Lagrange Points—Orbits to Nowhere

Milky Way in night sky above a lake shore

Without gravity, everything we see here—water, trees, rocks, and stars—would fly apart

Gravity glues our universe together. Without it, neither we nor any life could exist. The Earth spins at just over 1,000 miles per hour at the equator. Without gravity, anything loose on the surface like water, cars, picnic tables, and people would immediately fly off into space. Without gravity, Earth couldn’t keep an atmosphere. As soon as it drifted free from the surface, the fierce solar wind would blast it away. Without gravity the Sun itself would expand, dissipating in a fine, cold mist of dust and gas. 

In our solar system, eight (or nine, depending which side of the Pluto debate you camp on) planets orbit the Sun and nearly 200 moons circle those planets. Millions of asteroids, comets, rocks, and ice chips mingle among them. Every member’s gravity pulls—bigger ones like the Sun harder, smaller ones much less. All have negotiated their own place and work together in an intricate dance. Despite millions of gravity sources, order rather than chaos prevails and it all works.

Even so, odd things happen.

For example, take the interaction between just two bodies—the Sun and the Earth. The Sun’s gravity pulls the Earth inward, seeking to capture and devour it. But the Earth moves fast. Traveling at 67,000 miles per hour it manages to circle the Sun rather than fall into it. The deal seems simple enough. The Sun can’t consume the Earth, but neither can the Earth go its own way. The two agree to their parts in the long term choreography—fortunately for us.

Diagram of the five Lagrange points in the Earth/Sun relationship

The five Lagrange points in the Earth/Sun relationship.  Illustration Credit: NASA / WMAP Science Team

But, their simple pulling at each other also creates interesting side effects called Lagrange points. At each of the five Lagrange points, a mashup of centrifugal force, the Sun’s gravity, and the Earth’s gravity balance each other. An object placed at one of the points will tend to stay there—a convenient fact exploited by space engineers.

The first three points, L1, L2, and L3, all lay on the line drawn through the centers of the Sun and Earth. L1 is the obvious point between the two large bodies where their gravitational forces balance. About 1 million miles out from Earth, it provides the ideal spot to continually observe either the Sun or Earth’s daylight side. The Solar and Heliospheric Observatory (SOHO) currently resides there. Additionally, the Deep Space Climate Observatory (DSCOVR) produced this 2-minute video showing the Earth spin for an entire year.

L2, about 1 million miles from the Earth in the opposite direction, yields a pristine viewing spot always shielded from the Sun. The NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) lives there as will the James Webb Space Telescope starting in 2018.

L3 lies at the other end of the line where it crosses Earth’s orbit on the opposite side of the Sun. We can never see that spot from the Earth and, so far, it offers no useful function.

L4 and L5 also lie along Earth’s orbit. L4 is 60° ahead of the Earth and L5 60° behind. While no artificial probes occupy either of those spots, natural objects can settle in. Earth has one asteroid, named 2010-TK7, in L4, but some of the outer planets have sizable concentrations in their L4 and L5 positions.

Lagrange points furnish low-energy parking places. L4 and L5 are the most stable allowing objects to remain there with little station-keeping propulsion required. The more useful L1 and L2 spots work best when a probe enters what’s known as a Lissajous orbit centered, not on a planet, but on the Lagrange point itself.

Orbiting virtual locations works well for some space vehicles, but I found it’s not so useful for life. I can easily slide into orbit around some imaginary point. Doesn’t take much work to stay there.

But life—real life—demands choosing. I have three options.

I can choose to remain at one of life’s Lagrange points, slowly circling nothing, believing motion means progress, neither hot nor cold, just comfortably lukewarm. Or, I can choose to orbit the Earth, our world with its drama and death. Or, I can choose to circle the Sun, our source of light and life. My choice. But I must choose.

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