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Astronomy

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    $\begingroup$ escape velocity for Earth is 11.2kps; however, this applies to thrown objects; you have to throw a rock at 11.2kps (ignoring atmospheric drag) in order for it to leave the Earth and not fall back; however, if your rock has an engine that can apply thrust, it can leave the Earth at a much lesser speed. The longer it is able to apply thrust, the slower it can go when leaving. $\endgroup$
    – jmarina
    Commented Oct 30, 2013 at 9:20
  • $\begingroup$ jmarina, that's interesting, I haven't heard that. Would you mind providing a link with more info or the name of the effect you're describing? $\endgroup$
    – brentonstrine
    Commented Oct 30, 2013 at 16:29
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    $\begingroup$ @jmarina is kinda right but the explanation is more interesting. Escape velocity actually decreases with distance from the body you are trying to escape from. For instance at 9,000 km up the escape velocity is about 7.1 km/s. The reason is that if you are going that speed aimed to just miss the Earth then you would pick up extra speed from falling towards it. And while the escape velocity from the sun at the sun's surface is 617.5 km/s, at Earth's orbit it is only 42.1 km/s. $\endgroup$
    – Jason Goemaat
    Commented Oct 31, 2013 at 5:37
  • $\begingroup$ Ah, I see. The escape velocity is factoring in the "free" velocity you get from gravitational pull if going back down (but angled enough to miss the planet). Is that right? $\endgroup$
    – brentonstrine
    Commented Oct 31, 2013 at 8:11
  • $\begingroup$ @brentonstrine the free velocity you mention you get from a gravity assist: www2.jpl.nasa.gov/basics/grav/primer.php the orbital velocity of the Earth is about 30km/sec, it varies a bit down to 29kps because the orbit is not an exact circle, we are actually closer to the sun in northern winter and farther by about a million km in summer. nssdc.gsfc.nasa.gov/planetary/factsheet $\endgroup$
    – jmarina
    Commented Nov 1, 2013 at 14:28