Specifically, why are their two Venus flybys and two Mercury flybys? I understand one Venus flyby to send it on its way, but it’s going to make a very long, looping journey before it gets to a final orbit. What’s the deal. You’d think we could just fire that baby off in the general direction (hey, we’re going deeper into the sun’s gravity well, so speed shouldn’t be a problem, and we’d get an assist from Venus to boot), and it would arrive in practically no time. Obviously, it’s more complicated than that, though. What gives?
I don’t know the answer to the question, but here’s a Quicktime movie that shows how insane the orbit really is. It looks to me like they need two Venus flybys just to get the orbit down to Mercury.
I wish I knew enough of the physics to give a solid answer, but my understanding of it is that just shooting it in “the general direction” of where you’re trying to get it to is nearly impossible.
Remember we’re talking about HUGE distances here, and even tiny errors will lead to catastophic results.
I’d also expect that shooting things closer to the sun might make it more difficult to get it right, as there’s probably a greater probability that you’ll hit the sun (especially if you’re aiming for a small target like mercury).
I’m gonna shut up now and let somebody who actually knows what they’re talking about come along.
Thanks for the link, SmackFu, that’s really trippy! Them JPL guys sure are smart.
If you want to send a spacecraft to another planet, you need to change its energy and its angular momentum. It seems pretty obvious that when you send a spacecraft to the outer Solar System, you have to give it extra energy. But it’s also true that when you send a spacecraft to a planet that’s closer to the Sun you have to decrease its energy somehow.
The straightforward way to do this is to burn your engines. In essence, the rocket exhaust is carrying off some of the energy of the spacecraft. However, if you’re making a big change in the spacecraft’s energy, either increasing it or decreasing it, you need to use a lot of fuel.
The idea behind a gravity assist is to either steal a little energy from, or give a little energy to, a planet. In this case, the spacecraft burns its engines to get into an orbit that passes just ahead of Venus. Venus’s gravity gives the spacecraft a little tug backwards, taking some of its energy, thereby dropping the spacecraft into a lower orbit around the Sun.
This is how Mariner 10 reached Mercury, also.
In order to do a gravity assist, you have to wait for the planets to be lined up properly. You have to balance the amount of fuel you’re willing to use against how long you’re willing to wait for a good assist geometry. In this case, I’m guessing that the most efficient route turned out to be this double-Venus-flyby one.
Speed is a problem: there’s too much of it.
Right now, the probe is sitting on a lab here on Earth, orbiting the sun at in a relatively circular orbit a relatively constant 30 km/sec. A year from now, it is due to be lauched on an orbit to bring it closer to the sun. To do this, it has to be put in an elliptical transfer orbit, with an aphelion (furthest from the sun) point at Earth orbit, and perihelion (closest to the sun) at Mercury’s orbit.
To get the probe to start falling in towards the sun, its velocity needs to be reduced below 30 km/sec. This is accomplished by firing an engine to slow the probe. As it falls into an ellpitical orbit towards the sun, it speeds upas gravitational potential energy is converted into kinetic energy. Because energy is conserved, it’s going far too fast to maintain a circular orbit by the time it reaches Mercury, and would swing back out towards Earth’s orbit if it was not slowed down again.
All of this deceleration would consume quite a bit of rocket fuel if it was done directly. Since it’s expensive to launch anything into orbit, project managers feel a lot of pressure to reduce the weight of their vehicle. One way to reduce fuel needs is to use the gravitational pull of planets to slow down the probe. Upon preview, I see that Podkayne has already explained how it’s done.
Here’s another page that discusses the trajectory, but still not much detail.
According to that, one Venus flyby is from the night side and one is from the day side. Also, they plan to use the early Mercury flybys to plan the real mission. They are really close (200 km).