26-March-2008, 09:12 PM
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Senior Member
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Join Date: Feb 2004
Location: Austin Tx
Posts: 979
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I honestly have no idea. I will watch those video's tomorrow. If you have 30 minutes to spare, might want to listen to the Mercury Episode of Astronomy cast. They touched a bit on this talking about gravity braking. I looked up the transcript for the show, this is what they had to say:
Quote:
Fraser: I guess it's very different from the spacecraft they send to mars. When they go to Mars, they can use the atmosphere to aerodynamically break their orbit. I know the mars spacecraft come through the atmosphere several times, skimming the top of the atmosphere slowing themselves down a little bit more until they're in whatever orbit they want to be in.
With Mercury, that non-atmosphere isn't going to participate, so they've got to be doing it entirely with rockets.
Pamela: They actually do it almost entirely with gravity, that's one of the cool things.
A better way to think of it is when we send things out to Jupiter and Saturn, we often use some of the inner planets to give gravity boosts. We'll send things into an orbit where they go once around the Sun and then they start to catch up on Earth. As they catch up on Earth, its gravity pulls them in and they eventually fly past Earth. As they fly past, the Earth tries to slow them down, but Earth and this object are moving in the same direction, so the amount of push we can give an object heading out toward the outer solar system, that's going in the same direction of orbit we're going in, is a lot more than the pull we give it as it goes past us.
This is called gravity-assist. It's away to speed things up by allowing the Earth's gravity to pull in the direction we're all orbiting.
If you try going around the Sun in the opposite direction, such that it comes around the Sun and is headed into a head-on collision with the Earth, the planet's gravity will still pull it toward the Earth, but as it starts to go past the Earth, because we're now moving in the other direction, we slow the object down more than we speed it up.
So you can use gravity-assist to slow things down if you try and go against the flow of orbits, or to speed things up if you go in the flow of the orbit.
Fraser: So that's what Messenger's going to do: orbit in the "wrong" direction and use that gravity to actually slow it down until it can put itself into orbit.
Pamela: Exactly, so they're gradually breaking themselves (in this case using Venus and Earth to break themselves) to get to Mercury. They're going to have to go past Mercury a couple of times before they settle into a nice orbit and then just image, image, image that entire planet.
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