By Dr David Whitehouse
BBC News Online science editor
The Galaxy Evolution Explorer spacecraft (Galex), designed to explore star formation when the Universe was young, has been launched from a Pegasus rocket released from a carrier aircraft off the coast of Florida, US.
Looking towards other stars
The small satellite will observe millions of galaxies across 10 billion years of cosmic history, helping astronomers determine when the chemical elements were formed inside the first stars.
It will map out the first stages of the chemical evolution of the cosmos - how it evolved from a fireball of hydrogen and helium into a Universe full of heavier elements.
In a sense, it is a search for life's origins as well, because without such heavy elements as carbon and oxygen, life as we know it could not exist.
After the space observatory separated from the rocket's third stage - 11 minutes and five seconds after release from the carrier aircraft - it entered into Earth orbit at an altitude of 690 kilometres (429 miles).
After a one-month period of in-orbit checkout, the science mission will begin, lasting for up to 28 months.
Looking for surprises
Galex will focus on distant galaxies containing young, hot, short-lived stars that emit a great deal of ultraviolet radiation.
Air-launched by a Pegasus rocket
Astronomers are attracted to these galaxies because star formation is active there, so studying them will help astronomers learn more about how, when and why stars form.
"This mission will provide the first comprehensive map of a Universe of galaxies under construction and bring us closer to understanding how they, and our own Milky Way, were built," says Christopher Martin, Galex's principal investigator .
"Galex is designed specifically to measure the distances and star formation rate in a million galaxies," says the US space agency's James Fanson
"It is going to help us put together a picture of the Universe we see today - how stars formed in galaxies, how galaxies evolved through time, what caused star formation and led to the development of heavy elements of the periodic table."
He adds: "As a project manager, there are few opportunities in a career to be associated with a mission that will give you a fundamentally new view of the Universe.
"Typically, when you see the Universe for the first time in a new way, there are unexpected surprises that come from the data. So that is one of the things we are looking for - the surprises."
The Galex mission was the 33rd flight of the Pegasus rocket and the second of four missions in 2003.