Mission Description

The Herschel Space Observatory's mission has been designed to unveil a face of the early Universe that has remained hidden until now. Thanks to its ability to detect radiation at far infrared and sub-millimetre wavelengths, Herschel will be able to observe dust obscured and cold objects that are invisible to other telescopes.

Herschel's major objective will be discovering how the first galaxies formed and how they evolved to give rise to present day galaxies like our own. Additional targets for Herschel will include clouds of gas and dust where new stars are being born, disks out of which planets may form and cometary atmospheres packed with complex organic molecules.

What is the Herschel Space Observatory?

The Herschel Space Observatory is a space-based telescope that will study the Universe by the light of the far-infrared and submillimeter portions of the spectrum. It is expected to reveal new information about the earliest, most distant stars and galaxies, as well as those closer to home in space and time. It will also take a unique look at our own solar system.

Herschel is the fourth Cornerstone mission in the European Space Agency's Horizon 2000 program. Ten countries, including the United States, are participating in its design and implementation. Herschel was launched in May 2009, and is expected to remain an active observatory for at least three years.

Why "Herschel"

Originally called "FIRST," for "Far InfraRed and Submillimetre Telescope," the spacecraft was renamed for Britain's Sir William Herschel, who discovered in 1800 that the spectrum extends beyond visible light into the region that we today call "infrared."

Herschel's namesake will give scientists their most complete look so far at the large portion of the Universe that radiates in far-infrared and submillimeter wavelengths.

With a primary mirror 3.5 meters in diameter, Herschel was the largest infrared telescope sent into space as of its launch date. It will focus light onto three instruments called HIFI, SPIRE, and PACS, which will enable Herschel to be the first spacecraft to observe in the full 60-670 micron range.

More Than Meets the Eye

The far-infrared and submillimeter wavelengths at which Herschel will observe are considerably longer than the familiar rainbow of colors that the human eye can perceive. Yet, this is a critically important portion of the spectrum to scientists because it is the frequency range at which a large part of the universe radiates.
Much of the Universe consists of gas and dust that is far too cold to radiate in visible light or at shorter wavelengths such as x-rays. However, even at temperatures well below the most frigid spot on Earth, they do radiate at far-infrared and submillimeter wavelengths.

Stars and other cosmic objects that are hot enough to shine at optical wavelengths are often hidden behind vast dust clouds that absorb the visible light and reradiate it in the far-infrared and submillimeter.

There's a lot to see at these wavelengths, and much of it has been virtually unexplored. Earthbound telescopes are largely unable to observe this portion of the spectrum because most of this light is absorbed by moisture in the atmosphere before it can reach the ground. Previous space-based infrared telescopes have had neither the sensitivity of Herschel's large mirror, nor the ability of Herschel's three detectors to do such a comprehensive job of sensing this important part of the spectrum.

Two-thirds of Herschel's observation time will be available to the world scientific community, with the remainder reserved for the spacecraft's science and instrument teams.

A Special Orbit

Herschel and Planck will spend their first four months traveling about 1.5 million kilometers (about 931,000 miles, roughly four times the distance of the moon) from earth, in the opposite direction from the sun. During that journey, they will be calibrated and checked out to make sure they're in perfect working order.
Each spacecraft will then go into a separate orbit around the Earth-Sun L2 point, a relatively stable location where the gravitational pulls of the Earth and the Sun combine to keep spacecraft in a uniform position relative to Earth as they orbit the sun.

As it orbits L2 at an amplitude of about 700,000 km, Herschel's distance from Earth will vary from 1.2 to 1.8 million km. Small correction maneuvers will be performed each month to compensate for drift.

With its back to the Earth, Moon, and Sun, Herschel's telescope will point outward into the Universe without interference from the strong infrared radiation these bodies emit. It will focus light onto three instruments:

  • PACS, a camera and medium-resolution spectrometer sensitive to the wavelength range from 60 to 210 microns.
  • SPIRE, a camera and spectrometer sensitive to the wavelength range from 200-670 microns.
  • HIFI, a very high resolution heterodyne spectrometer sensitive to 480-1250 and 1410-1910 GHz (which corresponds to about 157-625 microns).

Keeping Its Cool

Keeping HIFI, SPIRE, and PACS at a temperature near absolute zero is critical to Herschel's mission for two reasons: the detectors work only at very low temperatures, and heat from the instruments could drown out the faint far-infrared and submillimeter light they were designed to detect. Maintaining that ultra cold temperature depends on the superfluid helium that serves as the coolant.

The cryostat that houses the instruments is expected to hang on to enough coolant to enable Herschel to perform its scientific observations for at least three years. At some point after that, the helium will evaporate into space, the instruments will warm up, and the mission will end.

Herschel is expected to offer about 7,000 hours of science time per year.

Communication with the spacecraft, to receive data and convey instructions, will be done via the ground station in Perth, Australia.