Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Things in orbits

  1. Dec 22, 2013 #1
    Hello. I asked a question a couple of weeks back about things in orbit. Now, I am watching a video on youtube about when mankind disappears, and they mentioned that the artificial satellites in space will 'run out of batteries and plummet to earth'.

    So, I am curious again. Why will these things come back to earth, yet asteroid belts can exist without batteries, and not descend? What is the difference? The moon? Why do they not require maintenance to maintain their orbits, but artificial things do?

    Thank you
  2. jcsd
  3. Dec 22, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Can you link the video?

    If I had to guess, I'd guess they mean that without power the satellites will be unable to do stationkeeping and will eventually drift from their normal orbits due to drag and gravitational interactions. Some will likely fall to Earth eventually, but I don't know if ALL will.
  4. Dec 22, 2013 #3
    Why do rocks in asteroid belts not drift from their orbits due to drag and gravitational interactions? Really this is confusing me.
  5. Dec 22, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Satellites near the Earth experience drag because there is still a small amount of air extending into space. For example, the International Space Station and Hubble Space Telescope have to regularly receive boosts to keep themselves in orbit because of this drag. The asteroid belt is very, very far from anything with an atmosphere, so the density of gas and dust in the interplanetary medium is extremely low, far too low to have any noticeable effects on orbits. (We're talking a couple of atoms per cubic meter on average)

    As for why gravitational interactions don't cause asteroids to drift from their orbits, I don't think I know the answer to that well enough to explain it. I'll let someone else tackle that.
  6. Dec 22, 2013 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Who says that asteroids don't drift from their orbits? They do it all the time, but we don't take special notice because they are usually small and very far from earth. However, an occasional asteroid does come close to the earth. A very small one (about 3 m long) just came between the earth and the moon only three months ago.


    One asteroid about 15 m long entered the atmosphere in February 2013 over Chelyabinsk in Siberia and detonated with the force of a nuclear weapon. At the same time, there was a much larger asteroid which passed within 18,000 miles of the earth.

    The gravitational interactions between the various planets in the Solar System are complicated due to their varying masses and distances from one another. It is thought that having a massive planet like Jupiter is essential for clearing out most of the asteroids between Jupiter's orbit and the sun and keeping the remaining asteroids in their present orbits. It is thought that life was able to obtain a foothold on the earth and begin to evolve because Jupiter kept many asteroids from hitting the earth due to its gravitational influence.
  7. Dec 22, 2013 #6
    They do, but the asteroids (or anything else) sort of have to be in the wrong place, and it only happens over long time scales. Most of the crud in the Solar System to which this kind of thing could happen has already been either 1) ejected from the system 2) pushed into a stable orbit, where the gravitational interactions sort of cancel out and they remain in place long term. This is why asteroids occur in well defined belts.
  8. Dec 22, 2013 #7


    User Avatar
    Science Advisor
    Gold Member

    Objects orbiting very close to the surface of objects can be destabilized by gravitational interactions because the object they are orbiting is not perfectly spherical. But the farther you get from an object, the more it behaves like a perfect sphere. Although the Moon has no atmosphere, objects orbiting the Moon in low orbits get destabilized and crash to the Moon quickly because of mass concentrations (mascons).
  9. Dec 22, 2013 #8
    Thank you for the information.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook