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.

Blog Hop: What Makes a Book Good or Bad?

book blogger hop

I have just discovered the Book Blogger Hop at Ramblings of a Coffee Addicted Writer, and I found this week’s question an interesting one.

To you, what qualifies a book as good or bad? Are some books objectively better than others, or is it purely a matter of opinion?

I would say there’s more of the latter, but a bit of the former.  It’s not an even mix, nor is it all one or the other.

There is some objectivity involved in determining if it’s a good or bad book.  Considering my scientific background, if I read a science fiction novel with particularly bad science, it makes it bad, unless it’s for humourous reasons.  Now, that’s subjective.  But I think the main objective reason a book may be bad is rampant bad spelling and grammar.  That can ruin a reading experience.

But most of it is subjective.  There are a few reasons a book may be good or bad.  First, the characters can make it or break it for me.  If I can’t feel anything for the characters, it’s not a good book.  I need to like the protagonist.  I need to feel their tragedies and triumphs.  They need to be three dimensional with plenty of depth.  They need to seem real to me.  Stereotypes can make a reading experience worse, but not necessarily.  Satire can take stereotypes and use them well.

Well-described settings can help make a book good, but it’s not entirely necessary.  It depends on the book.  Some books use descriptions that are far too long and dry.  Now, this is also subjective.  Lord of the Rings has incredibly long descriptions of scenes that some people can’t take in.  Information overload.  But I find it’s not a problem.  I enjoy it.

The plot needs to be like mozzarella cheese, not swiss cheese.  No holes.  Complex or simple, that doesn’t matter.  Both work depending on the book.  But for a book to be good, the plot needs to make sense and keep me interested.  If it’s predictable, it’s not a good book in my mind.  I want to be surprised.  I want tension.

So, what makes a book good or bad for you?