As astronauts on space shuttle Atlantis perform what is likely the final tune-up of the Hubble Space Telescope, a 19-year-old workhorse that has greatly advanced astronomy and the profile of space science, the European Space Agency (ESA) is readying two more heavy-hitting astronomical spacecraft for deployment into space.

On Thursday ESA will launch Planck and Herschel, two separate observatories that will share a rocket into space. Although their capabilities and goals are different, both spacecraft will peer at radiation emitted early in the universe's history to shed light on our cosmic origins.

Herschel boasts a 3.5-meter (11.5-foot) telescope mirror, the largest yet flown in space. (Hubble's, by comparison, is 2.4 meters, or 7.9 feet, across.) Its overarching theme as an observatory is investigating star formation, says Paul Goldsmith, Herschel project scientist at the NASA Jet Propulsion Laboratory (JPL) in Pasadena, Calif. (Both spacecraft, though primarily ESA projects, were built in conjunction with NASA.)

Billions of light-years across the universe—and therefore billions of years back in time—stars are being born in great quantities. These so-called starbursts are difficult to observe from Earth, as their dusty shrouds absorb much of the optical light from the stars and re-radiate it as longer-wavelength radiation to which Earth's atmosphere is mostly opaque. (Even photons that are not affected by dust see their wavelengths stretched as they make their way across an expanding universe.)

By sampling long-wavelength light outside the obscuring blanket of the atmosphere, the spacecraft will have an unprecedented view of those stars and galaxies taking shape. (Herschel and Planck will both take up residence about 930,000 miles, or 1.5 million kilometers, from Earth, well beyond the moon's orbit.) "Having Herschel will be like opening a dirty window," Goldsmith says, allowing us a clearer look at the star formation process. "We are woefully lacking in understanding how the cool, diffuse gas and dust of an interstellar cloud are converted to the much hotter, denser ball of gas that is a star."

Goldsmith adds that Herschel will also be able to inventory the individual species of molecules in interstellar clouds to firm up the chemistry of star formation. In particular it will examine the role—predicted to be a critical one—that water plays in the process. The observatory should be able to look into regions where stars are forming and, he says, "unambiguously determine how much water there is, and what it is doing."

Nestled just below Herschel on ESA's Ariane 5 rocket in French Guiana is Planck, a spacecraft that will build on the work of the seminal Wilkinson Microwave Anisotropy Probe (WMAP), launched by NASA in 2001. By measuring subtle variations in the cosmic microwave background (CMB), the remnant radiation from the early universe that pervades the sky, WMAP refined the estimated age of the universe (13.7 billion years, give or take), among other key cosmological parameters. Planck will have three times the resolution of WMAP and will be far more sensitive than its precursor to the variations in temperature across the CMB.

"Whereas WMAP has measured roughly 10 percent of the information that we can obtain from the CMB temperature fluctuations, Planck should measure essentially all of it," says project scientist Charles Lawrence of JPL, Goldsmith's counterpart on the Planck mission. Lawrence expects that the spacecraft will significantly shrink levels of uncertainty for such cosmological parameters as the age of the universe and the relative proportions of ordinary matter (all that we can see and touch) and that mysterious stuff known as dark matter, which is believed to make up far more of the universe than its ordinary counterpart.

Planck also may be able to confirm or challenge the theory of inflation, the reigning explanation for the universe's rapid, post–big bang expansion. "Inflation is a powerful idea that accounts in a general way for many key properties of the universe," Lawrence says. "But we do not know how inflation worked in terms of fundamental physics. There are lots of ideas, but not much data." Planck will look for the imprint of inflation in the CMB, helping to fill that data gap.

Ulf Israelsson, project manager for both missions at JPL, says that the two spacecraft will work in concert to help refine our understanding of the universe's birth and evolution. While Planck probes the radiation emitted when the universe was only about 0.003 percent of its present age, Israelsson says, Herschel will "look at how the first levels of galaxy formation occur in the early universe" and even examine more recent processes such as the birth of planetary systems. In doing so, he says, Herschel will "sort of pick up where Planck leaves off."