Cassini Question of the Week Archive submit a question!

September 29, 2004
What is the size necessary to determine whether an object is considered a moon as opposed to space debris? Are there any other characteristics that would be used in distinguishing moons from space debris?
Both human-made and natural satellites can be found in orbit around selected planets of our solar system. Those human-made objects that orbit planets other than the Earth are space probes sent on missions of scientific research. The human-made objects that orbit the Earth itself may be operational satellites, fragments from booster rockets, tools accidentally dropped by astronauts, etc. If the orbiting human-made objects are no longer functional, they are traditionally considered space debris and are monitored in order to avoid the possibility of collisions with astronaut crews moving to and from orbit in the course of their work. It is also worthy to note that space debris may be found in a variety of orbital paths that differ widely from those in which one would normally expect to find natural satellites (i.e. moons). Indeed one could find space debris in virtually any orbital path in which humans have decided to launch their spacecraft. Equatorial orbits, polar orbits and every conceivable orbital inclination between these two are all candidates for space debris. Furthermore, the orbital paths of space debris can be circular or very elliptical—again depending upon the flight path chosen for the spacecraft from which the debris originated.

On the other hand, if the orbiting objects are natural satellites (as opposed to human-made objects) they may be considered moons depending upon their mass and size. This question is especially difficult for Saturn because the rings are made of thousands of individual particles ranging in size from smaller than a grain of sand to that of a house. The smallest known moon of Saturn measures just 4 kilometers (2.5 miles) in diameter. Apparently, this is significantly larger (by about 100 times) than even the largest particle expected to be present in any of the rings. It also appears that the moons of Saturn, even the very small moons, have the ability to influence the rings in some way. Some of the moons are even called shepherding moons because they seem to herd the material in the rings and shepherd it along such that the ring material strays neither too far nor to close to the planet. It is also possible to think of a moon in terms of its material composition. It may be similar in composition to the material of the planet which it orbits. This would indicate that it was formed perhaps as a result of a large collision that broke off a massive chunk of the planet, thereby hurling the ejected material into space. On the other hand a moon could be a stray object that was captured by the gravitational field of the planet and settled into a stable orbit over many years. Sometimes these types of moons (i.e. captured objects) stand out from other moons because they follow orbital paths that are more elliptical than the paths of the other moons or because the orbital inclination departs from that which is observed for the other moons of the planet's system.

To help us learn more about what interests visitors to the California Science Center, we invite you to submit a question regarding the Cassini-Huygens mission to Saturn or the full-scale model of the spacecraft that is on display in the Air and Space Gallery. We will select one question for each weekly update.

July 30, 2004
What was the source of the sounds sent back by Cassini during its orbit insertion on July 1?
When Cassini flew through the ring plane (which it did twice—once ascending from the southern hemisphere up through the rings and diving downward again through the ring plane after its successful orbit burn) it turned its large high gain antenna in the direction of flight just in case some tiny orbiting particles hit the spacecraft. This turned out to be a good idea. The sounds that NASA recorded were created by tiny particles (about 100,000) that were vaporized upon impacting the spacecraft and picked up by the antenna as a resulting radio signal. To hear the sound and see an animation of the spacecraft as it crossed the ring plane, check out this animation with sound.

May 25, 2004
“Is that an actual hole or a dark spot at the south pole of Saturn?”
This is a great question that perhaps makes it a little easier to understand how the scientists on the Cassini project must feel when viewing such images. Imagine sending a probe to a distant location and seeing evidence of something quite unexpected—but being unable to offer a confident explanation. Scientists are very intrigued about this spot on Saturn primarily because it is so small and centered right at the location of the south pole of the planet. This implies that Saturn’s magnetic field could be responsible for creating or maintaining the spot.

Unlike on Earth, the magnetic field of Saturn is closely aligned to the axis of rotation for the planet. The actual composition of the spot remains a mystery; however, it’s not unreasonable to assume that the spot is not a hole in the atmosphere but rather a concentration of material that has a different physical composition or reflectivity. Last week’s image was taken using a filter that’s sensitive to light near one of the absorption bands of methane gas, which is an ingredient in Saturn's atmosphere. In general, bright areas in that photograph show thick high-level clouds that obscure the view of the darker areas below. So the small spot on Saturn’s south pole could be the result of a magnetic field presence that selectively concentrates material of low reflectivity—at least when viewed through the appropriate filter. A thorough and accurate explanation awaits further study by the project team using data from the Cassini-Huygens spacecraft.

April 9, 2004
"What are Saturn's rings actually made of?"
Based upon ground-based studies of the A and B rings (mostly using infrared wavelengths) it appears that they are composed mostly of water ice. However, the color distribution within the main rings suggests that they are not uniform in composition, and this poses a mystery which hopefully the current mission can address.

April 2, 2004
"What is a composite image and how is it made?"
The images released by NASA during the past 6 weeks (including today's image) were created by the narrow angle camera (NAC). That instrument has filters that can select wavelengths ranging from 200 - 1100 nanometers. (The human eye is sensitive only from 400 - 700 nanometers). The different wavelengths allow the NAC to see vertically down through different layers of the atmosphere of Saturn. When combined into a composite image, the resulting picture shows the natural color of the planet.

March 26, 2004
"I've been taught that Saturn is a gaseous planet. Is any part of it solid?"
The details of Saturn's interior remain a mystery, but using data from the Voyager flybys in the 1980s, along with the principles of physics, scientists have pieced together a fascinating picture of the ringed giant's interior. We believe that Saturn has a molten rocky core about the size of the entire Earth in volume, but with three times more mass than Earth's core. A thick layer of metallic liquid hydrogen surrounds the molten rocky core. Next is a layer of molecular liquid hydrogen. Finally, a huge layer of gaseous hydrogen surrounds the planet, and it's that we see in Cassini photographs.

In contrast to Earth, which has a distinct separation between land, oceans and atmosphere, Saturn only has layers of hydrogen that transform from gas high in the atmosphere (which we see) to liquid deep inside where tremendous pressures and temperatures build up. Due to these conditions, there is no clearly defined boundary between gaseous and liquid states.

March 12, 2004
"How is it that we see such well defined lines/circles since the rings are made of so many particles?"
It's a matter of resolution. Views of Saturn, even the close-up shots from the early Voyager flyby, have been taken at such distances that the granularity of the rings simply doesn't show up. The ring system looks smooth to us because we are so far away. As an analogy, think about a distant galaxy. From a great distance its smooth shape (perhaps a spiral or an ellipse) stands out clearly.

"Do other planets in our solar system have rings?"
Yes, indeed they do. Although Saturn clearly has the most complex and beautiful ring system, all of the other gas giant planets have rings as well. These include Jupiter, Uranus and Neptune. The rings of these planets are too faint to be seen with an Earth-bound telescope and were discovered by the Voyager 1 and Voyager 2 spacecraft.

Saturn's rings in detail

"Is the image from the Cassini spacecraft better than that from the Hubble Space Telescope at this point?"
The resolution of the Cassini spacecraft images will not exceed Hubble capability until late April or early May.