NASA's 2013 budget request is quite close to what Congress appropriated for 2012: it's down only .3%. So why does Planetary Science take such a big hit, dropping from $1.5 billion to $1.2 billion?
Here's the problem. ISS and Space and Flight Operations are up $300 million over 2012, which was anticipated in previous budgets. The essentially complete station takes more money to run, our logistic flights will be carried under the ISS budget rather than the Space Shuttle's, and aging communication systems need to be replaced. James Webb Space Telescope has been suffering serious overruns and is up over $100 million over 2012.
There are no good options. NASA could ask for a bigger overall budget, but in the current fiscal and political climate, Congress seems unlikely to grant one. I think we can live without the big SLS launcher, but Congress has mandated one and on a tight schedule. The JWST is another target, but a lot of money has already been spent, and it should be an amazing capability when finished. A commercial manned spacecraft is the fastest and cheapest way to restore U.S. access to ISS.
That leaves unmanned exploration, unfortunately.