planetary geodesy

planetary geodesy The geodetic study of the Moon and of the other planets in the Solar System has been restricted by the time and expense required for interplanetary missions, but significant discoveries have been made with techniques similar to those used in spaceborne terrestrial geodesy. The gravity field of a planet can be measured by observing its effect on a satellite in low orbit. Accelerations of the satellite caused by the planet's irregular gravitational field will cause a change in the Doppler shift of the radio signals transmitted back to Earth by the satellite. Only the component of gravitational acceleration in the line of sight from the orbiter to Earth will have an effect on the Doppler shift, so this method is often referred to as line-of-sight Doppler. Over many orbits, measurements of the line-of-sight Doppler shift can be combined to reconstruct all components of the acceleration and hence the gravity field. This technique has been applied since the 1960s on the NASA Lunar Orbiter and the Mariner missions to Mars and Venus, and more recently on the Magellan mission to Venus, the Voyager and Galileo missions to the outer planets, and will be used on the Mars Global Surveyor programme and the Cassini mission to Saturn.

The other major geodetic observable of a planet is its topography, which when combined with the gravity data will allow the planet's internal structure to be modelled and investigated. For the nearer planets, topography at a very coarse scale has been determined using Earth-based radar measurements since the late 1960s. The Mariner 9 mission to Mars was able to improve on this information using the method of radio occultation. As the spacecraft passes behind the planet (as seen from Earth), the radio signals will be blocked, and the precise time of radio eclipse can be used to give details of the topography. Unfortunately this method is limited in accuracy because of the refracting effect of the planetary atmosphere on radio wave propagation. The Magellan mission to Venus used radar altimetry, a much more accurate topometric technique similar to that used by some Earth-orbiting satellites to measure the geoid. A downward-pointing radar is used to determine the height of the satellite above the ground, and this information is combined with the satellite position data derived from line-of-sight Doppler to give the actual topography with resolution of a few kilometres horizontally and a few tens of metres vertically. Because the atmosphere of Mars is thinner than that of Venus and is also transparent to visible light, the Mars Global Surveyor will incorporate an even more precise laser altimeter, which works on similar principles but uses the two-way travel time of a laser pulse to determine the spacecraft altitude. This technology has also been used on the Clementine lunar orbiter. Laser altimetry gives much more precise measurements than radar, with a horizontal resolution of around 100 m.

Peter Clarke