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If Earth whirls around the center of our galaxy at some 220 kilometers per second, how can a spacecraft, which is much slower, enter Earth's atmosphere when returning from Mars for example?

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    $\begingroup$ The center of the solar system (which is inside the sun) goes around the center of the galaxy at 220kms/s. Earth goes around the sun at roughly 29km/s and Mars goes around the sun at about 24km/s, so the re-entry velocity of a spacecraft returning from Mars is a little larger than the difference between those two smaller numbers than like the velocity of the solar system around the galaxy. Indeed, a more precise value is around 14km/s for a free-flight trajectory, where we go out to Mars and then come back within about 500 days. $\endgroup$
    – CuriousOne
    May 26, 2015 at 3:32
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    $\begingroup$ If you're standing at the equator, you're rotating with ~1600 kp/h around the earth's center. If you jump up, earth doesn't suddenly rotate away from underneath you at supersonic speeds either. ;) $\endgroup$
    – DevSolar
    May 26, 2015 at 14:54
  • $\begingroup$ @DevSolar wait a minute.. you're right.. Stop the presses DevSolar just broke physics!! $\endgroup$ May 26, 2015 at 19:20
  • $\begingroup$ @easymoden00b: No need to be sarcastic about it. Just visualizing the concept of relative speeds. $\endgroup$
    – DevSolar
    May 27, 2015 at 6:39

2 Answers 2

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It all comes down to the fact that we are moving too.

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How can the bird drop down and catch the worm, if the earth and the worm are moving so quickly. It can because the tree, the bird, and the air are moving at the same speed, cancelling out.

If we launch a spacecraft from Mars to the earth, the spacecraft is zooming around the sun at the same speed Mars does. This is about the same speed that the earth zooms around the sun, so it almost completely cancels out.

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Because when you hear mission control mention a spacecraft's velocity, which may be well below the speed the Earth is actually moving at through the galaxy, the velocity in question is actually it's velocity relative to Earth, not it's actual velocity which will be much higher. i.e. all velocities are relative to the Earth's frame of reference.

Although it would have been funny if the first object in space had, once it had left the Earth's atmosphere, suddenly appeared to zoom off at enormous speed having dropped to an actual velocity of only a few thousand miles per hour...

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    $\begingroup$ In fact, there's no such thing as it's "actual velocity", all velocities are measured relative to something else. $\endgroup$ May 26, 2015 at 15:11
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    $\begingroup$ Indeed. I should have said relative to the Sun, or relative to whatever the Sun's velocity is relative to. I was trying to convey the effect of the object disappearing like an apple thrown out through a car window. $\endgroup$
    – Steve Ives
    May 26, 2015 at 15:21
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    $\begingroup$ @Steve: You shouldn't throw things out of car windows. $\endgroup$ May 26, 2015 at 17:42
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    $\begingroup$ Apparently the space shuttle's re-entry velocity is 17,000mph (about 7.5 km/s). So it's actually travelling at 7.5km/s + 220km/s + (possibly) 29km/s. But relative to the Earth, it's only doing 7.5km/s. $\endgroup$
    – Steve Ives
    May 26, 2015 at 20:24

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