New spin on spatial orbits

New spin on spatial orbits

Space technology now forms an integral part both of our modern economy and our everyday lives. For example, the blue dot on a Google maps smart phone app uses a satellite navigation chip to receive positioning signals from a constellation of satellites some 20,000 km above the surface of the earth. The individual user therefore has direct access to several billion dollars worth of space hardware to provide low-cost positioning services, whether to help drivers find the shortest route to a destination, or to help business people navigate in an unfamiliar city. While key space services such as satellite navigation are now pervasive, they are based on a deep understanding of satellite orbits. It is the underlying mathematics of orbits which allows their design to be optimised, to ensure the best satellite navigation coverage patterns, for example.

Orbits currently used for earth orbiting spacecraft are based on Kepler’s laws of orbital motion, named after Johannes Kepler who provided an early understanding of orbital motion 400 years ago. Once it is launched, an unpowered satellite will glide along a natural Keplerian orbit. However, the University of Strathclyde is devising families of so-called non-Keplerian orbits, which do not obey the usual laws of orbital motion. These orbits require a gentle, continuous push from a small electric thruster, or a solar sail which exploits the pressure of sunlight on a large reflective film.


For example, ‘pole-sitters’, a term coined by space pioneer Robert Forward, can use continuous low thrust to balance the gravitational pull of the earth and the sun to allow a satellite to remain on the earth’s polar axis, high above the Arctic or Antarctic. These orbits could be used to provide new vantage points to view the earth’s polar regions for climate monitoring, or to provide low bandwidth communications to support future high latitude oil and gas exploration.Other families of orbits include so-called ‘levitated’ orbits.

A satellite moving along a geostationary orbit makes one complete circle of the earth every 24 hours, and so appears stationary in the sky when viewed from the earth. These are ideal for direct broadcast satellite TV services to fixed dish antennae attached to homes and offices. However, geostationary satellites need to be separated along the geostationary ring and popular slots above population centres are beginning to get crowded. In order to create more slots, families of levitated orbits could be used that circle the earth every 24 hours, but are displaced north or south of the earth’s equator. The pressure from sunlight reflecting off a solar sail, for example, could push the satellite just above or below the geostationary ring, thereby creating additional slots.

The use of a continuous push on a satellite from a solar sail or a small electric thruster may open up new families of orbits such as ‘pole-sitters’, ‘levitated orbits’, etc. This new understanding of the mathematics of orbits could unlock new, entirely unforeseen space applications. 

(The writer is Director of the Advanced Space Concepts Laboratory at the University of Strathclyde.)