According to the lecture, the minimum escape velocity must be 11.2km/sec, but the space crafts never even reach near it, then how they go out of earth's gravity? The fastest space craft is only 7 km/sec.

Asked by arkabanerjee | 1st Oct, 2010, 12:00: AM

Expert Answer:

Dear Student

The escape velocity calculation assumes that none of the sources of gravitational potential are moving. In the case of the Earth-Moon system for example the Moon is in orbit around the Earth, and it turns out that an object that passes behind the moon can gain speed from the encounter (a gravitational assist).

Therefore there are certain launch angles and launch windows that will give the assist and this will give an object extra speed in-flight and still permit it to escape Earth's gravity even though it was launched with less than escape velocity.

We hope that satisfies your query

Answered by  | 2nd Oct, 2010, 08:28: PM

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