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Earth Fly-by
Posted: 21 April 2006 07:43 PM
Senior Member
Total Posts:  356
Joined  2004-11-10

The Messenger space probe is on its way to orbit Mercury in 2011. To catch up with its target, it will do a number of close fly-bys of Earth, Venus and Mercury using gravity assist to pick speed. Last August it came by Earth and mission controllers took a series of photos from 66,000 km to 436,000 km away (in 24 hours) and put them into this amazing animation:

Earth Fly-by

It’s a 5 MB download but worth the time.


Alas, to wear the mantle of Galileo it is not enough that you be persecuted by an unkind establishment, you must also be right.
-Robert Park

Posted: 22 April 2006 08:40 AM   [ # 1 ]
Five Star Member
Total Posts:  1313
Joined  2006-02-05

Thanks!  Fun video!

It is a very interesting process, the fly-by to gain (or lose) velocity.  I never understood how it really worked until I read an article in Scientific American that explained it properly (the maneuver, and how it works).

For those interested:

To gain velocity:

The vessel approaches the orbiting planet from behind, in other words, catching up to it in its orbit.  The vessel is then nearly captured by the planet’s gravitational field, and is accelerated accordingly.

The vessel comes as near the planet as it can without actually succumbing to the gravitational field, then zooms off into its own trajectory again, only it is travelling significantly faster.

The part that I did not understand, and a mis-conception by most people:  the planet’s gravity pulls the vessel closer and thus accelerates it, but then, as the vessel leaves the planet, that same force would slow the vessel back down by the exact same amount, thus leaving no gain in velocity.

But what happens is, and this is quite beautiful in its concept, is that the vessel, since it approached from behind the planet (‘behind’ in the sense it caught up to the orbiting planet), the vessel takes the planet’s orbiting velocity (as it orbits the sun) and gains speed from that.  Thus, it slows the planet in its orbit a minute amount, and gains in velocity that same amount, in a proportion directly related to their relative masses.  Then it leaves that planet and zooms away.  Thus, the term ‘Sling-shot Effect’.  It basically steals some of the planet’s orbital momentum.

To slow down, which is what is needed to be done to get to Mercury, approach the planet from a trajectory forward of the orbit path.

Dan the Super-Boring-Scientist

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