[bbcentral]

The skins look amazing, so much so that I'm inspired to "borrow" some of the elements for my own forum's skin
(I'd like to go with a background image, but then I realise that I'll be adding a gradient on my new skin anyway, so it's a bit pointless).

As far as what trifive is saying above, he's completely right.
I've been struggling with a lack of posts for a while, and I've been working on rebuilding my site to better integrate with my forum.

Here's what I'm doing, not sure how many of these will help you:
1) In order to download any files on the main site (videos, mp3s, etc) you'll need to login first (not too applicable to your site)
2) Whenever I post a news item, there will be a link to "Discuss this in the forum". Some will have a dedicated thread, others will simply link to the most relevant forum category.
3) Don't worry about a chatroom, resist that temptation. I had people begging for one for 2 years, I finally caved in and now nobody uses it. I will be hosting some scheduled chats later on though, which may help.
4) Send a welcome PM and welcome email to every new member that registers. Mine basically says: "Welcome to the site, we're happy to have you here, please introduce yourself in the introductions forum, don't be afraid to ask any questions here, we look forward to having you around, if you need any help don't hesitate to ask. Sincerely, your new friends at Forum Name" I've automated this using the "User Registration Options" page in the AdminCP.
5) I can't do this for my own site because I have an "obscure" topic, but you'll probably be able to get some decent traffic by submitting to Digg.com whenever an interesting topic comes up. Those guys are all tech-heads, and many of them love space stuff. Here's a topic that was posted today: http://digg.com/space/M81_Galaxy_image . You can either manually submit the topics yourself, or install one of the many "Digg This" mods available here, then you can leave it up to your members to submit your stories. Just be warned, you'll need a good server if you're going to submit to Digg, or else you'll suffer the "Digg Effect" from all the heavy traffic :P
6) "Discussion" and "Astronomy and Space Travel Forum" might not be the best names for your forum. "Forum", "Message Board", "Discussion", they're all very generic names. I renamed mine "The Soul Food Cafe", and it gives it a bit of "oomph" What about something like "The Space Place", or "The Launchpad", I'm sure you can come up with something much cooler though. It's cool when my members receive emails from "The Soul Food Cafe", sounds like some exclusive club!
7) Make sure you enable the feature to send Happy Birthday emails, and even try customizing the email body. I get 3-4 emails a week from members I've never even heard of saying "Thanks for the Happy Birthday email!". I haven't got the heart to tell them that I didn't send it, it was automatic! I'm going to change the text to include some links back to the forum, hopefully to show that we'd like them to come back.
8) You're already doing this here, but make sure you keep your site in every forum signature you can.
9) Instead of linking to your home page on other sites, keep it relevant, so if you can link directly to a related topic on your own forum then do that instead.

And finally:
http://www.nasa.gov/rss/breaking_news.rss
You could set your forum to automatically create topics based on the items there (RSS Support is available in vB)

I hope this helps you or anyone else who is trying to boost their activity.

The only other thing I sometimes do, which may or may not help you, is I run a lot of links to Amazon.com products. I earn decent commission for sales generated through them, and a few times a year I have enough available in gift certificates to run a competition for my members. I send a mass email to every member letting them know, with a link to the competition page. The page itself is a list of prizes (all Amazon.com products), and a simple form that allows them to select the answer to a multiple choice question, and enter their username, real name, address, country, and select which prize they would like.
When the competition is over, I just select a random winner from the list (who has the right answer), and then send them an email to let them know. I confirm their details and choice of prize, login to Amazon.com and buy the prize using the gift certificate, and have it shipped directly to their address.
Whenever I do this my Adsense traffic shoots up, I get more traffic (people forward it to their friends and family), more new members, and ironically I end up earning more back from Amazon than I spent on the prize!
It's a neat little trick that makes everyone happy