Lagrange points are not unique to the Earth. Every pair of large bodies, such as the Sun and Mars, has its own set of five Lagrange points. They are interesting for science, and NASA researchers have studied them as possible locations for relay satellites that could keep communications with Mars running when the Sun is in the way.
What Lagrange points are
According to NASA, Lagrange points are positions in space where the gravitational forces of two large bodies and the orbital motion of a small object balance each other, so that a spacecraft can remain there with little propellant. There are five such points, called L1 to L5. L1, L2 and L3 lie on the line through both bodies and are unstable. L4 and L5 lie 60 degrees ahead of and behind the smaller body along its orbit and are stable. For the Sun and Earth system, see our guide to Lagrange points.
The Sun–Mars Lagrange points
The same geometry applies to the Sun and Mars. The Sun–Mars L4 and L5 points travel ahead of and behind Mars along its orbit around the Sun. Because these points are stable, small bodies can collect there. Such objects are called Trojans. According to ESA, astronomers have identified a few dozen Trojans at Venus, Mars, Uranus and Neptune combined, compared with almost 10,000 at Jupiter.
The communication problem: solar conjunction
About every 26 months, the Sun lies directly between Earth and Mars. This is called solar superior conjunction, and it interrupts communications with Mars missions. A NASA study by Robert L. Howard Jr. and Marc Seibert estimates that, in the 2030 to 2041 timeframe, outages can last up to 78 days for optical links, up to 24 days in X-band and up to 7 days in Ka-band.
Lagrange points as relay locations
The same study examined relay satellites at the L4 and L5 points of Mars, Earth, Venus and Mercury with the Sun. Such relays would stay out of line with the Sun when Earth and Mars cannot see each other directly. The authors recommend the Earth–Sun L5 point as a starting location, noting that it avoids known Trojan asteroids and costs less to reach than points associated with Mercury or Venus. They conclude that eliminating these outages is physically possible, using a three satellite solution based on existing commercial satellite buses with optical communications.
Why it matters
As more missions go to Mars, and eventually crews, weeks without contact become a serious constraint. Relay satellites at Lagrange points are one option being studied alongside a dedicated Mars telecommunications network.
Sources
- NASA Science: What is a Lagrange Point?
- ESA: Newly discovered asteroid just second of its kind
- NASA Technical Reports Server: Lagrange-Based Options for Relay Satellites to Eliminate Earth-Mars Communications Outages During Solar Superior Conjunctions
Cover image: Artist's concept of NASA's Mars Reconnaissance Orbiter above Mars. Credit: NASA/JPL (PIA04916), public domain.