I wrote last week that the important part about the White House's vision for NASA and President Obama's speech was the asteroid mission, not Mars, simply because the asteroid mission is much easier, will happen much sooner, and has a much lower chance of being cancelled. Fortunately, after the furor over the speech has settled down it looks like the discussion is starting to shift towards the asteroid mission. An article here from New Scientist goes over what it will take to select a destination, and why we haven't decided on one yet.
The biggest reason is that we simply haven't found all the possible targets yet, not even close. A study in 2009 referencing numbers from 2006 only found seven possible targets during that time, but updated numbers increased this to 42. When WISE is done its mission we should see about 300,000 more asteroids added to those we know, and there are a lot of other sky surveys going on right now doing the same thing. Pan-STARRS is one of the newest ones.
In the article we see another one of the issues for a mission to an asteroid: gravity. Since asteroids have such low gravity, a spacecraft approaching one will have to use its own fuel to slow down. In addition to that the lumpy shape most asteroids have makes for an uneven gravitational pull, and (though not mentioned in this article) dust could be a problem as well, especially the possibility of it clinging to spacesuits due to static electricity. It's safe to say that the more massive, rounder and smoother an asteroid is the better. Taking a look at
the close approaches page, we can see quite a bit of variation with asteroids ranging from just a few dozen metres to up to 3 km or so in diameter. Mass and gravity for these would be approximately:
50 metres - 260000 tonnes, surface gravity 0.0003% that of Earth, escape velocity 0.13 kph
400 metres - 134 million tonnes, surface gravity 0.002% that of Earth, escape velocity 1.1 kph
3 km - 56 billion tonnes, surface gravity 0.02% that of Earth, escape velocity 8 kph
The fact that we don't know exactly where we'll be going in 2025 is probably a good thing, since it's a reminder that this is an entirely new field, and perhaps a welcome change from the discussion over more well-known destinations. NASA could easily generate interest for the mission by setting up a page with some potential destinations, providing the information we have on them so far, and inviting visitors to vote on their favourite.
