatening -20 degrees which had been expected. However, even as we set up, the Sun disappeared back behind the clouds.
The light thinned and silvered as 8:48, the onset of totality, approached. We realised that a race was on — there was a gap in the clouds scudding gently eastward, taunting us as to whether or not it would reach the Sun in time. Both to north and south we could see clear skies in the distance — impossibly far away. The seconds ticked down, the light dimmed perceptibly moment by moment. Then to the south the sky suddenly darkened ominously. The eclipse was about to begin without us. Just how close we were to seeing the Sun was evident by the skies immediately in front of us. Radial crepuscular rays in blue and black streamed across the higher clouds. Then the light level abruptly dropped to deep twilight, the Moon's shadow roared across the hills ahead of us and, behind the veil of cloud, Rakh gobbled the last morsel of the Sun. The total eclipse was under way.
To the north the sky was still clear, but as we watched despairingly, the northern, trailing, rim of the Moon's shadow caught up and wiped it out, leaving only a remote strip of sunlight from one hundred miles away, outside the totality track. Toward the Sun, the view through telephoto lenses and binoculars showed nothing but cloud. We looked around in despair, taking in the peripheral sights, but praying for the cloud to move over and reveal the solar corona. The seconds ticked away agonizingly. Two minutes crawled by — then suddenly, whoops and photo clicks erupted from the Japanese observers barely quarter of a mile away. Was there to be a last reprieve? To the south the sky began brightening with pink and purple light — totality was all but over. Then through the clouds a dim burst of light — the diamond ring. The Moon's shadow swept majestically over us and hurtled off into the east. Even as we watched, the Sun was clearing the clouds — we had missed relatively clear skies by two minutes, and a view of the corona — watery perhaps, but a view nonetheless — by no more than a few seconds.
The sense of crushing disappointment was overwhelming — some people (me included) cursed, but most people stayed silent and subdued. A few photographers rattled of shots of the partial phase of the eclipse but the seeing was hardly ideal — the light was still too weak even to cast a decent shadow on the snow. Grudgingly we packed up our equipment and returned to the coach to drown our sorrows.
We returned downhearted to Darhan to compare our stories with the rest of the party. The Explorers' convoy had been strung out over several miles by the speeds of the buses and the decisions of individual coachloads. Some had seen as little as our group, or less. A vanguard of two vehicles had driven nonstop northwards, almost reaching the Russian frontier, halting barely a minute before totality. John Mason, the tour lecturer, reported sprinting up a hill, binoculars in hand, to gain a cloudy view of the corona during totality. And those who stayed behind in Darhan had seen as good a view as anyone. As the locals sounded their car horns, and the lamas in the buddhist temple clashed cymbals and chanted sutras to try and drive away Rakh, the glorious sight of the corona was dimly visible through the clouds.
It turned out that the snow cloud system which had covered northern Mongolia did not affect the entire totality track. The southern portion of the track, which we had crossed six hours previously, was reported clear by the locals, and to the east of Mongolia, conditions became steadily better. An article in the Beijing people's daily (in Chinese) reported that 300 people, some foreign, had seen the eclipse from Mohe in the Heilongjiang province in the far north of Manchuria. In Beijing we met a party, led by Wasyl Moszowski, who had seen the eclipse from Mohe. They reported clear skies, shadow bands, and a beautiful corona, more rectangular than the previous eclipse in 1995, but with fewer equatorial streamers.
I bear many happy memories of my visit to Mongolia — the extraordinary hospitality of the Mongolians, so anxious to show off their country to visiting tourists; the barren beauty of the Mongolian steppes; the lovely view of Hale-Bopp in a black night sky; even the sight of the onrushing Moon shadow. But to be beneath an eclipse, unable to glimpse the corona, is a deeply frustrating experience. This is, unfortunately, an occupational hazard of the eclipse watcher. Maybe Rakh extracted his revenge on the astronomers.
