On Saint Patrick’s day, the MESSENGER spacecraft will slip into orbit around Mercury, the planet closest to the sun, and the smallest of the eight classical planets. It will be the first to do so.
You would think this would have happened before now. After all, there are numerous spacecraft in orbit around Mars, and one currently active in orbit around Saturn. Saturn is a lot farther away than Mercury (800 million miles versus 50 million miles), so why the delay? What’s so hard about this? It has to do with the direction of travel, not so much with the distance.
First you need to recognize that traveling between the planets is not along a straight line. The trajectories requiring the least energy are long, curving ones. The spacecraft gets a brief but energetic boost from its rocket at the very beginning of its trip, coasts for months or years, then goes into orbit around its target planet by means that depend on that planet’s characteristics. Here is a typical trajectory for a trip to Mars. (Clicking on any of the images in these posts will bring up a full-sized version.)
Secondly, a trip to Mars—or to any planet farther from the sun than the Earth—is uphill. Your trajectory puts you into orbit around the sun, and since you are moving away from the sun, you are fighting its gravity and slowing down all the way. Moving slowly once you arrive at the planet is a good thing! This makes it easier to slow down enough to slip into orbit around it.
Traveling to a planet closer to the sun than we are (Venus or Mercury) is a downhill journey as you fall ever faster toward the sun. You aren’t falling directly toward the sun (at least you hope not), but your curving path will move you closer to it, and your speed increases the closer you get. It reminds me of soapbox racers we used to build in my old neighborhood, racing them down the very steep hills that were part of that area. Navigating the sharp turn at the bottom of the hill sent many a kid into a skin-scraping crash, as that is when we were moving the fastest. The problem isn’t getting to Mercury. The problem is slamming on the brakes once we get there so we can go into orbit around it. Slowing down with a rocket engine requires fuel, and every pound of fuel we carry reduces the scientific payload of the craft.
The solution employed by MESSENGER is an elaborate game of celestial billiards. The paths of interplanetary spacecrafts can be altered to change direction or speed by sending them very near planets. The spacecraft actually “borrows” from or “loans” energy to the planet. Given the huge disparity in mass between the spacecraft and the planet, the effect on the latter is negligible, while the effect on the spacecraft is considerable. Trajectory engineers have become quite skilled in threading the needle for these maneuvers. MESSENGER was launched in August 2004, and has flown by Earth (once), Venus (twice), and Mercury itself (three times) in its perambulations around the inner solar system. It is finally moving at a speed that will allow it to move into orbit without having to shed enormous quantities of energy. Here is its rather more complicated trajectory.