[Space] NASA-ESA - Cassini-Hyugens

[Thell]

Originariamente inviato da GioFX
non è detto che affondi subito...

e cmq in bassa frequenza riesci a trasmettere anche in un mare di cacca...

e che ci mettono? la ciambella con la paperella?

a bassa frequenza i filmini e le foto non ce le manda però

non possono usare la correlazione quantistica come nell'università di zurigo?

[gpc]

Veramente ci contano che cada in un oceano, tant'è che è progettata per galleggiare e studiare il modo di eventuali onde.
Anzi, se cade in un oceano è meglio, perchè così sono sicuri che l'antenna risulta puntata verso l'alto.

[GioFX]

Cassini mission hinges on Wednesday's engine firing

BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 29, 2004

After a seven-year voyage from Earth, NASA's $3.3 billion Cassini probe is racing toward a make-or-break rocket firing Wednesday, a 96-minute maneuver designed to put the craft in orbit around the ringed planet Saturn for a four-year scientific odyssey.

Flight controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., sent final commands to Cassini over the weekend, setting the stage for main engine ignition at 10:35:42 p.m. Wednesday.


Cassini fires its engine to enter orbit around Saturn as illustrated in this artist's concept. Credit: NASA/JPL

Operating more than 930 million miles from Earth - so far it takes radio signals an hour and 23 minutes to make a one-way trip - Cassini's on-board computer system must carry out the all-important rocket firing on its own.

At this point, flight controllers can only sit and wait. And chew their nails.

"I think about the Cassini mission as having three primary segments and then two rather hair-graying events that connect those segments into one continuous mission," said project manager Bob Mitchell. "The segments are designing and building the spacecraft, flying the spacecraft to Saturn and then conducting the science mission at Saturn.

"And the hair graying events are launch and orbit insertion, which is coming up tomorrow. Now for the launch event, I think we've all recovered from that very nicely, primarily because it was just so outstandingly successful. ... We're about to go through our second hair-graying event."

At a news conference today, he told reporters "I think I can speak for all the team members when I say that while we're all at least a little bit nervous, we're also very excited. It's an event we welcome very much and are pleased to have here."

The goal of the Cassini mission is to study Saturn's windy atmosphere, its complex ring system, several of its icy moons and how the planet's magnetic field interacts with the space environment. In what promises to be one of the most exciting phases of the mission, a European-built probe called Huygens will be released from Cassini on Christmas Eve for a parachute descent into the thick nitrogen atmosphere of Saturn's moon, Titan, on Jan. 14.

In all, Cassini is expected to complete 77 orbits of Saturn over the next four years, requiring 157 trajectory-nudging rocket firings. The gravity of Titan will be used for major course changes, with 45 planned flybys. Seven close flybys of smaller, icy moons also are planned.

But first, Cassini must execute the Saturn Orbit Insertion maneuver, or SOI.


This graphic shows the ring plane crossing and orbit insertion burn. Credit: NASA/JPL

To achieve orbit around Saturn, the 12,600-pound Cassini must reduce its velocity by about 1,400 mph using a rocket engine that only produces 100 pounds of push. As a result, the engine must fire for 96.4 minutes to put Cassini into the desired orbit.

If the engine shuts down early, the computer will switch to a spare. But the end result must be roughly the same - 96 minutes of braking - or Cassini might not be able to achieve its long-awaited mission.

"There are no problems, we have no indication of any problems with the spacecraft that would have any adverse effect on SOI," Mitchell said. Added Julie Webster, lead spacecraft engineer: "This spacecraft, this whole mission has been an incredibly smooth one to fly."

"This orbit insertion sequence is self contained on the spacecraft," she said. "We loaded up the last command we're going to send to it late Saturday night, Sunday morning, and we've just been clocking it out ever since and getting no indications of anything. We expect this to go very, very smoothly."

The propulsion system has worked flawlessly since Cassini's launch aboard a Titan 4B rocket on Oct. 15, 1997. The only issue of any consequence was a leaking helium regulator that forced engineers to change the way they pressurize the system for major rocket firing.

Helium is used to push propellants through Cassini's plumbing and into the main engine's combustion chamber at a constant pressure. The regulator controls that pressurization, which is needed for long firings like the upcoming Saturn Orbit Insertion burn.

In this case, Cassini's complexity and built-in redundancy came to the rescue. By delaying the opening of a downstream latch valve to just 70 seconds or so before main engine ignition, engineers were able to work around the regulator issue with no impact to mission operations. The procedure was used for a major 88-minute Deep Space Maneuver rocket firing back in 1998 and again in late May for a six-minute burn that set up a flyby of the moon Phoebe.

"We've got a real nice propulsion system," lead propulsion engineer Todd Barber said in an earlier interview. "It's a plumber's nightmare, there are so many valves and alternate paths and contingency paths available that basically, we're able to handle a lot of anomalies. And the regulator leak we saw was right after launch and we've been able to accommodate that with the way we time the opening of valves, etc."

Even so, Barber will feel much better after Cassini successfully brakes into orbit. "It's been a long time coming and the hopes and dreams of thousands of engineers are resting on that one evening. When we get the signal back is when we'll all go take a deep breath. Hold your ears, because they might pop."


Cassini's two engines are seen in this pre-launch photo. Credit: NASA/JPL

Cassini is equipped with two rocket engine assemblies, REA-A and REA-B. Rocket Engine Assembly B, however, is strictly a backup. It has never been fired.

