I’m still not happy about NASA scrapping Ares and the manned lunar/Martian plans, but I’m less unhappy than I was. As long as unmanned planetary science picks up most of the slack I’ll grudgingly deal with it. The extra earth science is still stupid; if you want more of that, get NSF or NOAA to do it.
Mars exploration has been back in the news recently too, with the Spirit rover finally breaking down and getting stuck in place permanently. This is pretty impressive – it was originally designed to work for 90 days but here it is still functioning 6 years later. Even as a stationary platform it can continue to do interesting science. The main concern is keeping it sufficiently powered during the coming Martian winter. The rovers are solar powered, and solar energy is increasingly hard to come by as you move farther from the sun. It’s why probes that go to the outer planets are pretty much never solar powered.
Here’s the reason. The sun dumps its energy out evenly in all directions. By the time it gets to Earth, that energy is spread over a sphere with the same radius as the Earth’s orbit. The surface area of a sphere is 4*pi*r^2, so the sunlight energy flowing in through each square meter of sun-facing solar panel is the sun’s total output divided by that area. You can do the math, and it’ll turn out to be around 1300 watts per square meter.
But if you move to Mars, the radius that the sunlight is being spread over is much larger. Mars is about 1.5 times farther from the sun than the Earth is, so the power is reduced by a factor of (1.5/1)^2 = 2.25. That’s less than 600 watts per square meter in the ideal case, which a rover on the Martian surface assuredly is not.
What about the Cassini probe orbiting Saturn? Saturn is about 9.5 times farther from the sun than the Earth, so the sunlight there is reduced by (9.5/1)^2 = 90.25. That’s about 14 W/m^2 in the ideal case, which is why Cassini is nuclear-powered. The New Horizons mission to Pluto? By the time it gets there the available solar power will be reduced by a factor of around 900. It’s nuclear-powered as well.
Conversely, probes to Mercury and Venus have all the solar power they want. Unfortunately those planets present their own unique challenges, and adequate power is pretty far down the list.