For a generation, the Hubble Space Telescope has been exposing the universe's deepest, darkest secrets. From imaging the volcanoes of Jupiter's moon Io, to watching the dramatic breakup of comets, imaging baby galaxies, and helping to nail down the universe's age, its data has been instrumental in today's understanding of the cosmos near and far.

But it's an old telescope, unmaintained since the last space shuttle mission visited in 2009. While the observatory is in excellent health today, it's expected to stop collecting data sometime in the 2020s. What will we lose when the telescope finally dies?

NASA is quick to point out that the James Webb Space Telescope, expected to launch in 2018, will enhance Hubble's capabilities in many ways. But for the telescope's higher resolution and ability to peer back to the very early days of the universe, there is one key thing it doesn't have: ultraviolet capabilities. (It also will lack some of Hubble's fine spectral resolution, and ability to observe a special spectral line called H-alpha that is useful for nebulae and stars.)

Astronomers are being urged to submit as many UV proposals to Hubble as possible because once it dies, there are no immediate plans to launch a successor. (Astronomers could then pull from the archive as needed in future decades.) Earth's atmosphere filters out UV, which is great for protecting life, but bad for UV astronomy, so it needs to be done from space. Astronomers say they don't think another UV telescope will fly until the 2030s, at the earliest.