In part 1,
I said I'd love to see a manned vehicle capable of an acceleration of even .001g - which if I live to a ripe old age(*1) I may see at the end of my lifetime.
The only problem with this spaceship is it can't take off from Earth and it can't (safely) land on anything too big. Details, details!
Takeoff isn't a big problem, we do that with booster rockets today. Hopefully in a couple of decades we'll have a space elevator going... but we don't truly need it. Either way, takeoff is do-able.
But .001g puts some constraints on where we can go... they're not likely to have space elevators anywhere but the Moon! The problem is we couldn't land or take off from anywhere with a gravitational pull more than .001g - one thousandth the pull you're feeling now. (Assuming you're near the surface of the Earth!).
OK - so now how big is too big? Somewhere around 25 miles in diameter(*2) a body starts to have more self-gravity than .001g. This, unfortunately, rules out all the planets, the interesting moons, and some of the asteroids.
Earth-crossing asteroids! Almost all of them are way smaller than this (there's only one that isn't). So of we had the Ronco .001g Spaceship we should and could visit a small hunk of rock nearby. This mission is do-able today & I'd love to see it done.
While we're at it, we should start focusing on is asteroid detection and deflection. We're not bad on the detection side; it would be extremely unlikely for anything 1/2 km in diameter or larger to get close without our knowing it. But even a half-kilometer rock hitting in the "right" place (unlikely, but possible) (and by 'right', I mean 'wrong'!) could kill tens or hundreds of millions of people.
And, it would be nice to detect them well in advance of them hitting us... you know that hunk of rock that we recently thought could hit us in 2029? Turns out it'll miss, but even if we found it would hit, we have the time to do something about it. The real concern is finding one that'll hit in, say, 3 weeks. All we could try to do is evacuate the impact area (if it hit land).
So - we need to experiment with deflection techniques. There are a few ideas out there & it would be great to test-drive them before we need them.
Remember, the dinosaurs died out because they didn't have a space program!
(*1) Which may happen - both of my grandfathers are alive today at ages 94 and 97 & they're both going strong for 100!
(*2) I assumed a density of 2 for this calculation - typical for an asteroid.