Microwave Anisotropy Probe

The Trip to L2: How MAP Gets There

MAP will observe from the vantage point of the "second Lagrange point of the Sun-Earth system", or simply "L2". This point, 1.5 million km from Earth along the Sun-Earth line, is an ideal location for MAP because 1) it is far from near-Earth disturbances, and 2) it offers an unobstructed view of the full sky - without needing to look near the Sun, Earth, or Moon - over the course of one year.

What is L2?

L2 is an abbreviation for "Lagrange point #2." This is the second of 5 so-called Lagrange points in the combined gravitational field of the Sun-Earth system. The combined gravitational pull of the Sun and Earth at one of these points is such that a body at (or near) one can rotate around the Sun at the same rate as the Earth. The L2 point is 1.5 million km beyond the Earth along the Sun-Earth line. (Normally, as Kepler discovered, a body farther from the Sun takes longer to orbit the Sun - a martian year is 1.88 Earth years - but in the case of L2, the additional pull of the Earth is just enough to make it orbit at the same rate as the Earth.) A satellite placed at or near this point can always face away from the Sun and Earth and, over the course of one year, will have an unobstructed view of the full sky without ever needing to look near the Sun or Earth. Additionally, the L2 point is far enough from Earth that it is well protected from the Earth's microwave emission, magnetic fields, and other disturbances, but near enough to communicate with - it takes light only 5 seconds to reach L2 from Earth.

How does MAP get to L2?

MAP will use the Moon's gravity to "slingshot" around the moon to get to L2. In order for the Moon to be in the correct location to take advantage of this boost MAP will spend some time in a series of "phasing loops" - highly elongated Earth orbits that reach nearly to the Moon - until it is in a favorable position to continue on its trip to L2. The total trip will take about one hundred days, or a little more than three months. This trajectory is designed to consume as little fuel as possible.

Once at L2, the gravitational pull is such that MAP will orbit about the L2 point once every six months, as shown in the picture below.

MAP Trajectory to L2:

Trajectory to L2

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Last updated: Friday, 05-21-1999