Mitchell said in an interview he had high confidence the SOI maneuver will work normally and that REA-B will not be needed "based on all of the testing, all the elaborate work that we have put into this, as well as our experience with the spacecraft to date. We've done 15 or 16 maneuvers using the main engine ... and so we have every reason to believe this thing is going to work just fine."

But, he added, "the software the thing flies is all complex and I just worry about what bugs are still in there. I think it must be inevitable that there are still bugs in there for something this complex. We've tested it extensively, we have a test bed here in the basement of our building that is a quite high fidelity spacecraft simulator and the sequences have been run through there many, many times. We have injected faults, we've had various components break, where we simulated a break in the test bed and looked to see what response we got. And at the moment, everything works. All the tests that we've done, all the simulations indicate that everything is just fine."

The maneuver has little margin for error. Cassini first must avoid any crippling debris impacts when it crosses the ring plane between the F and G rings, moving from the lower side of the rings to the upper side as viewed from Earth.

Pioneers 10 and 11, along with Voyager 2, flew through the gap with no problems but Cassini flight planners are taking no chances. Before traversing the ring plane, the spacecraft will be oriented with its high-gain dish antenna facing the direction of travel to act as a shield.

Voyager 2 went through outer edge of the G ring and its instruments recorded "lots of evidence of micrometeoroid hits when going through, but nothing serious," said Voyager veteran Torrence Johnson, a Cassini imaging team member and chief scientist for the Galileo mission.

"So we have that maneuvering to do and then there's the fact that the place is just a junky system," he said in an interview. "We're going in close, we're skimming right over the rings, everybody thinks we've modeled all this right and we're being reasonably cautious. But I told some of the guys early on, if they're going to be scared of ring particles they ought to remember John Paul Jones' letter to Congress, 'give me a fast ship for I intend to sail in harm's way.'

"I don't think anybody's real complacent about this thing," Johnson said. "I think we've done everything we can to make sure we don't have any human or systems screw ups, but nature can still get you."

Jerry Jones, Cassini's chief navigator, said flying through the ring plane relatively close to Saturn will save propellant and makes the rocket firing more efficient. "But the real clincher, given all that, is the science in that close. We're going to be sitting there looking right down on those rings."

"We've got one class-A camera on this spacecraft," he said in an interview. "It's a beautiful telescope, it has great resolution, very sharp edges and for optical navigation, I'm just pleased as punch. ... The science opportunity going over the rings should be just fantastic, to say nothing of showing the public what they've paid for."


This graphic shows Cassini's track as it enters orbit around Saturn. Credit: NASA/JPL

Once safely through the ring plane, Cassini will re-orient itself once again, swapping ends to put the main engine forward for the SOI burn. NASA originally planned to carry out the rocket firing "in the blind," with Cassini focusing on science observations while the engine put on the brakes. But in the wake of back-to-back Mars mission failures in 1999, NASA management ordered engineers to figure out a way for Cassini to provide at least some information about the start of the burn, its progress and its termination.

"So we went back and scrambled then," Barber said. "We had a compromise solution. We could have pointed the high-gain antenna to Earth during the whole burn and have telemetry but there was a large delta V (fuel) penalty to do so. So the plan is to switch to a low gain antenna and that will allow us to maintain Doppler during the burn."

While no actual data will be transmitted to Earth, analysis of the Doppler shift of a carrier signal from the spacecraft will tell engineers when the burn started, the precise deceleration it produces and when it stops.

"There were two key things we wanted to be sure we could differentiate between," Mitchell said. "One was in the event we just lost it entirely and never saw it again, we wanted to be able to differentiate between whether we had a problem going through the ring plane or whether we did that successfully and had a problem during the course of the burn itself. So with the data we have, we will know that quite well."

After re-orienting itself for SOI, Cassini will begin transmitting a carrier signal. Six minutes later, the burn will begin, showing up on computer screens at the Jet Propulsion Laboratory as a change in the slope of the carrier frequency. Thirty minutes later, the signal is expected to fade out for 25 minutes or so as Cassini passes behind Saturn's A ring as viewed from Earth.

Engineers then expect six minutes of carrier through a gap in the rings known as the Cassini division before another 28-minute communications blackout while the spacecraft passes behind the broad B ring. Closest approach to Saturn - 12,400 miles from the planet's cloud tops - is expected at 12:03 a.m. July 1, nine minutes before the SOI burn comes to an end. The first images and other data from the orbit insertion maneuver are expected around 8:39 a.m.

"We turn off of Earth line shortly prior to the ring plane crossing," Mitchell explained. "We turn to point the high-gain antenna in the direction we need to be in to shield the rest of the spacecraft and then there's a period of about an hour where we don't have any contact.

"Then when we turn back to go to the burn attitude, at about the time we get to that attitude, which is six minutes prior to the burn start, we will crank up a signal that comes from one of the low gain antennas. There's no telemetry, it's just a carrier.

"But that carrier will allow us to get Doppler and that'll tell us if the spacecraft is operating fine and has not had any safing events. And then during the course of the burn, that Doppler will let us see quite accurately what the acceleration levels are. So if the system is performing nominally or over performing or under performing, the Doppler will show that very well."

