The rings of Saturn are incredibly thin. The most visible portions of the rings span 280,000 kilometers, and yet they are only about a kilometer thick. The rings aren’t solid, but rather a collection of icy particles and moonlets. Because of this, starlight can pass through the rings. We don’t normally notice it because the rings are so bright, but when the Cassini spacecraft passes into the shadow of the rings, it can watch a star as its light twinkles through the rings. 

The optical depth of Saturn’s rings compared to their visible appearance.

Stars have a known brightness, so when a star is seen passing through Saturn’s rings, the amount the light dims is a measure of how much light is absorbed by the rings. Known as the optical depth, it is a good measure of how thick and dense the rings are at different distances. This allows us to look for patterns that aren’t easy to see just by looking at the rings. While we’ve done this type of thing with stars as seen from Earth, Cassini has the advantage of watching different stars as their light passes through the rings at different angles, which gives us a much richer picture than Earth-based observations alone.

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One of the things astronomers are good at is gathering data in unexpected ways. In astronomy you can’t just go to what you want to study, nor can you do all of your experiments in the lab. Gathering useful data can be difficult, so when you get an event like the transit of Venus or the fortune of Jupiter having four large moons, you take advantage of it. For example, new work being published in MNRAS proposes a way to use the Cassini spacecraft and the rings of Saturn to resolve stars.

An image of the star Mira using the technique.

As Cassini orbits Saturn, it’s view of background stars are often occulted by the rings of Saturn. By observing a star as the rings pass in front of it, we can gather information about the star. Occultations have long been part of the astronomer’s toolbox, such as using the Moon’s occultation of distant stars to discover binary stars, or the occultations by asteroids to determine their shape.  The power of this type of approach is that since an occultation gradually blocks and unblocks the distant object, we can gather data about the object “a slice at a time.”

In this paper, the authors demonstrate how this data can be used to reconstruct an image of the star, which provides data on its size and brightness profiles. They then used occultation data gathered by Cassini to produce an image of a couple stars, such as Mira. The resolution of the images produced varies a bit depending on the occultation, but it is roughly on the order of milliarcseconds, which is pretty impressive. What makes it more impressive is that Cassini was never specifically designed for this kind of thing.

It just goes to show what ingenuity and a wealth of freely accessible data can get you.

Paper: Paul N. Stewart, et al. High Angular Resolution Stellar Imaging with Occultations from the Cassini Spacecraft II: Kronocyclic Tomography. arXiv:1502.07810 [astro-ph.IM]

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Titan is the largest moon of Saturn, and the second largest moon in our solar system. It has a greater diameter than Mercury. It is also the only moon with a thick atmosphere. It has liquid methane rivers and lakes, and has a seasonal climate.

And like our moon, we have landed a probe on its surface. In 2005 the Huygens probe made a one-way journey to the surface of Titan. You can see a video of that landing above.

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