"The Mongolian Myths about the Solar and Lunar eclipses and the constellations" (M.Monkhzaya, trans D.Altangerel, Interpress of Ulaan Bataar 1997)
Beijing People's Daily, International edition 10th March 1997
UB Post (Ulaan Bataar), 6th March 1997 edition
By Pam Draper
l've based this article on information from a book I read recently, "The Chemically Controlled Cosmos"
by Thomas Harquist of the Max-Plank InstItute fur Extraterrestrische Physik and Prof David Williams, its printed by the Cambridge University Press. I can recommend this book for anyone who is interested in the nitty-gritty workings of the universe. Although difficult to read, it was thoroughly fascinating and an eye-opener to the workings of nebulae at the molecular level. It was not easy condensing the main issues of the book into a page, while still keeping it coherent and building up a picture of the processes involved. I hope its understandable. On Earth, most matter is molecular rather than atomic. The air we breathe is mostly molecular nitrogen N2 and molecular oxygen O2 rather than atomic nitrogen N and atomic oxygen O. On Earth terrestrial conditions allow atoms to arrange themselves easily into molecules. However extraterrestrial conditions differ in that the often intense ultra-violet radiation from very hot stars powers the system towards atoms rather than molecules. This destructive process of molecules is called photdissociation. Molecules do not generally survive in gas that is more than ten times hotter than the Earths atmosphere. Most of the sun's surface (6,000° kelvins) is too hot, but sunspots and the envelopes of highly evolved stars are marginally cool enough for molecules to exist. Most of the interstellar gas that is molecular has temperatures of only a few tens of kelvins, (absolute zero or 0 kelvins is -273°C or -459.4°F). The hydrogen atom is the simplest type of atom, helium being the next most abundant. Basically, a rich chemistry of elements is present in many clouds of interstellar space and permit the existence of dust in which molecules are able to stick together with darkening effects. The ashes of thermonuclear processes from super-nova explosions and cool carbon rich stars are carried in stellar winds. Stars viewed through this dust appear redder as does the sun through the Earths atmosphere when low to the horizon. Dust particles absorb and scatter light, radiation heats the dust grains so that these radiate in the infrared part of the electromagnetic spectrum. The centres of thick molecular clouds are cooler and offer more protection from radiation than their outer surfaces. Carbon monoxide CO observations show that dark or diffuse clouds have primarily molecular centres with mostly atomic rims. A giant molecular cloud has a typical mass of a hundred thousand to a million times that of the sun. Most of this mass is in the clumps of dust and gas with high molecular number densities from which most star formation occurs.
The exact chemistry of these elements is far to complicated to include here. Cosmic ray protons have the most important effects on the gas in clouds causing ionisation of molecular hydrogen H2 giving a new ion H+3. This ion is reactive and can donate a proton to many species, creating more reactive molecules and atoms. Metal ions such as sodium and magnesium are unreactive at low temperatures and once created exist for a very long time. These processes are effected generally by cloud temperatures and magnetic pressures, the effectiveness of which determines wether collapse will lead to regions of stellar birth. This is the Jeans Mass (the minimum mass of a cloud, with a given density, temperature and often magnetic field strength that will collapse due to its gravity).The magnetic force acts only on charged particles and tends to push them outwards, while neutral particles are pulled inwards due to gravity, this is called Ambipolar Diffusion. This process occurs over a few million years and collapse leads to star formation regions. Similar mechanisms were at work in the formation of the Proto-solar nebulae and the Proto-intergalactic medium (before the first galaxies) following the big bang.
A Matter of Some Gravity
By Mike Frost
In the last edition of Mira, Pam Draper raised some very deep and perceptive questions about gravity, which I'd like to put some thought to. What is the speed of gravity? Well, any time Einstein ever answered a question like this, he usually started with a "thought experiment" — and what was good enough for Albert is good enough for me. Suppose the speed of gravity was instantaneous. What would this imply? Any change in the position of an astronomical body would instantly be detectable at any distance, by a sufficiently sensitive detector. In principle, this would allow faster than light communication. For example, if you had a convenient black hole, you could send a "dot" by dropping a small object into it (which releases gravity waves), and a "dash" by binning a larger object. This would allow a sort of instantaneous intergalactic Morse code. But it's an accepted tenet of modern physics that information cannot be transferred faster than the speed of light (that doesn't mean it's true, of course, but all I'm trying to do is explain what modern physics has to say about the speed of gravity). If instead of manipulating black holes, you sent messages by manoeuvring electrical charges, these messages would travel at the speed of light. Morse code messages sent by jiggling electrical charges were first achieved by Marconi at the beginning of this century — we know them better as radio (wireless) transmissions. So is the speed of gravity the speed of light? I think it is — but I'm not certain. You see, the only particles which can travel at the speed of light are massless particles. The "gauge particles" which transmit information about electromagnetic fields — photons — are massless. The gauge particles for subatomic forces are not massless, however; so consequently the speed of, say, the strong nuclear reaction, is less than that of light. Does the gauge particle for gravity, the graviton, have mass? I don't think anyone knows for certain, because I don't think it has been observed. The expectation is that it IS massless. But gravity is so much less powerful than any of the other fundamental forces, that the graviton will be very difficult to detect. (Why then is gravity the predominant force over astronomical distances? Because electrical charges cancel out, and nuclear forces are only strong over subatomic distances.) One last point. We have a good, accurate theory of the way gravity works on astronomical scales — Einstein's General Relativity. We have an excellent theory of how all the other forces behave at the subatomic level — Quantum Mechanics. What we don't have is a good theory of how gravity behaves at a subatomic level. All attempts so far to combine General Relativity with Quantum Mechanics, to give a successful coherent theory of Quantum Gravity, have failed. Maybe the really exotic branches of physics — string theory, GUTs, and other things I don't understand, will bridge the gap.Or maybe there's some physics going on that nobody understands. Or maybe everything I've written above is completely wrong! (it wouldn't be the first time). If you think I haven't understood the situation, well, you know where to send your article. Any takers?
PS And does time slow down in the vicinity of a large mass such as a black hole? Yes it does, and I hope to address this in the next issue of Mira.