Expert: Philip Stahl - 3/5/2011

Question
QUESTION: Dear Sir,
Thanks of Answering my Question.

Our earth is moving with 19 miles/sec on its orbit so the atmosphere around it and every object is actually moving with the same speed, It is due to frame of reference we dont realizes this, but what if the space shuttle goes out of the earth's gravitational influence it actually leaves the earth's frame of reference then how it attains that much speed to come back to earth. As per my understanding space shttles are not moving with 19 miles/sec.

ANSWER: Hello,

It is impossible for the Space shuttle to "go out of" the Earth's gravitational influence, because it lacks the ability to travel much more than to an altitude of ~ 320 km (e.g. to occasionally rendezvous with the space station).

In terms of the distance from the center of the Earth this amounts to roughly 320km + 6400 km = 6720 km.  Meanwhile, the acceleration of gravity at a distance 2r (12, 800 km) from the center of the Earth would still be g/4. This is also a distance the space shuttle could never achieve because of fuel constraints etc.

The key issue here is not so much the shuttle's speed as the distance from the center of Earth it would be able to attain and for which it could sustain orbit. Since it could never attain an altitude that would totally free it from the Earth's gravitational influence then it isn't going to "leave the Earth's frame of reference" to use your parlance.



---------- FOLLOW-UP ----------

QUESTION: Dear Sir,
Thanks a lot for your answer
It was the big misconception about Earth’s orbit and frame of reference that is cleared now.
However i am still interested in knowing if any space craft (like Cassini, new horizon, voyager 1, voyager 2 etc…) those are travelling in deep space is it possible to bring them back to earth?
•   If YES, then as per my first question how they will come back to earth’s orbit and match the earth’s orbital speed for successful landing
•   If NO, then how humans can travel to distant planets & moons (e.g. Mars, Europa, Gliese 581 c etc…), or whether it will be a one way journey?

Answer
Hello,

Re: your most recent questions, the responses are as follows-

1)NO, it's too late to 'recall' any of those spacecraft, and besides we lack the technology to do any kind of mission on that level of complexity.

2) Traveling to distant planets will basically be a one-way journey. However, on a Mars trip, for example, there will possibly be a turn around point midway through the trajectory (assuming all other factors are equal, e.g. no exhorbitant fuel loss from a meteorite strike or other mishaps).