"How useful really are ultra-high speeds? 1,000,000mph would get you to Saturn in fairly short order, but the flyby would be so fast that observations would be severely compromised."<br /><br />I don't know about 1 Million/hour, but certainly 100K/hour would be very useful in terms of a manned misssion to Mars. 1 Million/hour would have a couple of problems, slowing down, as well as increased danger from small impacts. 100k however, would greatly speed up the trip to Mars, not only would that mean less supplies, less radiaiton exposure, but also less distance to travel because Mars would not have rotated away as far from its closet point to Earth.