[Jack Hammer]

Uh... this may be a dumb question, but if it’s ninety-three million miles to the sun, why is it four and a half billion miles to Mercury...?

[The Cajun]

NASA overshot and had to turn around :^)
Just guessing they used a lot of gravity assist by coming close to other planets to get there. Long on flight time, but easy on fuel.

[ForGod'sSake]

...why is it four and a half billion miles to Mercury...?

Good question. Another ARTICLE says Messenger traveled 96 million miles. Much more plausible, eh. Amongst other things it made three passes around Mercury before being inserted into orbit.

[Little Pig]

Orbital mechanics. Our spacecraft don’t have the engines to ignore Newtonian laws yet, so we have to do everything using gravity assist from various other planets. Plus, getting something close to the sun without having it fall in is really tricky. We had to fly the craft by Earth once, Venus twice, and Mercury twice (including the final orbital insertion). That doesn’t include a couple of solar orbits as the craft traversed the intervening space between each of the other planets. You can see the whole trajectory at http://upload.wikimedia.org/wikipedia/commons/5/5e/MESSENGER_trajectory.svg

[Cyman]

The first time, we get halfway across
when we run out of gasoline.

We got to go back.

Then I take twice as much gasoline.

This time, we were just about to land,
maybe feet...

when, what do you think,
we run out of gasoline again.

Back we go and get more gas.

This time, I take plenty gas.

We get halfway over,
when what do you think happened?

We forgot the airplane.

[bert]

A Washington DC cab driver was at the controls