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Cassini Mission to Saturn Celebrates 10 Years Since Launch
Celebrating the 10th anniversary of its launch from Cape Canaveral, the Cassini-Huygens mission to Saturn is once again at the centre of scientific attention. Its latest discoveries about the ringed planet are a leading topic of conversation among the nearly 1,500 scientists gathered this week at a major astronomy conference in Orlando, Florida, US.
Cassini rode into space Oct. 15, 1997, atop a U.S. Air Force Titan IVB. Its mission: to orbit and study the Saturnian system for four years and to put the European Space Agency's Huygens Probe in position to parachute down to the frozen surface of Saturn's Earthlike moon Titan. Since entering orbit around Saturn, Cassini's scientific instruments, powered by radioisotope thermoelectric generators, have returned immense amounts of new information via NASA's global Deep Space Network to the international team of scientists working on the mission.

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The international Cassini spacecraft went into safe mode this week after successfully passing over a Saturn moon that was the mysterious destination of a deep-space faring astronaut in Arthur C. Clarke's novel "2001: A Space Odyssey."
Cassini flew within 1,000 miles of Iapetus on Monday and snapped images of its rugged, two-toned surface. As it was sending data back to Earth, it was hit by a cosmic ray that caused a power trip. The spacecraft was not damaged, but had to turn off its instruments and relay only limited information.
Mission controllers recently sent commands for Cassini to resume normal transmission and the spacecraft could be fully functional by week's end.

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The most recent spacecraft telemetry was acquired Wednesday, October 4, from the Canberra tracking stations. The Cassini spacecraft is in an excellent state of health and is operating normally.

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It was reported in the Cassini Significant Events last week that the atmospheric density measured at Titan 16 was higher than expected. The Titan Atmospheric Modeling Working Group (TAMWG) met this morning to discuss this result and assess whether the altitudes selected for upcoming encounters are still acceptable. At the conclusion of the meeting, the TAMWG recommended no changes to the altitudes of upcoming encounters. The new T16 data point has not fundamentally changed our understanding of Titan's atmospheric profile, except to indicate a less steep falloff in density at high latitudes. Cassini only has one future Titan flyby at very high latitude, Titan 32. Much like T16, this encounter is at 84 deg N latitude with a closest approach at 950 km, so it should be safe as well.

One of the things that presents a challenge to Cassini scientists and sequence developers is attempting to respond to new scientific discoveries as they arise. Background sequence S24 completed Science Operations Plan Implementation in March 2004. At that time the basic sequence was delivered to the project file repository. Science Planning was chartered to archive a sequence that if necessary would fly on the spacecraft "as is." Now roll the clocks forward to July 27, 2006. S24 has been through both the Aftermarket Process and the Science Operations Plan Update Process. It is currently in final development prior to uplink. The first of four phases of that development process is complete. But, due to the discovery of lakes near Titan's north pole over the weekend, RADAR has requested re-pointing of their T19 observation to cover more of the same region. What to do?

It was decided to stick with the process and have RADAR submit their pointing changes in the third phase. Although this decreases the number of iterations available to get the pointing right, it allows the other instruments and AACS time to properly assess any impacts they might have. To support the development of this observation, an interim set of sequence products was produced with the new RADAR pointing, to give the Attitude and Articulation Control Subsystem (AACS) time to evaluate it and make a c-kernel available to teams that might wish to examine it. At this time S24 is on track for final approval in September, and it is hoped to have lakes in its future.

Friday, July 28 (DOY 209):
Science data archive deliveries for data acquired during the period of July through September 2005 are now complete. The next archive delivery port is October 1.

The official port occurred today for S25 as part of the Science Operations Plan (SOP) Update process. The merged products are currently being run through end-to-end pointing validation by AACS. The Project Briefing and Waiver Disposition Meeting is scheduled for August 9. The SOP Update product is handed off to the sequence leads on August 11 for the final development process.

Monday, July 31 (DOY 212):
The Cassini RADAR image of Titan's methane lakes is Astronomy Picture of the Day today.

Tuesday, August 1 (DOY 213):
Orbit Trim Maneuver (OTM) #69, an apoapsis maneuver setting up for the Titan 17 encounter on September 7, was performed today. This main engine burn began at 2:30 PM PDT. Telemetry immediately after the maneuver showed a burn duration of 33.8 seconds, giving a delta-V of 5.4 m/sec. All subsystems reported nominal performance after the maneuver. A contingency DSS-15 track that had been retained for the DOY 214 backup OTM-69 window in case the DSS-14 antenna went red was released at the successful completion of the burn. Cassini thanks MGS, MER and Odyssey for their help in making this track available. The next OTM is scheduled for September 4.

Solar Conjunction occurs when the Sun is between the spacecraft and Earth. This year it will last from August 2 through August 12, and is a time of reduced commanding and downlink capability. Cassini enters Solar Conjunction tomorrow with a separation angle of 4 degrees. During conjunction, communications with the spacecraft become degraded due to interference from the sun. For the next ten days the Spacecraft Operations and Mission Support and Services offices will participate in a campaign where a command file consisting of 10 no-op commands will be uplinked to the spacecraft ten times daily. This will allow the teams to obtain link characterizations and accumulate statistics for uplink reliability at decreased separation angles. With the exception of these no-op command files, Spacecraft Operations has asked for and received a command moratorium. Additional commands will only be sent in the event of an emergency. On Saturday, the spacecraft will be turned so that the High Gain Antenna is continuously pointed to Earth, and 1896 bps telemetry continues while separation is less than 2 deg. Normal playback downlink rates will resume on August 10.

