Fusion being 20 years away was contingent on receiving an appropriate level of funding. Given the funding it's received (below the "fusion never" level), it's exactly on track. In fact, if anything it's ahead of "never" given the advances in superconducting materials we've made, since the director of MIT's fusion program seems to think that we're now only 5 years away from energy-positive fusion.

http://i.imgur.com/sjH5r.jpg (source: https://hardware.slashdot.org/story/12/04/11/0435231/mit-fus...)

MIT director: https://www.youtube.com/watch?v=KkpqA8yG9T4

Manned missions to Mars are also perfectly feasible. Like fusion, we just don't devote enough resources to it.

Pretty much anything space-related would be much easier with two simple steps: first, you build a launch loop to drop launch costs. It's all the capabilities of a space elevator but doesn't require any exotic materials. Launch costs are estimated at ~$3 per kg for a 20GW system, figuring $30B construction cost with 5-year payback if you can hit 100% utilization. And no that's not a typo, that's 5 years to profit at $3/kg - where's Elon Musk when you need him, right? :P

https://en.wikipedia.org/wiki/Launch_loop

http://launchloop.com/LaunchLoop?action=AttachFile&do=get&ta...

Then, you drag an asteroid into orbit and mine it, so you have a bunch of material that's not at the bottom of a gravity well. Both of those steps are in a similar category to a manned mission to Mars - feasible, but would take some serious engineering and serious resources. But once you do them, you also open some serious doors. There is a massive quantity of literally any metal you want in an asteroid. One small rocky asteroid has about 10x the total amount of minerals ever mined on Earth, and why stop at one?

https://en.wikipedia.org/wiki/Asteroid_mining

Even if we couldn't feasibly return them to earth, they're infinitely more valuable in orbit where they are. We are talking about just a mind-boggling amount of resources exactly where we need them.

If you have large amounts of lead and other radiation-resistant material in orbit you've solved one of the main impediments to a Mars mission, and large-scale space construction in general. Then it just comes down to lifting enough fuel to get you on your way, or finding a way to refine it in orbit.

Heck, you can even use the husk of the asteroid itself as an Aldrin cycler. You kit it out as a radiation shelter, then you can put it on a periodic orbit that will make a 146-day trip to Mars every 2 years and then you never have to touch it again, you just dock your payload to it and ride it along. That way you don't need to accelerate and decelerate all the mass of your radiation shelter every single trip. There are even some orbits that will make faster trips (but less frequently)

https://en.wikipedia.org/wiki/Mars_cycler

Another possibility you open up there is using some of a rocky asteroid's uranium for nuclear-thermal rocket engines. The problem with them has always been that nobody wants to use them for takeoff, or even really risk lifting them off with a regular rocket. But if you source uranium in orbit, that problem is solved. Firing them in orbit is still kinda questionable, but it's much more plausible to be able to give them a good kick to build distance if you're not starting from a terrestrial launch. Having orbital infrastructure just makes everything so incredibly much easier since you are saving at least 2 stages, maybe three.

Nuclear-thermal rockets use hydrogen as a reaction mass, which you may know as "the most abundant element in the universe". Ramscoops could even allow such craft to have a plausible mechanism to acquire more fuel (reaction mass, technically) on long-duration or interstellar missions. You could even combine nuclear thermal propulsion to get you up to ramscoop speeds, switch to ramjet for cruising, and then brake on the nuclear-thermal engine again - all on a single fuel source.

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket

Viable interstellar probes are the marquee selling point of the uranium, but of course you would also be able to power reactors in orbit to run all this infrastructure, breed more Pu-239 for RTGs for probes safely in orbit without the risk of a launch accident, etc.

At this point the solutions exist on paper to go anywhere we want within our solar system, and even start exploring beyond. You can even make it pretty easy by laying down this infrastructure. But we could do it the hard way right now as well, if we wanted to. There's nothing stopping you from using big rockets to lift big parts into orbit and then assembling a huge ship suitable for a long-duration interplanetary mission. We just don't want to. Bummer, especially since we could get unlucky tomorrow and have another K-T extinction event.

Aging is a tough nut to crack and we're not going to plausibly break Death anytime soon (maybe in a few hundred more years - but sorry kiddos, that still means that in the grand scale of the human timeline you still are some of the last people that are ever going to die). But there's animals that are capable of living much longer lives than you would expect, and we can certainly adopt some of their mechanisms for maintaining genetic homeostasis. There's no reason to think that our current lifespan is the limit, after all people living to 100+ was absurd not too long ago. Nature and science undoubtedly still have some tricks up their sleeves.

You don't suddenly discover one weird trick to live forever (doctors hate him!). It's a debugging process, we are going to crack one problem and then move onto the next one, getting a little farther each time as we go. Someone will figure out how to switch cancer cells off, someone else will figure out how to cure dementia, then the average lifespan will start creeping up to 150 years old. At that point the predominant cause of death will shift to skin failure or boneitis or some other failure point, we will spend another 50 years curing that, and the cycle repeats. (a word of warning: do NOT use squirrel bones)