The thing about Duna is that it’s actually not that much of a stretch to go there after you’ve done a Mun landing. Your typical Mun landing takes about 5500 dV to get there, and about 900 to get back, so rounding up to give us a smidge of slack, it’s about 6500 dV. To get a probe onto the surface of Duna will cost you about… 4700 dV or so with parachutes. It seems intimidating because of the planning involved in picking the right window, getting an encounter node, etc. but rocket-wise, it’s easier since Duna has an atmosphere. Now, to bring a ship to Duna and BACK takes a bit more dV, like another 2400 or so. But if you don’t have the rockets to make that happen right away, then send a crash probe, and get that science! There’s a ton of it there.
Now, if you just can’t get to Duna or are waiting for the clock to spin because you have life support mods or something, there are a few other opportunities for advancement that you should consider:
- Do you have a Keosynchronous satellite array for communicating with your Kerbals while they transition to Duna? If not, I’d suggest putting one in place. It can be a challenge in 1.3.x to setup a working com relay network. Scott Manley has a great video on how to do this in one easy launch: https://youtu.be/Z3FHOQFVV0k
- Have you captured an asteroid? Mined an asteroid? put a base on an asteroid? This usually comes up right before Duna missions do.
- Like Eric said, you can simply hit fast forward, making sure to keep your Kerbals in the field either fed or frozen if you have a modded game going. If not, then fast forward is your friend! Kerbals live forever. There is no reason to worry about time in this game if you don’t have mods.
- Have you grabbed science and experience for your Kerbals from the Sun? Many people mis this step, but just leaving the SOI of Kerbin, doing a bunch of science and some EVAs, and then returning to Kerbin gains loads of XP and science points and only take a month or two to accomplish.
- Do you have an Ore lab on Minmus? Minmus is a great staging launch site because it is very cheap to get from the surface to orbit, and then very easy to get back to a Kerbin SOI for your next hop. So, make a gas station at Minmus!
- Next, this is usually the time I get the itch to build an SSTO space plane. They aren’t very practical, but if you can design one that can reach orbit, dock with a space station, and return safely it is quite an accomplishment.
- Finally if you are really just struggling to gain science, then I would recommend a science lab around Mun or Minmus. Load it with experiments from the surface (even science you have already gathered can be used again in a lab), add a couple of scientists and have them do research. The science will come flowing in. Science labs are kind of broken OP for gaining lots of science easily in this manner.
Edited because I forgot parachutes for Duna!