Frozen tundra throughout the winter, the Lena River Delta in Russia thaws in spring, emptying into the Arctic Ocean. NASA/METI/AIST/Japan Space Systems/ASTER Science Team An instrument called the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) has been taking pictures of Earth since it launched into space in 1999.

In that time, it has photographed an incredible 99% of the planet's surface.

Although it's aboard NASA's Terra spacecraft, ASTER is a Japanese instrument and most of its data and images weren't free to the public — until now.

NASA announced on April 1 that ASTER's 2.95 million scenes of our planet are now ready to download and analyze for free.

With 16 years' worth of images, there are a lot to sort through.

Here are our 21 favorites: