Editor's note: Meg Urry is the Israel Munson professor of physics and astronomy and chairwoman of the department of physics at Yale University, where she is the director of the Yale Center for Astronomy and Astrophysics. This piece was written in association with The Op-Ed Project, an organization seeking to expand the range of opinion voices to include more women.

Last weekend, another large piece of "space junk" tumbled to Earth, perhaps in Southeast Asia. Many people - if they noted the event at all - probably worried about being hit on the head, even though the odds are overwhelmingly against such a catastrophe (trillions to one).

But for thousands of astrophysicists around the world, the German Roentgen satellite ("ROSAT") was no mere rubbish; it was an old and important friend. Launched in 1990, a few months after the better known Hubble Space Telescope, ROSAT provided images of the sky in X-rays (very short wavelength light), as opposed to the red-green-blue light visible with Hubble, meaning it could see the most energetic phenomena in the Universe. Plus ROSAT had better image quality than any X-ray satellite had before, an improvement comparable to the superiority of Hubble imaging compared to ground-based telescopes.

A few thousand astronomers worldwide used ROSAT to study the universe, discovering where black holes are growing, when massive clusters of galaxies formed, and how neutron stars and black holes in our Milky Way Galaxy behave.

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