2004 has seen two more Mars rovers exploring the planet. Opportunity and Spirit, vital components of the Mars Exploration Rover mission have acted like robotic human field geologists. They have travelled over 0.6km climbing 25° rocky slopes on their way, used abrasion tools to drill into various rocks, returned images of the rock surfaces using on-board microscopes and taken (together with the lander) coloured stereo panoramas of their surroundings

Spirit landed in the 140-km diameter Gusev Crater and the returned data clearly show that this was once a lake. Data from Opportunity indicates that its landing site, Meridiani Planum, was once a salty swamp. The climate of Mars has changed drastically over time mainly due to slow variations in the eccentricity of the orbit and the angle of inclination of the spin axis to the orbital plain. It is quite likely that it will again become warm and wet some time in the future.

One of the next steps in Martian exploration, midway between a lander and an orbiter is NASA’s proposed remote controlled ‘model’ aeroplane ARES (short for Aerial Regional-scale Environmental Survey). This will fly around searching for possible sources of methane, trying to check whether they are volcanic or biotic.

Landing on Venus has always been a well-known problem ever since the USSR’s Venera 7 touched down in December 1970. Concentrated sulphuric acid from the all-enveloping cloud layers corrode any descending space probe and it quickly becomes inoperable. The last probes to land there were Vega 1 and Vega 2 in June 1985. No samples have been returned and no Venusian meteorites are known. It is certainly time to try again. Modern technology should surely be able to shield a space probe during descent. That said, with a surface temperature of about 460°C and a surface pressure 92 times that at the surface of Earth there are still serious challenges.

Landing on Mercury is also a job for the future. The lander that was to be part of the joint European / Japanese mission to the planet has been cancelled due to lack of funds.

Landers are important. They can be used to monitor atmospheric pressure, composition, winds and weather. It is also very important to have networks of landers scattered over planetary surfaces. Atmospheric parameters clearly vary with latitude and longitude and time of day and year. And having seismometers at different places enable the source and strength of planet-quakes to be monitored. Even though it is relatively easy to see what is on the surface of a planet, the interiors remain extremely mysterious and our knowledge of the composition and volumes of planetary cores is extremely rudimentary.