Our own beloved astrophysicist to the (radio stars), Summer Ash, sends a primer on the MESSENGER spacecraft's dance with Mercury. You can get more Summer on her blog, Newtonianism for the Ladies.
Special to the BPP from Summer Ash:
No, not the quicksilver kind. That will make you batty. I'm talking about the planet Mercury. NASA has a playdate with the first rock from the Sun coming up on Monday the 14th, its first in more than 30 years. The spacecraft MESSENGER (an acronym for mouthful called MErcury Surface, Space ENvironment, Geochemistry, and Ranging) will flyby Mercury on Monday on its way to eventually settling into orbit around the planet in 2011. Why isn't it going into orbit now instead of doing a flyby of its supposed target you may ask? Welcome to the world of gravity assists. You may think that gravity always keeps your feet on the ground, but in the case of cruising through the solar system, it can also give you a swift kick in the pants, metaphorically speaking of course.
Since its launch on August 3. 2004, MESSENGER has rendezvoused with Earth once and Venus twice. After dashing past Mercury on Monday, it will fly by the planet three more times over the next four years before finally dropping in to orbit it once and for all. While this sounds like a long and winding road all over the inner solar system (and it is), believe it or not, it is also the most efficient and therefore the cheapest route.
In the wonderful world of space flight, human or otherwise, to maneuver in space you need rockets, and rockets need fuel. Naturally, fuel for these rockets has to be stored on board the spacecraft as there are no gas stations after launch. There are warning signs at Cape Canaveral: No Gas — Next Gazillion Bagillion Miles. And just as gravity keeps your feet firmly on the ground, gravity tries its utmost to keep launch vehicles from leaving the earth, and once in space, gravity is everywhere, despite what you think you may have heard.
At launch, we overcome gravity with — you guessed it — fully fueled rocket engines. The majority of fuel on any space bound vehicle is used up during launch and most of the weight of the payload can be attributed to fuel as well; over 55 percent of MESSENGER's launch weight was due to propellant. This is what keeps long distance space travel just out of reach for us humans: to pack enough sustenance and fuel for the journey, you need to use even more fuel to start the journey — this becomes a runaway process resulting in an overweight space transport system that can't get its butt of the ground.
Once in space, we again overcome gravity with rocket engines to keep us on course. Vehicles sent out of Earth's orbit are tugged on by every other object in our solar system, some more than others. Naturally, the sun is usually the winner in this case, but the other planets, especially Jupiter, always want a piece of the action.
Therefore, traveling in a straight line from Earth to Mercury is a bit of a challenge, from both a mathematical and a fuel standpoint. And we haven't even gotten to the fact that Mercury is a moving target to say the least, orbiting around the sun roughly once every three months. Unmanned spacecraft don't share human sustenance requirements and aren't restricted to non-stop flights, so to speak. Therefore, they are free to wind their way around the solar system, however long it takes to get them to their destinations.
For example, check out the animation on my blog of MESSENGER's circuitous route from Earth to Mercury. Each time the spacecraft passes a planet, the path changes color — indicating that its orbit has been altered by the encounter and it has moved into a new orbit around the sun. These are the gravity assists I spoke of at the beginning of the post. They enable the spacecraft to tweak its trajectory with minimal use of fuel, sometimes none at all. The engineers in mission control only have to ensure that the spacecraft approaches the planet just right (in speed and angle) for the necessary assist, and gravity does the rest.
Severe changes require the spacecraft to brake, slow down, and come in close in order to be flung around by the planet's gravity, while high speed, high altitude flybys result in only slight changes of direction. However, as the animation reveals, sometimes only good old fashioned manual steering will do the trick and MESSENGER will do five deep space maneuvers to compliment the gravity assists it gets along the way to ensure it reaches its target.
Planet hopping is not an easy task. So keep your fingers crossed for Monday when MESSENGER comes within 125 miles of Mercury at the breakneck speed of 15,877 mph (that's 4.410 miles per second!). And stay tuned for the sequel next October when the second Mercury flyby takes place!