Illustration of Lagrange Points
Original Windows graphic, by Sarah Joseph

# Lagrange Points

We could learn a lot about the solar wind if we could fix a satellite in a certain location in space. Changes in magnetic fields and particle flows could be measured. Most importantly, if the satellite is between Earth and the Sun, it could function as an early warning system, letting us know about any changes in the solar wind about an hour before they reached the Earth.

The problem with fixing a satellite in a certain location is that it is impossible. In order to resist gravity, any satellite must be constantly moving in an orbit. Nearly as helpful though would be if the satellite were to orbit the Sun in a one-year orbit, it could maintain a fixed position relative to the Earth. However, Kepler's laws show that any satellite closer to the Sun must orbit more quickly than the Earth, and any satellite farther away would have an orbit longer than one year.

There is one way around this problem. The Lagrangian Point L1, named after French mathematician Joseph Lagrange, is one of five Lagrangian points in the Sun-Earth system. Also called libration points, these five points are locations in which a satellite may remain in a nearly fixed position relative to the Earth as the Earth revolves around the Sun. L1 is just inside Earth's orbit, the best place for an early warning system. At the L1 point, the Earth's gravity pulls in the opposite direction of the Sun's gravity. This cancels out the effects of the Sun's gravity, allowing the satellite to orbit the Sun with less velocity, in an orbit of one year matching that of the Earth.

The ACE and SOHO spacecraft are both at L1, which means they will stay in a relatively constant position with respect to the Earth as the Earth revolves around the Sun. This point is about one-hundredth of the distance to the Sun, or 1.5 million km from the Earth.

Last modified June 26, 2001 by Jennifer Bergman.

## ACE Mission Page

The ACE (Advanced Compostion Explorer) was launched aboard a Delta II rocket in August 1997. This unique NASA mission will run a minimum of 2 years (with an expected lifetime of over 5 years). This mission...more

## SOHO Mission Page

Have you ever wondered why your favorite radio station doesn't always come in? Solar activity, such as solar wind, sometimes causes this and other communication problems. Satellites experience disruption,...more

## How do Satellites & Spacecraft Monitor Space Weather?

Observations by Earth-orbiting satellites and other spacecraft have revolutionized our understanding of the Sun and space weather in recent years. A host of spacecraft provide a flood of information about...more

## IMF

IMF stands for Interplanetary Magnetic Field. It is another name for the Sun's magnetic field. The Sun's magnetic field is enormous and is carried by the solar wind. The solar wind and magnetic field are...more

## Coronal Mass Ejections

"Without warning, the relatively calm solar atmosphere can be torn asunder by sudden outbursts of a scale unknown on Earth. Catastrophic events of incredible energy...stretch up to halfway across the visible...more

## Solar Activity

The Sun is not a quiet place, but one that exhibits sudden releases of energy. One of the most frequently observed events are solar flares: sudden, localized, transient increases in brightness that occur...more

## The Solar Atmosphere

The visible solar atmosphere consists of three regions: the photosphere, the chromosphere, and the solar corona. Most of the visible (white) light comes from the photosphere, this is the part of the Sun...more

## The Solar Interior

To understand how our Sun works, it helps to imagine that the interior of the Sun is made up of different layers, one inside the other. The innermost layer, the solar core, is the region where the energy...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information.