The day after orbit insertion, Cassini will pass within 205,000 miles of Titan, the first official Titan encounter of the mission. Between July 4 and 11, the spacecraft will be out of contact as Saturn passes behind the sun as viewed from Earth. The SOI sequence will end on July 30 as tour sequence No. 3 begins.

Sometime around Aug. 23, Cassini's main engine is scheduled to fire in what will be the last fully helium-regulated burn of the mission: a 51-minute maneuver that will change the spacecraft's velocity by 877 mph. The Perigee Raise Maneuver, or PRM, will raise the low point of Cassini's orbit and set up the first close flyby of Titan in October. After another flyby in December, the Huygens probe will be released for atmospheric entry during the mission's third Titan encounter in January.

The SOI maneuver is one of only three so-called "critical sequences" built into Cassini's mission software. A critical sequence is one that simply must execute properly to ensure mission success. The launch to Venus was one such sequence and the only other one is the Huygens data relay sequence.

SOI is "the only maneuver that we will do throughout the entire course of the mission where we just absolutely have to do this burn right now," Mitchell said at a news briefing. "If this burn doesn't work, then we would have a Saturn flyby and that's not what we're here about. So we have designed what we refer to as a critical sequence where no matter what fault might occur, the spacecraft will not let the burn halt. Now in some modes it will stop the burn, swap to the other engine and then continue on with the burn. But the burn will continue even in the presence of faults."

During normal operations, a problem with a spacecraft system would trigger fault-protection software that would shut down unnecessary activity, a condition known as safe mode. There are numerous variations, depending on the nature and timing of the fault and whether the spacecraft still knows its orientation in space. The end result, however, is the same: Cassini shuts down, finds the sun (Earth will never be more than six degrees away), switches to low-data-rate communications and awaits instructions from Earth.

Because of Saturn's great distance and the slow-speed radio link used in safe mode, engineers would need at least 48 hours to restore Cassini to normal operations. Such a fault during the SOI maneuver would shut the main engine down, interrupting the all-important rocket firing and possibly dooming the mission. But in critical sequence mode, that will not be allowed to happen.

"In critical mode, if the spacecraft detects a fault - we get a glint in the eye of the star scanner or the thrusters don't like what they're doing or a piece of hardware doesn't work right - the spacecraft will detect a fault," Webster said. "The sequence will halt and the spacecraft is allowed through its autonomous fault protection to go off and fix the fault.

"And then it will come back and say I've detected a fault, I've fixed the fault. And then it'll come back and say OK, critical sequence, you can restart. And the critical sequence will say Oh, but I've gotta remember where I was. We have what we call a mark and rollback strategy. So it'll roll back to the last good mark point and it will resend all the commands necessary to execute the next states that it needs to be in. It'll recommend all of those and continue on. And so, if there's a fault anytime during the critical sequence, it'll stop, halt, detect the fault, correct the fault and then restart the sequence."

That's where Cassini's second main engine comes in. If a fault of any kind interrupts the SOI sequence, the computer will fire up rocket engine assembly B and continue the burn.

"In the burn, we have already disabled any fault protection activity that's not necessary to complete the burn," Webster said. "So we've got fault monitors, say, on the CDS (command and data system) computer and on the radio. Well, the radio's not necessary to complete a burn. Neither is the CDS, ironically, because the attitude and articulation control system also has its own computer and once the CDS has told it to go do a burn, it takes over and says I don't need you anymore, I'll complete this burn and I'll let you know when I'm done. So only the fault protection that's necessary to complete the burn is active.

"It would detect a fault in where it's pointing or the propulsion system wasn't acting right, maybe under thrusting or over thrusting. If there's a fault during the burn, then we terminate the burn. We try to fix the fault and then we mark and roll back, pick up where we did, restart but we're going to restart on the second engine. It takes at least two hours to cool down one engine. So 10 minutes later, we can restart on engine B and minimize our overall cost."

But any safe mode that would necessitate firing REA-B also would terminate priceless SOI science operations, a small price to pay if survival of the mission is at stake. Cassini has a seven-hour window in which to complete the SOI maneuver and "as long as we got the correct amount of burn at any time in that seven-hour period, we would get into orbit," Webster said. "We might not like the orbit, but we'd get into orbit."

[GioFX]

Cassini 'go' for Saturn orbit insertion burn

BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 30, 2004

Executing stored instructions, the electronic brain of NASA's Cassini probe made final preparations for a critical 96-minute rocket firing tonight that will slow the craft by about 1,400 mph and allow Saturn's gravity to pull it into orbit.

The make-or-break Saturn Orbit Insertion - SOI - maneuver was scheduled to begin at 10:36 p.m. EDT and end around 12:12 a.m. Thursday. If successful, the burn will put Cassini in a long orbit around Saturn, kicking off a four-year tour of the ringed planet, its magnetosphere, its largest moon, Titan, and a retinue of smaller, icy satellites.

If the rocket firing fails or falls short of its 96-minute target duration, the $3.3 billion spacecraft will sail past Saturn and into a useless orbit around the sun.

"Unlike the two Voyagers that flew by Saturn in the early '80s and obtained just days worth of Saturn close-in science, Cassini-Huygens will be for Saturn what the Galileo mission was for Jupiter: a long-term science observatory," said Ed Weiler, NASA's associate administrator for space science.

But, he cautioned, "we are not there yet. Although things have gone very, very well so far, Saturn orbit injection will be the most critical event in the mission's life since launch. This main engine burn must be performed as planned or the mission will be lost.