Source: Jet Propulsion Laboratory

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As the Cassini spacecraft reaches the halfway mark in its four-year tour of the Saturn system, discoveries made during the first half of the mission have scientists revved up to find out what's in store for the second act. Cassini has been orbiting Saturn since June 30, 2004, studying the planet, its rings and moons.

"The spacecraft has spent a considerable amount of time studying the moon Titan during 15 separate flybys so far. In the second half of its prime mission, ending June 2008, Cassini will swing by Titan 30 more times. The past two years have been just like a warm-up"- Robert T. Mitchell, Cassini program manager at NASA's Jet Propulsion Laboratory, Pasadena, California, US.

"We especially focused on Titan because we thought it could tell us something about the early Earth. Examining this world frozen in time, we find evidence that Earth may have begun with the same methane-ammonia atmosphere that marked the birth of Titan. Because of our world's closeness to the Sun, Earth has oceans of liquid water, which Titan lacks. The resulting chemistry in Earth's warm environment ultimately led to the origin of life, whereas on Titan we find only a frozen echo of early Earth: methane, nitrogen, and a suite of small organic molecules. Our planet's carefully balanced, warm global climate is the underlying reason that we are investigating Titan, instead of Titanians investigating Earth" - Dr. Toby Owen, Cassini interdisciplinary scientist at the University of Hawaii at Manoa.

Cassini's tour of the Saturnian system is about to take on a new pace.

"This summer we will begin our express-ticket ride. That's 11 months with 17 Titan encounters and 51 spacecraft maneuvers to adjust the flight path, more than one manoeuvre per week" - Jerry Jones, Cassini chief navigator at JPL.

The first of these encounters will be a Titan flyby on July 2, followed by the closest Titan encounter yet on July 22, at 950 kilometres above the surface.
Later in July, navigators will begin to flip the spacecraft's orbit orientation with respect to the sun by nearly 180 degrees, resulting in a bird's-eye view of Saturn's glorious rings. This gradual transfer will take about one year.

"One of the biggest mysteries confronting Cassini is the changes we've seen in Saturn's radio emissions. We've seen the radio period, the frequency of emissions that tell scientists how fast or slow the planet is rotating, change by as much as one percent (or a few minutes) over just 10 years, and we don't know why. Pinning down how long the day is on Saturn is key to understanding other things, such as wind speed" - Dr. Bill Kurth, Cassini scientist at the University of Iowa, Iowa City.

Cassini has quite a job to do during the second half of the mission to match the potpourri of discoveries in its first half.

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NASA mission controllers encountered a serious software problem with the Cassini spaceprobe last week.

Controllers at Jet Propulsion Laboratory sent a complex command sequence, but found the message had used so much of Cassini's memory that it could accommodate only 720 more words of code not enough to record the entire command.
The controllers then decided to send the sequence in pieces, and allow the spacecraft to reassemble them after it received the complete message.

However when controllers attempted to send the broken up transmissions, they found errors in the spacecraft's software prevented it from reconstituting the commands. Worse, controllers found that their additional transmissions would require up to 13,000 words of memory, Doh!

After further analysis, the controllers managed to find the software errors that appear to stem from an incompatibility between one of the sequence-transmission directives and the spacecraft's basic software package.
Controllers have now fixed the problem, by developing a new technique to send a single code sequence in as many pieces as necessary to accommodate the software limitations.

Currently, Cassini remains in "an excellent state of health and is operating normally"

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Titan flybys
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Titan flyby dates for the Cassini space probe.

Jan. 15, 2006 -- 2,042 kilometres
Feb. 27, 2006 -- 1,812 kilometres
March 18, 2006 -- 1,947 kilometres
April 30, 2006 -- 1,853 kilometres
May 20, 2006 -- 1,879 kilometres
July 2, 2006 -- 1,911 kilometres
July 22, 2006 -- 950 kilometres
Sep. 7, 2006 -- 950 kilometres
Sep. 23, 2006 -- 950 kilometres
Oct. 9, 2006 -- 950 kilometres
Oct. 25, 2006 -- 950 kilometres
Dec. 12, 2006 -- 950 kilometres
Dec. 28, 2006 -- 1,500 kilometres
Jan. 13, 2007 -- 950 kilometres
Jan. 29, 2007 -- 2,776 kilometres
Feb. 22, 2007 -- 953 kilometres
March 10, 2007 -- 956 kilometres
March 26, 2007 -- 953 kilometres
April 10, 2007 -- 951 kilometres
April 26, 2007 -- 951 kilometres
May 12, 2007 -- 950 kilometres
May 28, 2007 -- 2,425 kilometres
June 13, 2007 -- 950 kilometres
June 29, 2007 -- 1,942 kilometres
July 19, 2007 -- 1,302 kilometres
Aug. 31, 2007 -- 3,227 kilometres
Oct. 2, 2007 -- 950 kilometres
Nov. 19, 2007 -- 950 kilometres
Dec. 5, 2007 -- 1,300 kilometres
Dec. 20, 2007 -- 953 kilometres
Jan. 5, 2008 -- 949 kilometres
Feb. 22, 2008 -- 959 kilometres
May 12, 2008 -- 950 kilometres
May 28, 2008 -- 1,316 kilometres

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The Cassini spacecraft will fly-by Saturn's largest moon, Titan, on January 15.
Views from the Huygens probe and Cassini's eight flybys of Titan have revealed that every geologic process on Earth is active on Titan.
Scientists have seen evidence pointing to rivers and channels, a possible lake, a shoreline, what may be a volcano, and an abundance of sand dunes. During 2006, the spacecraft is scheduled to have 13 Titan flybys.


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The most recent spacecraft telemetry was acquired Wednesday from the Madrid
and Madrid tracking stations.



The Cassini spacecraft is in an excellent state of health and is operating normally.

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