MESSENGER -- home page See Mercury at Sunset --- Science@NASA 1Footnote: How can a spacecraft's speed increase with respect to the sun and at the same time decrease with respect to Mercury? Consider the following: Imagine you want to hop a train from a moving car (don't try this at home!) The train is traveling 60 mph along the tracks paralleling Main Street and will be passing the General Store at precisely 1 p.m. You're primed for action, parked along Main Street in your car. At 12:59:30 you start accelerating down the street to intercept the train. Your car's starting point is closer to the General Store than where the train is now, but the train is going faster (60 mph), in the same direction you're going. As you speed up to 10 mph relative to your starting point, you're going 50 mph in relation to the train. As you keep increasing your speed relative to your starting point, your speed relative to the train decreases. You're closing in! You time your acceleration to pass the store at 60 mph at the same moment the train passes it. At that moment you're going zero mph in relation to the train. Now leap! Similarly, once MESSENGER approaches Mercury at the right speed, it can fire its main engine enough to be captured by the planet's gravity and begin orbiting. More MESSENGER Q&A: Did you know?

It would take a team of people driving a car at 56 mph 10,000 years to travel the 4.9 billion miles traveled by MESSENGER around the Sun in only 6.6 years. What does MESSENGER stand for? It stands for MErcury Surface, Space ENvironment, Geochemistry, and Ranging spacecraft. Why orbit Mercury? Orbiting Mercury for a year will allow scientists to study Mercury in detail and answer tantalizing questions raised by our previous glimpses of the small world. Many of the answers to the questions require measurements at Mercury (such as characterizing the planet’s magnetic field) or long-term observations. Also, MESSENGER will move more slowly in orbit than in a flyby, allowing cameras to zero in on intriguing features seen only at a distance up to now. The cameras will have 250 meter resolution but can do 25 times as good as that when they zoom in. Why does Mercury travel so fast? The sun's gravitational pull is stronger the closer you are to the sun, so a body has to move faster to keep from being pulled down. Think of swinging a rock on a string. The shorter you make the swing, the faster you have to spin it faster to keep it going around. That's because any orbit is just the right balance between the inward pull of gravity and the innate tendency of objects to travel in a straight path. What would have to be done to enter Mercury's orbit with MESSENGER if you didn't use flybys?

McAdams answers: "About 54% of MESSENGER’s launch mass was propellant. Much more fuel would have been required without the flybys. Flybys save fuel. There are design and cost challenges with having to increase the percentage of propellant to over 85% of launch mass. Launch costs alone could have equaled the entire mission cost (development, spacecraft, testing, launch, and operations)." What would a direct shot on an intercept course with Mercury require? McAdams answers: "If you sent a spacecraft directly to Mercury on a huge rocket with stacked upper stages, you would need a rocket with many times the lift capability of the Delta II (i.e., Delta 2) that sent MESSENGER toward Mercury. After leaving Earth orbit, the spacecraft would need nearly 2.5 times (150% increase) the propellant loaded onboard to place the spacecraft into the desired orbit around Mercury." How many flybys were there and when did they occur? There were six flybys: one at Earth, two at Venus, and three at Mercury. The Earth flyby, a year after launch, and the Venus flybys, in October 2006 and June 2007, used the pull of the planets’ gravity to guide MESSENGER toward Mercury’s orbit. The Mercury flybys in January 2008, October 2008 and September 2009 fine-tuned and shrunk MESSENGER’s orbit closer to the size of Mercury’s orbit while allowing the spacecraft to gather data critical to planning the mission’s orbit phase.