"Unlike the Mars (rover) landings, where we had the 'six minutes from hell,' so to speak, in this case it'll be 96 minutes in purgatory. I hope the outcome will be as successful as our experiences with the Mars missions last January."

Engineers and managers at the Jet Propulsion Laboratory in Pasadena, Calif., believe it will. They said today the spacecraft has performed virtually flawlessly during the seven years it's taken Cassini to reach Saturn and they are confident it pull off the SOI burn, as Weiler said the most important single maneuver since launch from Earth on Oct. 15, 1997.

"Today, the Cassini spacecraft is in an excellent state of health and it is ready to perform the main engine burn tonight," said Julie Webster, lead vehicle engineer at JPL. "The spacecraft, the flight software and the on-board (computer command) sequences are now completely self contained and need no ground interaction from us to complete this burn. If a fault occurs on the spacecraft, the software will isolate the fault, identify the cause, fix it and continue on with the burn with no ground intervention."

The only issue of any significance is the possibility of high winds at a tracking station in Canberra, Australia. If the winds exceed about 47 mph, a 230-foot-wide dish antenna needed to pick up Cassini's radio signal will have to be stowed, delaying confirmation of a successful rocket firing.

"This has no bearing on the performance of the spacecraft," said Cassini program manager Bob Mitchell. "The spacecraft is perfectly capable of doing what it needs to do and weather on Earth is just not a factor.

"The thing that would be influenced is our blood pressure throughout the course of the evening," he joked. "However, a recent weather report from Canberra was favorable. It didn't eliminate the threat entirely, but it is favorable. So we don't believe it's going to be a problem."

Because of Cassini's enormous distance from Earth - some 934 million miles - radio signals from the spacecraft take an hour and 23 minutes to reach flight controllers at JPL. As a result, confirmation of events during SOI will be delayed by the same amount. In the timeline discussion below, "Earth-receive time," converted to Eastern Daylight Time, is used throughout.

"After 6.7 years we're at Saturn," said lead navigator, Jeremy Jones. "We're feeling the gravity well, falling into Saturn quite rapidly. ... This is the E-ticket ride, I'll tell you."

The numbers prove his point. At noon today, Cassini was 403,891 miles from Saturn and moving through space at 26,846 mph. By 6 p.m., the craft will be roughly half that far out - 233,014 miles - and its velocity will be up to 33,557 mph. Three hours later, the distance will be just 155,343 miles and its speed will be up to 42,506 mph.

The first critical milestone in the SOI sequence is the so-called ascending ring plane crossing. Approaching Saturn from below the plane of its rings, Cassini will cross that plane, in a broad gap between the F and G rings, at 10:11 p.m. That region is thought to be clear of any major pieces of ring debris, but no one knows for sure. At Cassini's velocity, an impact with anything larger than a very small pebble could cause major damage.

To be on the safe side, the spacecraft will re-orient itself an hour ahead of time, pointing its high-gain dish antenna in the direction of travel to act as a shield, cutting off telemetry from the spacecraft for the duration of the SOI maneuver. At the moment of ring plane crossing, Cassini will be just under 100,000 miles from Saturn - less than half the distance between the Earth and moon - and streaking through space at 50,335 mph.

"The antenna is a graphite-epoxy structure, so it's quite rugged and very capable of withstanding the kinds of small dust grains that we believe might be in this region," Mitchell said.

Once above the ring plane, Cassini will re-orient itself for the Saturn Orbit Insertion burn, pointing its main engine in the direction of travel. It also will begin transmitting a radio carrier signal using a low-gain antenna. The signal will not carry any data. But by monitoring how its frequency changes due to the SOI rocket firing's effect on Cassini's velocity, engineers will be able to confirm the start of the burn and the engine's overall performance through engine cutoff.

The moment of truth arrives at 10:35:42 p.m. when Rocket Engine Assembly A - REA-A - flashes to life, pushing against Cassini's 53,691-mph forward motion with a mere 100 pounds of thrust. Over the course of 96.4 minutes, REA-A will consume 1,874 pounds of rocket fuel, roughly one third of the 6,600 pounds Cassini was launched with, slowing the spacecraft by 1,396 mph and ensuring Saturn's gravity pulls it into the desired orbit.

Thirty minutes into the burn, at 11:06 a.m., Cassini will move behind the A ring as viewed from Earth and the carrier signal may be lost for up to 25 minutes or so. The signal should show back up for six minutes when Cassini "sees" Earth through a gap in the rings known as the Cassini division. Then communications may be lost again for 28 minutes or so as the spacecraft moves behind the B ring.

Saturn closest approach, or periapsis, will occur 10 minutes before the end of the burn, at 12:03 a.m. By this point, Saturn's gravity will have boosted Cassini's velocity to 69,350 mph. But by the end of the burn at 12;12 a.m., the velocity will have dropped to 68,000 mph and Cassini will be safely in orbit.

Weather in Australia permitting, engineers at JPL should be able to confirm the start of the rocket firing monitor its performance between ring blackouts and confirm the end of the burn. A few minutes later, Cassini is programmed to turn back toward Earth and transmit 20 seconds of engineering data starting around 12:30 a.m. The craft then will re-orient itself yet again to aim its cameras at Saturn's rings for 75 minutes of up-close science observations.

