The payload "stack" build-up for Ariane 5's second mission of 2009 marked another milestone with the encapsulation of its lower passenger — the Planck space observatory — in the SYLDA dispenser system.

SYLDA is a canister-shaped device that enables Ariane 5 to carry its trademark dual satellite payloads, and the dispenser has been positioned over Planck, which was installed atop the Ariane 5's core stage last week. The encapsulation took place inside Ariane 5's Final Assembly Building at the Spaceport in French Guiana, where the heavy-lift launcher is scheduled for a May 14th liftoff. With Planck now covered by the dispenser, all is ready for the placement of the Herschel space telescope atop SYLDA, along with the addition of the payload fairing that protects Ariane 5's two passengers during its ascent through the atmosphere's dense layers.

During Ariane 5's launch sequence on May 14th, Herschel will be deployed first, followed by the release of Planck.  Both payloads were produced by Thales Alenia Space-led industry teams for the European Space Agency's space science program. Planck is designed to map the sky in nine wavelength bands, helping to determine the universe's fundamental characteristics — including the overall geometry of space, the density of normal matter and the rate at which the universe is expanding. The observatory carries a telescope with an effective aperture of 1.5 meters, which will direct microwave radiation onto the spacecraft's low frequency and high frequency instruments.

Herschel will be the largest space telescope ever launched, and was conceived to provide astronomers with their best views of the universe at far-infrared and sub-millimeter wavelengths.  Data gathered with Herschel's 3.5-meter-diameter mirror will be used to study the formation of stars and galaxies, and to investigate the relationship between the two. After being lofted by Ariane 5, both Planck and Herschel will follow transfer trajectories for their 1.5 million-kilometer voyages to the Sun-Earth system's second Lagrange point (L2).