Encyclopedias serve to give general overviews and summaries of topics. They don't get into the in-depth information you should be seeking for a college-level research paper.
Examples of encyclopedias include: Encyclopedia Britannica, Wikipedia, and any other source that only provides that shallow level of information.
Shallow information is trivia: e.g. this satellite was built in 1974, cost $8m to create, launched in 1975, travels at 10k mph, and so on. This is good to know and informs your deeper analysis! But it's not the thoughtful analysis you want to focus on. Also, you can get that kind of info from e.g. NASA's own website, straight from the horse's mouth, which is better typically.
Research means more than just Googling some likely terms and picking something easy off the first page of results. Consider:
Besides keywords, there are an estimated 200 factors that affect Google's page rankings, including:
Notice that "accuracy" or "reliability" don't make the list.
You can streamline the quality of your Google searches by focusing on government (.gov) and education (.edu) domains in your results.
Google has some advanced search commands to make this quicker. Just add site:___ to your search! E.g. site:nasa.gov or even just site:.edu. Try it below!
The ebook collections offer several advantages: instant online-access, no need to check-out the book, and you can easily search inside to jump to your search terms.
Scholarly articles are some of the best, most authoritative kinds of articles you can read, and are typically how new knowledge is disseminated. They also tend to have a very narrow focus: if a Wikipedia article is just about Kuiper belt objects in general, a related scholarly article might be more specifically on "An exploration of the Kozai resonance in the Kuiper Belt" or "Formation of Kuiper-belt binaries through multiple chaotic scattering encounters with low-mass intruders."
For this assignment, honestly, this type of research might be a little bit overkill. (Or will it take your paper to new heights? I suggest taking a peek in at least Academic Search Complete just to see what's out there and then decide if you can make use of it.)
Chances are you're going to be reading articles online. Distinguishing between purely digital sources and the online versions of print magazines and newspapers is a bit of non-starter: visually, you're not going to notice anything different. Your citations also will not indicate what type of website this is exactly, either, other than the publisher probably being very specific.
You may use your textbook as a source, but it will not count towards your three required sources.
The Three Rs
Is your source
Recent? Reliable? Relevant?
Is this source up-to-date? Is it about my topic, and does it go into enough depth? Does it come from an authoritative source? Is the information accurate (and are there citations given to back it up)? And why was this information written in the first place?
While conducting research you will need to capture the articles and associated citations. Your process will be very individual, but be consistent and choose a tool to help organize your research. Some suggested tools below:
OneDrive - Cloud storage from Microsoft Office 365. 1 TB free for Lone Star College students.
Evernote - The Basic version is free. Works across all mobile devices. Create notebooks for each course or writing assignment. Be sure to download the Web Clipper as well.
EasyBib - Citation generator that is free for MLA.