The twin spacecraft are to lift off May 14 aboard an Ariane 5 rocket from the Guiana Space Center in French Guiana.
"The missions are quite different, but they'll hitch a ride to space together," said Ulf Israelsson, NASA project manager for both Herschel and Planck at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
NASA said Herschel's telescope mirror -- about 11.5 feet in diameter -- is the largest ever launched into space. The mirror will collect longer-wavelength light in the infrared and sub-millimeter range -- light never before investigated by an astronomy mission.
Planck will look back in time to just 400,000 years after the universe exploded into existence nearly 14 billion years ago in the event known as the Big Bang. The mission will spend at least 15 months making the most precise measurements yet of light at microwave wavelengths.
NASA's Herschel Science Center at the California Institute of Technology and U.S. and European Planck scientists will work together to analyze the data.
More information about the Herschel mission is available at http://www.nasa.gov/Herschel, while information about the Planck mission is available at http://www.nasa.gov/planck.