Sir

While joining in your celebration of achievements by Mars Global Surveyor and remaining Mars spacecraft (Nature 444, 519; 2006 and Nature 444, 526–527; doi:10.1038/444526b 2006), I disagree on priorities. The Editorial commends “the idea of a continuous presence” on Mars, with “various spacecraft calibrating and complementing each other's results”. Yet the related News story seems to accept the view that “other missions might be easier to finish, such as Venus Express — there's a limit to how much useful data can be gathered by continuing to orbit Venus with the same instruments”.

According to this 'science per dollar' principle, if continuous and intensive exploration is appropriate for Mars, then it is also appropriate for Venus, which is no more costly to reach and features a dynamically varying atmosphere and surface that have evolved in an intriguingly different way from Earth. The same principle also applies to the Moon. Although not dynamically changing, it holds valuable records of the early Earth's environment, and is reachable in a fraction of the time it takes to reach the planets.

Let us hope that the new US Congress and its international counterparts take the best ideas from all sources and assemble a balanced programme of lunar and planetary exploration for the coming decade.