It then will re-orient itself antenna forward for the descending ring plane crossing and then point back toward Earth to begin downlinking its treasure-trove of science data. The first pictures are expected Thursday morning.

"Once we're done with about a 75-minute data observation period, we turn again for the descending ring plane crossing, again point the high-gain antenna as a protective shield," Mitchell said. "Once that's complete, we're going to turn and look back up at the rings, now on the sunlit side, take another set of images, mostly around the outer extent of the visible part of the rings. We then turn back to Earth and play this data back over about an 18, 19-hour period of time."

While confident, Mitchell cautioned "this thing is not a slam dunk by any means."

"There are two general concerns," he said. "One is the environment, primarily the environment of the rings. We have studied this very carefully, we believe we've taken prudent actions, we believe we've got the safest possible course of action, but it's not a guarantee.

"And then for the spacecraft itself, the spacecraft isn't going to do anything tonight that it hasn't already done in flight. But a 96-miute burn, where all of the systems have to work right for this amount of time, is a concern. Our confidence in the spacecraft is high, we have no specific reason to be concerned, but this is not a assured, either."

In preparation for the rocket firing, Cassini's on-board software opened low-pressure helium latch valves Tuesday. A backup inertial reference gyroscope was turned on and warmed up for use as needed during the burn. This morning, the main engine gimbal system was activated so the engine nozzle can be pre-positioned and the spacecraft's accelerometer was calibrated.

In short, Webster said, all systems were "go" for SOI.

"If the burn goes fine and we just don't get the signal back on the ground for any reason, we will get about 20 seconds of the high-gain coming in right before we turn go off and do the images," Webster said. "So if everything's perfect through the burn, we should get that little 20-second blip. That's very good because that tells us that the spacecraft didn't do any safing event or didn't stop the burn for any reason because the background sequences are still going. That's the good indication. That'll come in about 12:30 a.m. (EDT)

"I'm an old telecom person, so I actually wouldn't be concerned about this until past 3 a.m. ... I would not get any panic level until 6 or 7 in the morning. That's not unusual in low gain safing to have a little bit of trouble, especially with the Doppler changes that are going on with the spacecraft. Sometimes it just takes a while to find the signal. But I have full faith in this spacecraft, that it's going to make this burn one way or the other."

[von Clausewitz]

Originariamente inviato da gpc
Veramente ci contano che cada in un oceano, tant'è che è progettata per galleggiare e studiare il modo di eventuali onde.
Anzi, se cade in un oceano è meglio, perchè così sono sicuri che l'antenna risulta puntata verso l'alto.

veramente si parla di possibile distese di metano allo stato liquido, per parlare di oceani bisognerà riscontrare l'esatta estensione sulla superficie di titano
cmq questa si che è una missione veramente interessante che ci potrà dare se non proprio risposte,almeno ulteriori elementi per capire l'origine e la formazione del sistema solare

[Frank1962]

Originariamente inviato da GioFX
Closing In: Saturn Fills Cassini Craft's Viewfinder

Saturn and its rings fill the full frame of the camera in this Cassini image taken March 27, 2004 as the craft is just weeks from going into orbit.

ma come mai in questa foto l'ombra del pianeta che si riflette suglia anelli è cosi netta?

[gpc]

Originariamente inviato da Frank1962
ma come mai in questa foto l'ombra del pianeta che si riflette suglia anelli è cosi netta?

Perchè? Come dovrebbe essere?

[Fede]

Originariamente inviato da gpc
Perchè? Come dovrebbe essere?

e' telmente perfetto che sembra fatto al pc

[GioFX]

Cassini successfully arrives at Saturn

BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 1, 2004; Updated at 2:10 a.m. EDT

NASA's $3.3 billion Cassini probe completed a seven-year, 2.2-billion mile voyage tonight, firing its main engine for a nerve-wracking 96 minutes to successfully brake into orbit around the ringed planet Saturn.


Credit: ESA

Throughout the all-or-nothing rocket firing, flight controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., could only sit and wait, monitoring events that had already taken place 934 million miles away.

At that distance, radio signals, moving at 186,000 miles per second, needed an hour and 24 minutes to complete a one-way trip between Saturn and Earth. As a result, Cassini's on-board computer was responsible for carrying out the most critical maneuver since launch Oct. 15, 1997, a maneuver that simply had to work or the mission would end in failure.

To everyone's relief, Cassini's main engine fired up on time at 10:36 p.m. EDT and shut down at 12:12 a.m., putting the craft in its planned initial orbit around Saturn.

"Flight, telecom," the Cassini communications officer called out. "The Doppler has flattened out."

Translation: Cassini's engine had shut down and Cassini was in orbit. Flight controllers burst into cheers, sharing hugs and high fives as Cassini lived up to its reputation for near flawless operation.

"It feels awfully good to be in orbit around the lord of the rings," said Charles Elachi, director of the Jet Propulsion Laboratory. "It's going to be a huge leap in our understanding of the Saturnian system."

Ed Weiler, NASA's associate administrator for space science, described the rocket firing as 96 minutes of purgatory during a news briefing Wednesday. Halfway through the burn, "I started to think gee, here we are sitting on this little pale blue dot, third rock from the sun. We just landed on Mars twice. We flew by a comet and picked up some comet dust (with the Stardust mission) and all within six months, we're about to go into orbit around a planet a billion miles away. How do we get away with having so much fun?

