[nradov]

At some point in the far future it might make sense to renegotiate that treaty, but for now we're so far from being able to build a practical Project Orion type spacecraft that the issue is moot. And in the meantime there are other nuclear rocket designs which don't rely on explosions and would comply with the treaty. Those engines would be perfectly adequate to send manned missions to Mars and the outer planets, if we were willing to spend a few trillion dollars.