At one of these points, the pull of the sun and the Earth combine in just the right way to keep a satellite from being flung out of the solar system. The new satellite, also known as DSCOVR, won’t be alone at its Lagrangian point. For 30 years, space agencies have been capitalizing on the unique properties of these quirky places. (See “Spacecraft to Watch Earth and Warn of Solar Storms.”)
A spacecraft in one of these pockets needs very little fuel to stay at a constant location relative to the Earth, helping extend the life of the mission.
What’s more, satellites that stay in this sweet spot between the sun and Earth avoid the dramatic temperature swings and periodic magnetic disruptions that Earth-orbiting satellites encounter as they pass behind the planet.
Five Lagrangian points, named for mathematician Joseph-Louis Lagrange, occur with any circular orbit, not just the Earth’s path around the sun.
The Earth and moon together create five Lagrangian points, for example. And some of the stable pockets created by Jupiter’s orbit have captured so-called Trojan asteroids.
Limited Real Estate
The DSCOVR satellite will travel a million miles, or roughly one
percent of the way to the sun, to its home at the Earth-sun Lagrangian
point known as L1. From there it will have unobstructed views of the sun
and Earth.If this is such a great location for a satellite, won’t it start to get crowded?
In fact, several other missions have also taken advantage of L1.
The first, launched in 1978, was the International Sun-Earth Explorer-3 (ISEE-3), which was diverted along an elaborate trajectory to allow it to make the first intercept of a comet's tail. Three decades later, a private group tried unsuccessfully to revive the dormant satellite when its complicated orbit brought it close to Earth again. (See “Zombie Spacecraft Rescue Planned by Private Group.”)
DSCOVR will be joining a few other satellites already located at L1: the Advanced Composition Explorer, which compared solar and interstellar particles; the Solar and Heliospheric Observatory, which is studying the sun; and the WIND mission to study the impact of solar wind.
Stacking Satellites
So won’t the new satellite send these others hurtling into space like balls on a pool table?
Fortunately, multiple missions can play together nicely at this spot because the spacecraft don’t sit directly on L1. Instead, they move around the point in one of two types of looping orbits, known as Lissajous orbits or halo orbits.
It’s as if they become tiny moons orbiting an invisible planet circling the sun at the same pace as Earth.
Marc Kuchner, an astrophysicist at NASA’s Goddard Space Flight Center, says it’s just like commercial airspace. “You can fly a lot of planes between Los Angeles and New York,” he said. “Just like you can stack a lot of 747s in 40,000 feet, you can also stack a lot of satellites at L1, L2, or any of these Lagrangian points.”
Mission commanders don’t avoid placing their satellites at the point itself out of pure politeness, but to keep an uncluttered line of communication.
“The problem is the sun is in the background then,” said Robert Farquhar,
a retired NASA mission design specialist sometimes known as the master
of getting to places, who figured out how to redirect ISEE-3 onto its
comet intercept course. Solar interference can make it hard for the
satellite to send data back to Earth.The equilibrium at L1 is not completely stable, so these satellites require periodic “station keeping” adjustments to keep them orbiting correctly. Eventually they will run out of fuel and will not be able to hold their positions.
The Other Sweet Spots
Another point, L2, is also located along the straight line that passes through the Earth and the sun. It is about a million miles away on the opposite side of Earth, in the direction of our planet’s shadow.
This second point is a preferred perch for missions that need to search deep into space and whose sensitive instruments would be particularly challenged by hot and cold extremes, the magnetic field, or solar wind.
The James Webb Space Telescope, launching in 2018, will peer out from L2 with a number of infrared sensors to study the period shortly after the big bang, the formation of solar systems like our own, and other topics. (See video: “Building the Largest Space Telescope Ever.”)
Both the European Space Agency and the China National Space Administration have already placed probes at L2.
The other Lagrangian point along the Earth-sun axis is called L3, but since it sits on the far side of the sun, it’s less useful for space probes: A satellite there would never have a direct line of communication with the Earth.
The final two Lagrangian points, L4 and L5, are the most stable of the group, and are located partially ahead and partially behind the Earth along its orbit.
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