Herschel in space, close up on its mirror

Herschel was launched on 14 May 2009, 13:12 CEST, together with another satellite, Planck, which is studying the Cosmic Microwave Background radiation, on an Ariane 5 ECA launcher. Almost 26 minutes after launch, and about two minutes from each other, they were released separately (Herschel first) on a trajectory toward a virtual point in space, called L2, some 1.5 million kilometres from Earth in the direction opposite to the Sun.

The satellite is a tall 'tube' 7.5 m high and 4 m wide, with a launch mass of around 3.3 tonnes; it carries the infrared telescope and three scientific instruments. The bulk of the spacecraft consists of a liquid helium thermos bottle inside which the instrument detectors sit and are cooled down to only a few degrees above absolute zero, some of them even to a fraction of a degree.

Herschel has been designed to perform routine science operations for a minimum of three years at its orbit around L2. The mission will end when the helium used to cool the focal plane of the scientific instruments is depleted.

The Herschel Mission Operations Centre (MOC) is located at ESOC, Darmstadt, Germany.

 
 

	Micha Schmidt

The Flight Control Team
 
The Flight Control Team works from the Herschel Dedicated Control Room located at ESOC. Spacecraft Operations Manager Micha Schmidt, from Germany, has been assigned to Herschel since 2002.

In the two-year period prior to launch, he assembled and trained the FCT and worked with the team and other experts at ESOC to develop the mission control system and practise and perfect flight operations processes and procedures. Since launch, the team has been actively engaged in operating the satellite and commissioning systems and instruments.

	Ariane-5 under testing at Kourou

The Ariane 5 ECA launcher burned its solid boosters for slightly less than 2½ minutes and its main engine for about nine minutes to lift Herschel and Planck into a ballistic coast orbit. After coasting for 107 minutes, the Ariane 5 upper stage ignited for just over 17 minutes to inject Herschel and then Planck into separate transfer trajectories. After separation, Herschel entered free flight as a three-axis stabilised satellite (Planck, in contrast, rotates to perform its scientific mission).

Herschel will take between four and six months to reach its orbit around L2, the second Lagrange point of the Earth-Sun system, 1.5 million km from the Earth. During the journey Herschel will perform a number of manoeuvres.

Time	Manoeuvre
Launch plus one days	Perigee velocity correction and trajectory correction to remove launch dispersion (change in speed of 8.7 m/s)
Launch plus twelve days	Fine correction to launch dispersion manoeuvre dispersion
--	Mid course correction manoeuvre, if needed

 
 

	Spacecraft on L2

At L2, Herschel will enter a large Lissajous orbit about the Lagrange point.

Lissajous orbits are the natural motion of a satellite around a collinear libration point in a two-body system and require less momentum change (i.e. thruster firings) to be expended for station keeping than halo orbits, where the satellite follows a simple circular or elliptical path about the libration point.

While complex, this orbit will provide an ideal environment for a space observatory: far from the Earth and its magnetic field (that traps the solar wind) and far away from both the Sun and Earth, whose emitted or reflected light can damage sensitive optical instruments.

	New Norcia: ESA's first 35m deep-space station

DSA 1 (Deep Space Antenna 1), located at New Norcia, Australia, hosts a 35-metre deep-space antenna with transmission and reception in both S- and X-band. The town of New Norcia is 140 km north of Perth, Western Australia.

For Herschel, the Earth-to-spacecraft distance will vary between approximately 1.2 to 1.8 million km. Note that the ESTRACK stations at Kourou and at Maspalomas and Cebreros, Spain, also supported Herschel during the Launch and Early Orbit and commissioning phases.

	This mission uses SCOS-2000

In addition to the MOC at ESOC, the ground segment comprises the Herschel Science Centre (HSC), located at ESA's European Space Astronomy Centre (ESAC), Madrid, supported by NASA's Herschel Science Center (NHSC), located at the agency's Infrared Processing and Analysis Centre (IPAC, JPL, California), which act as the point of interface to the American science community.

There are also three dedicated Instrument Control Centres (ICCs), one for each instrument on board the spacecraft, provided by the respective principal investigators.
 
 

	Block diagram of Herschel ground segment

The observatory schedule will be created based on a database of accepted observations, fed by requests from the scientific community. The long-term mission plan - plus short-term observing schedules - together with the corresponding instrument commands is produced in the mission planning system at the HSC, and transferred to the MOC at ESOC. The MOC adds in the current required satellite commands and produces the final detailed mission timeline that is uplinked to the spacecraft.

Observational data are stored on board, and downlinked to the New Norcia station (backed-up by DSA 2, Cebreros station) during passes lasting approximately three hours. The satellite is capable of storing two operational days of observational data in the on-board computer in case any downlink passes are missed. The down-linked satellite telemetry is transferred from the ground station to the MOC, where it is consolidated and transferred to the HSC.<

The SVM houses 'warm' payload electronics and provides the necessary 'infrastructure' for the satellite such as power, attitude and orbit control, the on-board data handling and command execution, communications and safety.

The payload

The Herschel science payload comprises three instruments that perform a combination of spectrometry, imaging spectrometry and imaging photometry covering a wavelength range from 60 to 670 µm.
 
 

• Photodetector Array Camera and Spectrometer (PACS), a camera and a low- to medium-resolution spectrometer for wavelengths up to about 205 micrometres. It uses two bolometer detector arrays in the camera and two photo-conductor detector arrays in the spectrometer.
• Spectral and Photometric Imaging Receiver (SPIRE), a camera and a low- to medium-resolution spectrometer for wavelengths longer than 200 micrometres. It uses five detector arrays: three to take images of infrared sources in three different infrared 'colours' and two to fully analyse the longer infrared light being released from the source.
• Heterodyne Instrument for the Far Infrared (HIFI), a highly accurate spectrometer that can be used to obtain information about the chemical composition, kinematics, and physical environment of infrared sources.