"This has just been an incredible ride," he said. "This wasn't NASA going into orbit around Saturn, it's the Earth going into orbit around Saturn because 17 countries made this happen. This is the way exploration should be done: by the Earth."

David Southwood, director of science for the European Space Agency agreed Cassini is a "world mission."

"But this evening I have to say, it's been the Americans' evening," he said. "This was America doing it right. ... There are Europeans involved in just about everything in the instrumentation, the science on Cassini and Huygens. It really is a mission where everybody is working together.

"But this evening, you guys did it right," he said. "Thank you JPL, thank you USA, thank you NASA."

Referring to ESA's Huygens probe, which will make a parachute descent into the atmosphere of Saturn's moon Titan in January, Southwood said the Saturn Orbit Insertion rocket firing would be a tough act to follow and "we have now to get it right, too."

"You really showed us how it's done. It was a very professional show and frighteningly on the nail. We've got a lot to live up to. Thank you everybody. It's been a great eveing."

The most sophisticated - and expensive - robotic spacecraft ever built, Cassini approached Saturn from below the plane of its rings. Using its high-gain antenna as a shield, the spacecraft sailed through the ring plane at 10:11 p.m., passing through a broad gap between Saturn's F and G rings. The region was thought to be empty of any debris larger than dust grains, but at Cassini's enormous approach velocity - more than 53,000 mph at that point - impacts posed a major concern.

But right on schedule, after Cassini re-oriented itself for the Saturn Orbit Insertion rocket firing, ground stations in Australia and California picked up Cassini's radio carrier signal at 10:27 p.m. EDT, confirming the spacecraft had survived the ascending ring plane crossing.

"One hurdle down, one to go," said Todd Barber, lead propulsion officer. "We're approaching two minutes before the SOI burn. The hopes and dreams of thousands of scientists and engineers are resting on the next few moments. So Godspeed, Cassini-Huygens. May we see you in orbit."

And with that, the moment of truth was finally at hand.

As timers counted down to the start of the Saturn Orbit Insertion rocket firing, engineers at JPL monitored computer screens showing a graphical representation of the carrier signal from Cassini. They were looking for a very precise, predicted change in the frequency of the signal due to the effects of the rocket firing, much like a siren changes pitch as a police car races past.

And right on schedule, at 10:36 p.m., the signal changed exactly as predicted. On computer screens, a horizontal line representing the carrier frequency suddenly bent sharply downward, matching the slope predicted for a normal rocket firing. Flight controllers burst into applause, relieved to know Rocket Engine Assembly A had fired on time to begin slowing Cassini's ever-increasing velocity.

Producing just 100 pounds of push against the enormous 54,000-mph velocity of the 9,970-pound Cassini, the main engine had to fire 96.4 minutes to produce the required deceleration and to ensure Saturn's gravity could capture the spacecraft and warp its trajectory into the planned orbit.

Thirty minutes into the burn, at 11:06 p.m., Cassini moved behind Saturn's A ring as viewed from Earth, dimming the carrier signal for about 25 minutes. After fading in and out as it was blocked by ring debris, relatively clear reception was established at 10:31 p.m. when Cassini had a brief, clear view of Earth again through a gap in the rings known as the Cassini division. Six minutes later, exactly as predicted, communications dropped out again for 28 minutes or so as the spacecraft moved behind the thicker B ring.

Still picking up speed from Saturn's gravitational attraction, Cassini reached periapsis, the closest it will ever be to Saturn - 12,400 miles from the cloud tops - at 12:03 a.m., just nine minutes before the end of the SOI burn. By that point, Saturn's gravity had boosted Cassini's velocity to a blistering 69,350 mph, four times faster than a space shuttle in Earth orbit and 32 times faster than the bullet from an assault rifle.

Waiting for the carrier signal to reappear from behind the B ring, Barber provided an impromptu Saturn weather report, predicting temperatures of "minus 226 degrees Fahrenheit, winds of 1,100 miles per hour or so, pressure highly variable depending on where you are in the atmosphere. At the top of the atmosphere, better than the best vacuum on Earth. Down in the depths, millions of atmospheres of pressure. Chance of helium rain inside the interior: 100 percent. Hurricanes the size of the Earth."

By the end of the SOI burn at 12:12 a.m., the velocity had dropped to around 68,000 mph as Cassini streaked away from the planet after close approach. While most reporters (including this writer) were not aware of it, navigators changed their prediction for the burn duration Wednesday, expecting 97 minutes instead of 96. Analysis of the carrier signal's frequency showed the rocket engine actually generated about 1 percent more thrust than expected. Cassini's flight computer compensated by shutting the engine down one minute early to achieve the planned deceleration of 1,400 mph. That translated into a 96-minute burn as originally expected.

With the conclusion of the SOI rocket firing, Cassini was finally in its planned initial orbit around Saturn. Over the next four years, the spacecraft will study Saturn's windy atmosphere, its complex ring system, several of its icy moons and how the planet's magnetic field interacts with the space environment. In what promises to be one of the most exciting phases of the mission, a European-built probe called Huygens will be released from Cassini on Christmas Eve for a parachute descent into the thick nitrogen atmosphere of Saturn's moon, Titan, on Jan. 14.

In all, Cassini is expected to complete 77 orbits of Saturn over the next four years, requiring 157 trajectory-nudging rocket firings. The gravity of Titan will be used for major course changes, with 45 planned flybys. Seven close flybys of smaller, icy moons also are planned.

