Travel to Mars, or "scientific exploration" are exactly the wrong objectives. They can become valuable adjuncts, but if one reviews the history of terrestrial exploration and settlement, you will find that it has ALWAYS been MAINLY about acquiring cheap and abundant resources -- mainly gold and land, with a few other assorted things like spices thrown in for good measure.
Petroleum is eventually going to become more scarce, but the people who think that just putting a solar collector on every roof and a wind generator in every back yard is the solution are hopelessly idealistic and naive. Solar energy might be part of the solution, but only if it can be harvested from massive facilities taking up a lot of room, in places where the sun shines 24/7/365, with no clouds or even atmospheric scattering to get in the way. Guess where that place is? The other big answer will be fusion. Again, we are NOT going to have little "Mr. Fusion" engines in our car like the guy had in "Back to the Future." If this technology is ever developed to the point of being economical, it is going to require huge facilities. The extremely high temps and pressures, and the extremely high levels of neutron release, are going to make these dangerous facilities to be around. Plus, it might just be easier to engineer them in a zero-g environment, so once again, space might be a good location. How to get the energy transported to earth. We'll probably need orbiting facilities that will take the energy produced by the solar arrays plus the energy produced by the orbiting fusion reactors to produce deuterium and tritium. Chill it down close to absolute zero, put it in a big stainless steel vacuum thermos, pop it in a re-entry vehicle, and send it to earth, where it becomes the feedstock for the terrestrial fusion plants. He3 (which can become the feedstock for the tritium production) is present in huge quantities on the moon, so we'll have to set up a major mining operation there.
Our terrestrial mineral supplies will eventually be getting scarce, too. There's a gazillion asteroids orbiting the sun, all made of all types of interesting minerals. It should be possible with a little bit of effort to send robotic spacecraft out that can attach themselves to an asteroid, and then through a series of carefully directed and timed thrusts gradually move an asteroid into the same orbit as the earth, but either leading or trailing us by a safe distance. We can then send crews out to mine these asteroids, and transport the ores to orbiting mills for refining. Much of the earth's heavy industry could eventually be relocated to earth orbit, which would provide two big advantages: 1) less pollution here on earth; and 2) development of a self-sustaining infrastructure in space (which will be absolutely essential before we can even begin to consider any exploration farther away).
The biggest advantage of pursuing a resource development/industrial infrastructure track is that this provides a logical and feasible pathway for private sector involvement and financing. Rather than just buring up dollars every time we launch stuff into space, we can actually turn space into a profit sector. That is the ONLY way that the long-term future of space exploration can ever be secured.
This is what our big-picture vision needs to be for at least the next 50-100 years. Once the infrastructure is in place and space development is a "going concern", THEN we will be ready to consider more adventurous steps to the planets and even nearby stars.