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Frequently Asked Questions - Mission

Frequently Asked Questions - Mission

What's the different between a 'targeted' flyby and a 'non-targeted' flyby?

A "targeted flyby" (as opposed to "non-targeted flyby") refers to the times when the navigation team is anchoring its trajectory design to the flyby -- tweaking the spacecraft's path to take full advantage of the opportunity. Usually targeted flybys are closer, and are therefore of more scientific interest (and have more scientific opportunity) than non-targeted flybys.

Targeted flybys include maneuvers to achieve a specific altitude that will allow scientists to gather as much science as possible. Typically, these flybys are within 3,000 kilometers (1,864 miles) from a moon's surface.

A non-targeted flyby is one that occurs without any maneuvers. Whenever a moon happens to be relatively close to the spacecraft's path, Cassini simply points the instruments most likely to gather scientifically valid results and keeps going to its next destination. The distance between Cassini and a given moon in non-targeted flybys is usually more than 3,000 kilometers (1,864 miles).

How was the altitude of 50 kilometers chosen for the March 2008 flyby of Enceladus?

A primary science objective for the March 2008 Enceladus flyby is to understand details about Enceladus' plume, including its source and composition. To accomplish this, closer is better. At the same time, it is essential that the safety of the spacecraft not be jeopardized in the process. The two threats to the spacecraft were identified to be an inadvertent impact with Enceladus, and damage resulting from the environment within the plume. With regard to the former, the Project studied its navigational capability and past performance together with its knowledge of Enceladus' position, and concluded that at 50 kilometers, and even somewhat closer than that, there was just no possibility of an impact. In assessing the environmental hazard, one of the main data sources used was an observation of a stellar occultation by the plume made by the Ultraviolet Imaging Spectrograph instrument on the spacecraft. In this observation, the instrument observed the star as it passed behind the plume, and measured the varying intensity of the starlight during the passage. This information gave the scientists considerable information on the size and distribution of the particles within the plume. A second source of information was dynamical modeling work done of the plumes. The result from primarily these two sources was that there is virtually no prospect of there being any particles large enough to damage the spacecraft at altitudes even well below 50 kilometers.

The previous closest approach of Cassini to Enceladus was at 175 kilometers. The science results obtained during this flyby were compared with what can be expected at 50 kilometers and closer, with the result that 50 kilometers will yield substantially better science return than was obtained at 175 kilometers, but that going even closer provided only a minimal further improvement. Another consideration was the fact that there are four Enceladus encounters in the proposed extended mission that go even deeper into the plume than the March '08 flyby does. Therefore 50 kilometers was chosen as an appropriate tradeoff between optimal science and risk to the spacecraft.

Why are the flyby names (e.g. T-99) not in sync with the actual count of the fly-bys?

In designing the original Cassini-Huygens mission, mission planners named every Titan flyby in a straightforward manner: T-1, T-2, T-3 and so on. However, during the spacecraft's seven-year journey to the Saturn, engineers uncovered a communication problem between the spacecraft and the probe that could have cost the loss of a significant amount of the data transmitted by the probe to the orbiter.

Engineers realized that to be able to receive the data sent by Huygens, the closing rate between the probe and the orbiter during the relay period had to be reduced. Planners solved the issue by shortening the first two orbits around Saturn and adding a third orbit. The third orbit provided the required geometry for a successful Huygens mission and brought the orbiter back to the original orbital plan as soon as possible.

At that point, mission planners could have renamed every orbit or only change the names of the modified first three orbits. Because a lot of products -- such as spreadsheets and lines of software code -- had already been written using the original naming convention, to avoid mistakes or confusion, the replacement orbits were renamed T-A, T-B and T-C -- leaving all following orbits' names intact. After T-C, Cassini performed T-3, followed by T-4, T-5 etc.

Interestingly, all the Cassini instruments were switched off during the Huygens mission at T-C. So, while T-99 is the 100th flyby of Titan, it is only Cassini's 99th opportunity to collect information on this intriguing moon.

When did Cassini arrive at Saturn?

Cassini successfully entered orbit around Saturn on June 30, 2004, Pacific Daylight Time. At 9:12 p.m. PDT on June 30th, flight controllers received confirmation that Cassini had completed the engine burn needed to place the spacecraft into the correct orbit. For more information, visit

Has any previous spacecraft orbited Saturn?

No. Pioneer 11 flew by Saturn in 1979, and the twin Voyager spacecraft flew by in 1980 and 1981, but no spacecraft have ever orbited Saturn before.

What will happen to Cassini at the end of its mission? Will it be deliberately crashed into Saturn, as the Galileo spacecraft was crashed into Jupiter?

If Cassini is in good health at the end of its primary four-year mission, NASA may extend its mission. Cassini's ultimate fate has not yet been decided.

What is to become of the spacecraft? Is it eventually going to degrade and smash into Saturn's atmosphere?

This is currently undecided. We are waiting until we see what targets may turn out to be environmentally sensitive, and then make a decision regarding the final disposition of the spacecraft. Going into Saturn's atmosphere as Galileo did at Jupiter may be difficult to accomplish because of the need to fly through the rings for an orbit or two and yet maintain a functioning spacecraft capable of going the rest of the way down to the atmosphere. The orbit of the spacecraft won't degrade by itself. We would have to actively control it to a Saturn impact if that is the way the mission is finally ended.

What it would take to insert Cassini into orbit around an interesting moon, say Enceladus or Titan? Would a gravity assist maneuver with the planet or other moon help?

Cassini's propulsive capability is orders of magnitude less than it would take for an orbit insertion at one of Saturn's moons.

Cassini currently uses Titan for gravity assists in almost every orbit. The spacecraft uses the resulting push to set up the geometry for each Saturn orbit. Titan assists will permit the orbit inclination to be increased greatly so the spacecraft will be able to "look down" on the rings rather than always seeing them "edge on" as has been the case ever since Saturn Orbit Insertion in July 2004. This inclination change will start in August 2006.

However, Titan cannot provide enough gravity assist to do anything like orbit capture. Nor can Saturn, since it's the body being orbited. The other moons of Saturn are way too small to provide any useful effect.

Orbiting Jovian planets' moons is of immense interest to the scientific community. Jupiter's moon Europa, Titan and Enceladus are all compelling targets, but in-depth studies of these moons will have to wait for dedicated missions -- carrying lots of propulsive capability -- to be funded, designed and launched in the future.

For more about "gravity assist," see A Gravity Assist Primer or Basics of Space Flight.

How can I find out where Cassini-Huygens is right now?

Go to You'll find computer-simulated views of Saturn and Earth as seen from Cassini, along with the spacecraft's position, distance traveled, distance from Saturn, and other interesting information. It's updated at least daily. You can also get there from NASA's Cassini-Huygens home page by clicking on "Where is Cassini now?"

What does the Cassini-Huygens mission cost?


These figures are from the press kit, "The Jupiter Millennium Mission," which was prepared in October 2000. You can see it at, where you can also find the press kit that was prepared for the launch in 1997.