Mercury Messenger (orbiter, not a car)

Factual Questions
1

The Messenger probe has just passed Mercury on it’s way to orbiting the planet in 3 years.

Now, I recognize that I don’t know the first thing about orbital balistics, beyond the fact that they’re complex. My question though, is why will it take another 3 years to put it into orbit? I assume it’s just moving too damn fast right now, but how do they plan to slow it down? Is it going to come back past Earth again or stay inside our orbit and just slow down moving around the sun?

2

These craft can’t carry the fuel needed to do a controlled orbital acquisition so they have to “slingshot” the craft around the solar system in just the right way so that the planet’s gravity eventually captures the vehicle and it falls into a reasonably stable orbit.

This process can take quite some time, possibly made even longer if the target body has very low gravity.

3

This page has some explanations, along with a diagram. Mercury Messenger has already had one fyby of Earth and two of Venus. This was the first Mercury flyby, but it has two more before it can go into orbit around Mercury. The spacecraft loses some kinetic energy at each flyby.

4

I’m no expert, but my understanding is that as Patty O’Furniture says, the reason is fuel. If you’ve got lots of fuel to burn, you point your craft when the planet will be when you cross its orbit, and when you get close enough you fire your engines until you’re at the altitude and orbit you want.

This takes a (comparatively) enormous amount of fuel.

So instead, you set up a path that you can get into for the littlest amount of fuel possible that will take you around a couple of giant speed brakes (known as the Sun and the planets) until when you finally sneak up on the your target, your speed and direction are as close to the orbit you want as you can get.

Then you give a gentle nudge on the engine and slip into orbit.

5

…most optimal amount of fuel, considering the constraints of payload, time and the orbital elements of the planets and Sun, and most likely…budget.

This was probably solved by using a form of Linear Programming.