Earth’s Big Moon and Little Moon

Here’s a nice picture of two of Earth’s satellites, the big natural one and the small, artificial, inhabited one, that NASA put on Twitter today. It was taken from a ground telescope in the U.S.. These are the three bodies in the solar system that have been inhabited by human beings, so far. The International Space Station (ISS) is that little black object crossing in front of the moon, that looks a bit like the letter Z.

Here’s some facts about how people would feel on each of these bodies, due to their gravitational attraction. They are courtesy of Newton’s Law

• Gravity = G *M1 * M2 / (R**2), where:
  1. G is the universal gravitational constant.
  2. M1 is the mass of one body.
  3. M2 is the mass of the other.
  4. R is the distance between them.
     
     

• Assume that you have a mass of 100 kg. Then, on the surface of the Earth:
  • The force of gravity that you would feel as a result of your interaction with the Earth’s mass, would be about 952 Newtons. In Imperial units, this would be described as a weight of 220 pounds.
  • If the moon was directly overhead, you would have a force acting upon you from it, acting upwards, of about .00322 Newtons. If the moon was on the other side of the Earth, that force would be directed downwards. This corresponds to a “weight” of about .001 pounds or about .01 ounces (.3 grams). That’s about the weight of a good sized drop of water, at the surface of the Earth. That seems like rather a lot, but that’s what the formula says. But, the moon’s gravitational force is largely responsible for the ocean tides (and even earth tides), so perhaps such a large force shouldn’t be a surprise.
  • If the ISS was directly overhead, you would have a force acting upon you from it, acting upwards, of about 4.2 X 10 -15 Newtons. If the ISS was on the other side of the Earth, the force would be directed downwards. This corresponds to about 9.6 X 10-19 pounds or about the equivalent weight of a single (very small) bacterium on Earth.
• If you were on the surface of the moon:
  • The force of gravity that you would feel due to the moon’s mass, would be about 158 Newtons, or about 36 pounds. That’s the well known one sixth gravity factor.
  • If the Earth was directly overhead, the gravitational attraction between you and it would be about .26 Newtons, directly upwards. That’s about .06 pounds, or an ounce. If you were on the far side of the moon it would add that to your lunar weight.
  • If the ISS was directly overhead, you would have a force acting upon you from it, acting upwards, of about 4.5 X 10 -21 Newtons. If the ISS was on the other side of the Earth, the force would be directed downwards. This corresponds to about 1.1 X 10-21 pounds or about the equivalent weight of a single HIV virus particle.
• If you were on the International Space Station, 400 km above the surface of the Earth:
  • The force of gravity that you would feel due to the Earth’s mass, would be about 843 Newtons, or about 195 pounds. That’s not a lot different from your weight at the surface of the Earth. So, why are the astronauts and cosmonauts “weightless”? It’s because they are in free fall, falling towards the Earth at the same rate that the Earth’s surface is falling away from them, due to curvature of the Earth and the particular path in which the ISS is moving. An object in free fall is equivalent to an object free of gravity (at least for a short time, until you hit the ground, or a sufficiently dense atmosphere to burn up).
  • If the moon was directly overhead, the gravitational attraction between you and it would be about .0033 Newtons, directly upwards. That’s about .0008 pounds, or .01 ounces. If the moon were on the other side of the Earth, that would add that to your weight (though you would still be in free fall, so you wouldn’t know it).
  • You would be attracted to the ISS itself, by a staggering 1.7 X 10 -7 Newtons, which is like .00001 ounces. That assumes that you are about 20 meters from the ISS center of mass, which is probably pretty far out.

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If you want to read about a planet with a really complicated system of moons, you should try Helena Puumala’s latest novel, The Witches’ Stones, Love and Intrigue Under the Seven Moons of Kordea. Seven moons is a neat balancing act, and a dangerous one, as the book makes clear.

http://www.amazon.com/The-Witches-Stones-Book-One-ebook/dp/B008PNIRP4

Of course you might want to try Book One first, Rescue from the Planet of the Amartos. Among other things, it features a planet on the edge of the galaxy.

http://www.amazon.com/The-Witches-Stones-Book-ebook/dp/B008PNIRP4

And don’t forget the Kati of Terra series, which also features plenty of romance and action (and space stations).

http://www.amazon.com/Kati-Terra-Book-One-ebook/dp/B00811WVXO