On this day in space! Dec. 23, 1672: Giovanni Cassini discovers Saturn's moon Rhea

a grey orb pockmarked with craters
The Cassini spacecraft looks toward the cratered plains of the trailing hemisphere of Rhea. (Image credit: NASA/JPL/Space Science Institute)

On Dec. 23, 1672, Italian astronomer Giovanni Cassini discovered Rhea, Saturn's second-largest moon. Rhea is made up of rock and ice and is covered in craters.

Rhea is also the only moon discovered to have an oxygen atmosphere. Rhea was the second moon that Cassini discovered. He discovered Saturn's moon Iapetus about a year earlier.

Rhea wasn't formally named until 1847.

The British astronomer John Herschel suggested that Saturn's moons should be named after the Titans. In Greek mythology, the Titans were the brothers and sisters of Cronus, who the Romans referred to as "Saturn."

Cassini originally named the moons he discovered after the Sidera Lodoicea, meaning the "Stars of Louis," after King Louis XIV. Because Cassini discovered four of Saturn's moons, NASA named the Cassini spacecraft after him.

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Hanneke Weitering
Contributing expert

Hanneke Weitering is a multimedia journalist in the Pacific Northwest reporting on the future of aviation at FutureFlight.aero and Aviation International News and was previously the Editor for Spaceflight and Astronomy news here at Space.com. As an editor with over 10 years of experience in science journalism she has previously written for Scholastic Classroom Magazines, MedPage Today and The Joint Institute for Computational Sciences at Oak Ridge National Laboratory. After studying physics at the University of Tennessee in her hometown of Knoxville, she earned her graduate degree in Science, Health and Environmental Reporting (SHERP) from New York University. Hanneke joined the Space.com team in 2016 as a staff writer and producer, covering topics including spaceflight and astronomy. She currently lives in Seattle, home of the Space Needle, with her cat and two snakes. In her spare time, Hanneke enjoys exploring the Rocky Mountains, basking in nature and looking for dark skies to gaze at the cosmos. 

  • The Exoplanets Channel
    Very interesting
    Reply
  • Arc Light
    Just so you know, the X-15 took off from Edwards AFB under the wing of a B-52 & was airdropped over Delamar Dry Lake, Nevada. The flight to the altitude record wasn't planned, but the rocket engine burned for two seconds longer than expected. The test flight was scheduled to peak at 280,000', but actually reached 314, 750' due to the extra rocket burn time.
    Reply
  • Fourth Root
    Misleading wording. Bob White did not set the world altitude record on July 17th, 1962. Four Astronauts and two cosmonauts had flown higher prior to his flight. One could say it was the highest manned flight of a winged craft. But that's not the wording that was used.
    Reply
  • DrRaviSharma
    On this date in 1969, I was part of NASA Apollo Team

    Contributed to Experiments in orbit and on Surface of Moon (ALSEP) etc.also trained astronauts

    Studied containation on and ouside Spacecraft.

    The Moon gave me employment to work for 5 Years on exciting Human Space flight Programs Skylab, Planning of Space Station and Space Shuttle

    See My picture taken with Buzz Aldrin in 2009
    https://www.space.com/india-moon-landing-not-a-failure.html
    I received Apollo Achievement Award from NASA dated July 20, 1969.

    Thanks Hanneke Weitering for today's Historic post

    Ravi
    Dr. Ravi Sharma
    Reply
  • Mergatroid
    "Allegedly, the moon turns green because of its close proximity to Uranus"

    I'm sorry about that. I hear they have been investigating x-rays from the same source. I had no idea. I'll get a doctor to check into it.

    Sorry everyone.

    Sorry...
    Reply