Herschel Space Observatory

Instrumentation

Herschel was the first space telescope to observe the full far-infrared and submillimeter range. Its 3.5-meter mirror, the largest optical telescope ever placed in orbit, was constructed from sintered silicon carbide rather than glass. The mirror blank came from Boostec in Tarbes, France; Opteon Ltd. in Tuorla Observatory, Finland, ground and polished it; and the Calar Alto Observatory in Spain applied the reflective coating via vacuum deposition.

The mirror focused light onto three instruments, all cooled below 2 K by more than 2300 liters of liquid helium boiling at about 1.4 K in near-vacuum. This helium supply limited the mission’s lifespan, originally expected to be at least three years.

One instrument, PACS, was an imaging camera and low-resolution spectrometer built by the Max Planck Institute for Extraterrestrial Physics. It covered wavelengths from 55 to 210 micrometers, with spectral resolution between R=1000 and R=5000, and could detect signals as faint as −63 dB. The camera imaged simultaneously in two bands, reaching a detection limit of a few millijanskys.

Service module

Herschel and Planck were built as a single program, sharing a common service module designed and manufactured by Thales Alenia Space in Turin. The two modules are structurally alike, both octagonal, with each panel assigned to specific warm units and arranged to manage heat from the units, instruments, and spacecraft. Their avionics, attitude control and measurement systems, command and data management systems, power subsystems, and tracking, telemetry, and command subsystem all share a common design.

Every unit on the service module is redundant. The power subsystem includes a solar array with triple-junction solar cells, a battery, and a power control unit. It connects to the array’s 30 sections, supplies a regulated 28-volt bus, distributes power through protected outputs, and manages battery charging and discharging. On Herschel, the solar array is attached to the lower part of a baffle that shields the cryostat from the Sun.

A three-axis attitude control system keeps the baffle pointed at the Sun. The baffle’s upper surface is covered with optical solar reflector mirrors that reflect 98 percent of solar energy to prevent heating the cryostat.

The attitude control computer handles pointing and slewing for both payloads. Herschel is three-axis stabilized, with an absolute pointing error of less than 3.7 arc seconds. Both spacecraft use a star tracker as their primary line-of-sight sensor.

Discoveries

Herschel made over 35,000 scientific observations and amassed more than 25,000 hours' worth of science data from about 600 different observing programs. It specialized in collecting light from objects in the Solar System, the Milky Way, and extragalactic objects billions of light-years away, such as newborn galaxies. The observatory was charged with four primary areas of investigation: galaxy formation in the early universe and the evolution of galaxies; star formation and its interaction with the interstellar medium; chemical composition of atmospheres and surfaces of Solar System bodies, including planets, comets and moons; and molecular chemistry across the universe.

During the mission, Herschel was capable of seeing the coldest and dustiest objects in space, such as cool cocoons where stars form and dusty galaxies just starting to bulk up with new stars. The observatory sifted through star-forming clouds to trace the path by which potentially life-forming molecules, such as water, form.

End of mission

The telescope's lifespan was governed by the amount of coolant available for its instruments. At the time of its launch, operations were estimated to last 3.5 years, to around the end of 2012. It continued to operate until 29 April 2013 15:20 UTC, when Herschel ran out of coolant. The observatory was carried into orbit by an Ariane 5 in May 2009, reaching the second Lagrangian point (L2) of the Earth–Sun system, 1500000 km from Earth, about two months later.

Herschel is named after Sir William Herschel, the discoverer of the infrared spectrum and planet Uranus, and his sister and collaborator Caroline Herschel. NASA was a partner in the Herschel mission, with US participants contributing to the mission, providing mission-enabling instrument technology and sponsoring the NASA Herschel Science Center (NHSC) at the Infrared Processing and Analysis Center and the Herschel Data Search at the Infrared Science Archive.

Did You Know?

• Herschel was the fourth and final cornerstone mission in the Horizon 2000 programme, following SOHO, Cluster II, XMM-Newton and Rosetta.
• NASA developed and built the mixers, local oscillator chains and power amplifiers for the HIFI instrument.