Cassini’s Last Close Flyby of Rhea

Rhea 4 Flyby, Credit: NASA / JPL
Rhea 4 Flyby, Credit: NASA / JPL

This targeted flyby marks the last time Cassini will get this close to Saturn’s second largest moon, Rhea. But don’t worry too much. Cassini will fly past Rhea again in 2015 for three more non-targeted flybys. (If you want to know more about targeted and non-targeted flybys, check out this link.) We will also gain valuable knowledge into what this moon’s interior looks like from this flyby.

Quick Facts:

  • Name Rhea (R-4) Flyby
  • Date March 9, 2013
  • Altitude 620 miles (997 kilometers)
  • Speed 9.3 kilometers/second (184,000 miles per hour)
  • Goal To understand the internal structure of Rhea

What will we be able to see?

Cassini’s camera system, also known as the Imaging Science Subsystem (ISS) instrument onboard the Cassini spacecraft, will image a crescent Rhea right before the flyby begins. Soon after, ISS will snap some close-up images of Rhea’s north polar and sub-Saturn regions (maximum resolution: 18 meters/59 feet per pixel). Next, ISS will take images to form a mosaic covering Rhea’s northern, sub-Saturn hemisphere (resolution: 370-520 meters/1,210-1,710 feet per pixel). As Cassini departs Rhea, ISS will image the moon at 269,000 kilometers/167,000 miles away. Below is a preview of what some of the images will look like.

Rhea Preview, Credit: CICLOPS
Rhea Preview, Credit: CICLOPS

What science will be coming out of this flyby?

The main goal of this flyby is to determine Rhea’s internal structure. Is Rhea homogeneous all the way through or has it differentiated into a core, mantle and crust? (What is currently known about Rhea’s interior.) Cassini’s Radio Sub-System (RSS) instrument will probe Rhea’s internal structure by measuring the moon’s gravity field. The effect of Rhea’s gravity on the Cassini spacecraft will be measured by looking at the Doppler Effect on Cassini’s radio signal.

That’s not all the science that will be happening during this flyby! Cassini’s Cosmic Dust Analyzer (CDA) instrument will try to count dust particles that fly off Rhea’s surface from tiny meteoroid bombardments. This will help us estimate the bombardment rate for the Saturn system and how often Saturn’s icy rings have been polluted by particles from other places in our Solar System. This will also help us improve estimates of the age of Saturn’s rings.

Also, Cassini’s Composite Infrared Spectrometer (CIRS) instrument will make a temperature map of Rhea’s south polar region.


Want to know more in-depth information on this flyby? Check out Cassini Imaging Central Laboratory for Operations’ (CICLOPS) website with information on Cassini’s activities every day: and the flyby page from Cassini’s main website. (All of my information from this post, unless otherwise specified, comes from these great sources.)

UPDATED: Images!

Narrow, Curious Arc on Rhea's Surface, Credit: NASA/JPL/SSI
Narrow, Curious Arc on Rhea’s Surface, Credit: NASA/JPL/SSI

So, what’s that huge line? At first glance it looks almost like it was drawn onto the surface! It’s actually what appears to be a fracture, cutting across many craters, suggesting that the fracture is relatively recent. There are a few more strange lines off to the right side of the image. Don’t worry — they aren’t a natural phenomenon on Rhea. (They are a result of lossless compression of the image from Cassini to Earth. More information here.)

Rhea's Shadowy Craters, Credit: NASA/JPL/SSI
Rhea’s Shadowy Craters, Credit: NASA/JPL/SSI

Rhea is covered in craters, as seen in the recent images from this flyby. Rhea’s surface can be divided into two different regions: (1) areas that have craters larger than 40 km in diameter and (2) polar/equatorial areas that only have craters below 40 km in diameter. This indicates that during Rhea’s formation, a major resurfacing event occurred. In addition, “wispy lines” can be found throughout Rhea’s surface, as well as on Dione and Tethys.  They can stretch tens to hundreds of kilometers, often cutting through plains and craters (similar to the image above: “Narrow, Curious Arc on Rhea’s Surface”). “In 2006, Cassini spacecraft images showed that the wispy areas are subsidence fractures that make canyons (some of them several hundred meters high). The walls of those canyons are bright because darker material falls off them, exposing fresh bright water ice. These fracture cliffs show Rhea may have been tectonically active in its past.” (From Cassini’s main website and more information from Wikipedia.)

A Portrait of Rhea, Credit: NASA/JPL/SSI
A Portrait of Rhea, Credit: NASA/JPL/SSI

Read more about these images here, as well as a few more selected images from the flyby. If you want to see ALL of the images, check out this link.

Cassini’s next flyby: Titan (T-90) Flyby occurs on April 5 so get excited! (Scroll down to the very bottom of the page to see a countdown to Cassini’s next flyby.)


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