Midnight Planets hosts huge amounts of data, and an epic amount of work goes into the app. While most of the data comes from NASA or NASA partners, Midnight Planets is not affiliated with any government agency and is not funded by taxes. So if you use the app, you help support it by occasionally looking at an ad.
People complain about advertising, and it's true ads are often overused. On the other hand, people really like free apps. Midnight Planets tries not to overuse ads. The author uses the same app that you do.
A subscription option doesn't seem realistic at the moment. Midnight Planets is an ongoing experiment being undertaken by one developer. At the moment the developer prefers to keep things simple and focus on expanding the app.