Safely in orbit, Cassini turned so that its high-gain antenna was aimed back toward Earth for a brief, 20-second burst of carrier signal at 12:30 a.m. That switch from the low-gain to the high-gain antenna confirmed the spacecraft was operating normally and had not suffered any "safing" events during the burn that could have shut down science operations during Saturn close approach.

"We've got it!" Barber reported as yet another round of cheers and applause burst out. After sending the brief call home, Cassini turned away to begin a 75-minute sequence of ring observations.

"I feel great!" said program manager Bob Mitchell. "It was kind of a nail biter throughout."

One hour and 46 minutes after the end of the SOI burn, Cassini was expected to turn once again, orienting itself so the high-gain antenna could act as a shield during a descending ring plane crossing. Once safely through the ring plane, Cassini was expected to begin transmitting science and engineering data back to Earth. The first pictures were expected around 8:40 a.m. Thursday.

On July 2, Cassini will make its first official flyby of Titan, passing the cloud-shrouded world at a distance of 205,000 miles.

Larger than Pluto and Mercury, Titan's thick nitrogen atmosphere is thought to mirror Earth's shortly after the planet's formation. Based on approach photos, Cassini's cameras should be able to "see" the surface through specific spectral "windows." But just how well the cameras will be able to image the surface won't be known until after the Friday flyby.

Data playback from the Titan flyby is expected to begin around 6:15 p.m. Friday. If all goes well, a minor trajectory correction maneuver is scheduled Saturday at 8:30 p.m. to fine tune the orbit with a predicted velocity change of just 11 mph. Starting July 6, Cassini will be out of contact while Saturn passes behind the sun as viewed from Earth, completing the initial phase of Cassini's orbital mission.

In late August, a major rocket firing is planned to raise the low point of Cassini's orbit well beyond the rings and to set up the second Titan flyby Oct. 26. After another Titan flyby Dec. 13, the European Space Agency's Huygens probe will be released from Cassini on Christmas Eve for the three-week trip to Titan.

Huygens will slam into Titan's atmosphere on Jan. 14 for a two-and-a-half-hour parachute descent to the surface. Data from Huygens, including panoramic pictures of its enigmatic surface, will be beamed back to Earth through Cassini's radio system.

After that, Cassini will continue on its own, flying through a ballet of ever-changing orbits and beaming down up to four gigabytes of data per day.


GO CASSINI!!!!

[spinbird]

Originariamente inviato da Fede
e' telmente perfetto che sembra fatto al pc

e l'uomo non è mai andato sulla luna?

[gpc]

Originariamente inviato da Fede
e' telmente perfetto che sembra fatto al pc

Perchè, tu credi di vedere delle imperfezioni o sbavature in una foto fatta da qualche milione di km con una risoluzione di qualche centinaio di km per pixel?
Ma ripeto la domanda: come dovrebbe essere?

[gpc]

Originariamente inviato da spinbird
e l'uomo non è mai andato sulla luna?

Ovvio
Ci sono gli americani di mezzo

Questa volta però c'è anche l'ESA, forse qualche brandello di verità ci arriverà...

[cionci]

Se guardi bene ci sono delle tempeste in alta quota...

Il fatto è che quelle immagini hanno una risoluzione mediocre...ed il risultato che vediamo è una interpolazione fatta al computer di vari frame successivi...

[gpc]

Originariamente inviato da cionci
Se guardi bene ci sono delle tempeste in alta quota...

Il fatto è che quelle immagini hanno una risoluzione mediocre...ed il risultato che vediamo è una interpolazione fatta al computer di vari frame successivi...

Mediocre?
La risoluzione è la migliore che si sia mai ottenuta, è che essendo fatta credo di ricordare a più di un milione di km di distanza non puoi pensare di vedere dei particolari di qualche metro, come un'ombra frastagliata sugli anelli...
E non credo che sia interpolazione di frame successivi, se guardi sul sito della missione tra le foto raw trovi tutte le singole foto.

[Frank1962]

Originariamente inviato da gpc
Perchè? Come dovrebbe essere?

dovrebbe essere orientata come l'ombra sul pianeta e non in orizzontale, presumendo che l'unica fonte di illuminazione sia il sole

[GioFX]

Ehm, se nessuno l'ha notato... Cassini-Huygens, la più costosa e complessa macchina scientifica creata dall'uomo è la prima sonda ad orbitare Il Signore degl Anelli, e la seconda ad orbitare un pianeta del sistema solare esterno.

3,3 miliardi di $ non sono andati persi, perchè una delle l'operazione più delicata in assoluto delle tre fasi centrali della missione (flyby di Venere, inserimento nell'orbita di Saturno, e la discesa di Hyugens su Titano), e l'unica la cui perfetta esecuzione era fondamentale per il successo dell'intera missione, è andata a buon fine, senza il minimo problema.

Parliamo di un evento scandito in minuti, preciso come un orologio, che avveniva a 880 milioni di km di distanza con una macchina che viaggiava a 69.000 km/h attraverso miliardi di massi e rocce degli anelli di Saturno.

[gpc]

Originariamente inviato da Frank1962
dovrebbe essere orientata come l'ombra sul pianeta e non in orizzontale, presumendo che l'unica fonte di illuminazione sia il sole

Mi sa che ti devi fare un ripassino di geometria...
In quella foto il sole è in basso, l'ombra è un cono che parte dal pianeta in direzione diagonale da basso-dx ad alto-sx.
Se metti gli anelli di mezzo ottieni una proiezione, ovviamente sul piano degli anelli, dell'ombra... ed ecco l'immagine che viene fuori. Normalissima geometria... che ha di strano?

