Video can be very effective in the classroom. For example, you can go to many science-oriented web sites for kids and select specific topics to view. These videos will be most effective as part of a well-designed unit that includes standards-based objectives. Hands-on activities are also useful. For example, after watching the NASA video, your students could design their own rockets using household materials.
The first link below is a NASA webinar that you will find extremely interesting. The video is 2 hours and 25 minutes, so you will want to preview it and select parts to show or possible show it in half hour segments, depending on your use.
I have also had a lot of luck with Discovery Channel for kids. There are games and videos on almost every topic. National Geographic for kids also has great content.
You can also simply go to YouTube and type in the topic. If your district does not block the video, you can find great videos there. For a more classroom friendly version, most districts don't block TeacherTube.
Good luck! I am a big champion of video in the classroom. It adds interest and also helps your students explore all kinds of content in a more interactive way.
NASA offers a lot of video but individual videos are rarely that long. You could easily put together multiple videos on a similar topic though. Much of the video is uploaded to YouTube but the NASA Video Gallery link below is different in that it allows you to download the videos for later use .