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LAUNCH WINDOW: THE PERFECT SPACE IN TIME

There are many factors that engineers must consider when determining a launch window for a mission to Mars. Two major parts of the equation involve celestial mechanics and the performance capability of the selected launch vehicle.

Trajectory About every 26 months, Mars and Earth reach a position in their respective orbits that offers the best trajectory between the two planets. But timing is critical. The Earth is a moving launch pad and Mars is a moving target. Even though the two planets are traveling in the same direction, they are moving on different orbits. Engineers must plot a separate orbit for the spacecraft that connects the position of the Earth at launch with that of Mars months later. The launch needs to be timed so that Mars and the spacecraft converge at exactly the same point in space. Think of intersecting ellipses rather than a straight line.

But before a spacecraft can coast to Mars on its own solar orbit, it has to break free from the grip of Earth's gravity. By launching in the same direction as the Earth's spin, the spacecraft takes maximum advantage of its moving "launch platform." It uses the spin of the Earth for an added boost and follows the orbital motion of the planet. This conserves rocket fuel. The point at which a launch vehicle uses the least amount of fuel to push a spacecraft onto the proper trajectory for Mars identifies an ideal launch date.

The length of a launch window is often determined by the type of launch vehicle that is selected for a mission. There are many different sizes and capabilities of rockets. The payload indicates the type of equipment needed. When a launch vehicle is selected for a mission to Mars, it comes with a set of parameters for optimum performance. It has been designed to launch a certain amount of mass at a certain velocity. Think of a shortstop throwing out a runner. The shortstop can most easily make a good throw if moving toward the "target" base. It is much harder, and requires a stronger arm, to throw the ball to the proper spot if the shortstop is falling backwards or leaning in the wrong direction. The same applies to a launch between Earth and Mars. The farther the launch is from the optimal time for lift-off, the more energy needed to get to the proper target. If a rocket can barely do its assigned job under the best launch conditions, the launch window becomes very narrow. The more energy a rocket delivers for a particular payload, the wider the range of conditions it can handle. Then the launch window widens. But too much rocket wastes fuel and money. There has to be a balance between the mass of the payload and the energy capabilities of the launch vehicle.

For a mission to Mars, lift off is usually scheduled prior to an ideal launch day, but within acceptable conditions for the launch vehicle. This allows for weather-related delays or delays in getting the vehicle ready. A range of days after the ideal launch date make up the rest of the launch window. After this period of time, the trajectory between Mars and Earth becomes more difficult, the thrust of the launch vehicle becomes inadequate, and the window of opportunity for a mission to Mars closes for another two years.

Web content editor/writer: Pamela R. Smith

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