[gpc]

Originariamente inviato da GioFX
Parliamo di un evento scandito in minuti, preciso come un orologio, che avveniva a 880 milioni di km di distanza con una macchina che viaggiava a 69.000 km/h attraverso miliardi di massi e rocce degli anelli di Saturno.

E sai cos'hanno usato come parasassi? L'antenna italiana

[GioFX]

ARRIVAL! Cassini Enters Orbit Around Saturn

By Leonard David
Senior Space Writer
posted: 01:20 am ET
01 July 2004

BOULDER, COLORADO -- The Cassini tour bus to Saturn has arrived. After a nearly seven year journey, the spacecraft swung into an orbit around the giant gas globe tonight, ready to spend the next four years performing scientific investigations of the Saturnian system.

Beginning Wednesday evening, an engine on the nearly six-ton spacecraft throttled up and fired for more than 90 minutes, slowing Cassini down and placing it into an initial orbit around Saturn. That maneuver is called the Saturn Orbit Insertion, or SOI.

The SOI burn was critical to the success of the mission at Saturn. Cassini approached Saturn from below the planet’s ring plane, crossing through the large gap between the F Ring and G Ring.

Saturn's ring system is divided up into 7 major divisions, with the innermost ring to the outermost ring designated as D, C, B, A, F, G and E Ring. Each major ring division is further subdivided into thousands of individual "ringlets". These are made of ice particles and rocky material.

The spacecraft’s main engine was turned to face the direction of travel, and the resulting thrust from the engine acted as a braking device, slowing down the spacecraft as it entered Saturn's orbit.

Price tagged at over $3 billion, the mission is the most ambitious planetary mission ever conducted.

More moons?

Cassini’s engine burn lasted 96 minutes, placing the spacecraft into a highly elliptical orbit around Saturn.

The spacecraft’s closest approach to Saturn during its basic four-year tour occurred during the engine firing. The spacecraft's distance from Saturn was about 11,184 miles (18,000 kilometers), or less than a sixth of Saturn's diameter.

Cassini is now continuing to coast above the rings for approximately one hour and 44 minutes before its descent back through the ring plane.

As Cassini begins surveying the Saturnian system, ahead for the spacecraft is at least 76 orbits around the ringed planet, including 52 close encounters with seven of Saturn's 31 known moons. Scientists speculate that more moons orbiting the planet may still await discovery.

Extended mission

Program managers and scientists have already begun to discuss an extended Cassini mission, beyond the four-year primary mission. That extended mission might last 4 to 6 years, perhaps as much as 8 years if onboard fuel holds out.

Along with an array of science instruments, Cassini is toting the European Space Agency’s Huygens probe. The probe’s task is to parachute into the thick atmosphere of Titan -- Saturn's largest moon -- in mid-January of next year.

The Cassini-Huygens mission is an international undertaking led by three space agencies: NASA, the European Space Agency and the Italian space agency, Agenzia Spaziale Italiana (ASI). Seventeen nations contributed to building the spacecraft. The project is managed by the Jet Propulsion Laboratory (JPL) in Pasadena, California.

Anxious scientists

Numbers of anxious space scientists have gathered here tonight at the Colorado University’s Laboratory for Atmospheric and Space Physics (LASP). And for good reason.

Onboard Cassini is a $12 million CU-Boulder instrument -- LASP's Ultraviolet Imaging Spectrograph, or UVIS. It is one of the 12 scientific instruments that the craft has hauled to Saturn.

The UVIS instrument package has a set of telescopes to measure UV light reflected by or emitted from Saturn's atmosphere, its rings and its moon atmospheres and surfaces. The data collected can determine their compositions, distribution, aerosol content and temperatures.

Name plate

Kip Denhalter, an electronics engineer at LASP helped build the UVIS instrument.

"There’s a sense of accomplishment. In the case of UVIS, an added feature on our flight instrument is that there’s a plate on the back to cover all the cables. All the people that worked on it, our names are nicely engraved on that plate. So we’ve got our names in orbit around Saturn, Denhalter said. "It’s kind of neat to point to Saturn and say I’ve got something I worked on out there," he told SPACE.com.

Alain Jouchoux, operations team leader for the UVIS, said he expects the device to keep working for many, many years – far beyond Cassini’s initial exploration goal of four years. "It can work forever," he confidently added.

Best photos in our lifetime

"Officially, we’re in orbit," said Jim Crocker, Vice President, Civil Space for Lockheed Martin Space Systems in neighboring Denver, Colorado. "We really needed to nail it and it looks like we did. This will give us a lot of science," he told SPACE.com in a phone interview.

Lockheed Martin built the spacecraft’s propulsion system, 16 thrusters, the nuclear power generators, and assembled a camera that is onboard the Huygens probe.

Crocker said imagery from Cassini is forthcoming. "These will be the best images of the rings that we’ll see in our lifetime," he added. "We’re very happy. Of course this is only the beginning. We’ve got four years to go and we’ll be firing up the engine a number of times."

[GioFX]

Originariamente inviato da gpc
E sai cos'hanno usato come parasassi? L'antenna italiana