Tag Archives: Jupiter

Encyclopedia Entry #5 – Adrastea

This is the first in the Solar System Moons series.  Each entry will go through the moons of the solar system in alphabetic order.  This time, we will start with Adrastea.

Adrastea is also known as Jupiter XV.  The smallest of the inner moons of Jupiter, it was the first moon discovered by a spacecraft, Voyager 2.  It was discovered on July 8, 1979.  Very little is known about the surface of Adrastea, as it is very small and was not photographed very much at all.  It’s known to be tidally locked to Jupiter and orbit at the edge of Jupiter’s Main Ring.


  • Dimensions: 20 x 16 x 14 km
  • Mass: ≈ 2×1015 kg
  • Mean density: 0.86 g/cm³
  • Surface gravity: 0.0004 g
  • Albedo: 0.10±0.045
  • Temperature: ≈ 122 K
  • Mean orbit radius: 129000 km
  • Orbital period: 0.29826 d
  • Inclination: 0.03°
  • Eccentricity: 0.0015

Name Origin

Adrastea was named after the nymph who nursed the infant Zeus in Greek mythology.  This was in order to protect him from his father Cronus.

5 Interesting Facts

1. It was the first natural satellite to be discovered not by a telescope, but by a spacecraft.

2. It’s thought to be the main contributor to Jupiter’s rings.  It orbits at the edge of the Main Ring.

3. Very little is known about Adrastea, but if it’s like Amalthea, it’s thought to be composed mainly of water ice with a porosity of 10-15%, as its density is lower than water.

4. It’s one of only 3 moons that orbit its parent planet faster than the planet’s day.  The other two are Metis, the innermost moon of Jupiter and Phobos, the innermost moon of Mars.

5. As it lies within Jupiter’s synchronous orbit radius, it will eventually crash into Jupiter as its orbit decays.  However, as it appears to be just outside the Roche limit, it hasn’t been been broken up.

And that is Adrastea. It’s not a well-known moon, since it hasn’t been studied much at all.  The Galileo probe didn’t study it much, as you can see by the image taken by it.  It’s pretty fuzzy.  I hope you learned something about Adrastea.


Celestial Rendezvous

As many people know, I am a bit of a science geek.  I was the science geek in high school, and I even got a degree in physics and astronomy in University.  Science is something that drives a lot of my thoughts.  I’m always reading about science, looking around at nature, and looking up at the sky.  I love science, yet I enjoy reading science fiction, even if it isn’t very scientifically accurate.  But on the topic of science, I saw something amazing earlier this week.  I’m sure many of you saw the same thing.

There was a conjunction of 3 bright lights in the sky, Venus, Jupiter and the crescent moon.  I hadn’t seen anything this wonderful in the sky since the planetary alignment of 2002, when all 5 visible planets could be seen at above the horizon after sunset.  Incidentally, that was the first time I saw Mercury.  For the past month, Venus and Jupiter have been hanging out together in the evening sky to the west after sunset.  I managed to get a couple of pictures of this sight with my phone.

Venus and Jupiter over the Fujisawa sky. Venus is the brighter planet.
Here's a closer view of Venus and Jupiter.

I was quite impressed when I saw this.  At the same time, red Mars was shining brightly overhead, while Saturn was rising to the east along with the moon.  But this wasn’t the best to be seen.  The moon moved its way across the sky as it always does, and settle right between Venus and Jupiter.  Perfect!  It was a wonderful opportunity to take another picture of a celestial meeting.

A crescent moon slipped between Venus and Jupiter.

Mars and Saturn were also visible at the time, so 4 of the 5 visible planets were above the horizon.  Unfortunately, Mercury is on the wrong side of the sun to be visible in the evening.  With good timing, it could be visible just before sunrise, though.

This has been an incredible winter and spring to watch the skies.  I was treated to a total lunar eclipse and I was able to see a few meteors during a meteor shower.  I hope you’ve all enjoyed watching the same amazing events.