Herschel Space Observatory

Herschel Space Observatory

General information
NSSDC ID	2009-026A
Organization	European Space Agency (ESA) NASA
Major contractors	Thales Alenia Space
Launch date	2009:05:14, 13:12:02 UTC
Launched from	Guiana Space Centre French Guiana
Launch vehicle	Ariane 5 ECA
Mission length	planned: 3 years elapsed: 1 year and 9 months
Mass	3,300 kg (7,300 lb)
Type of orbit	Lissajous orbit
Orbit height	1,500,000 km (930,000 mi)
Orbit period	1 year
Orbit velocity	7,500 m/s (27,000 km/h)
Location	Lagrangian point L2
Telescope style	Ritchey-Chrétien
Wavelength	60-670 µm (far-infrared)
Diameter	3,500 mm (140 in), f/0.5 (Primary Mirror)
Collecting area	9.6 m2 (103 sq ft)
Focal length	28.5 m (94 ft), f/8.7
Instruments
HIFI	Heterodyne Instrument for the Far Infrared
PACS	Photodetector Array Camera and Spectrometer
SPIRE	Spectral and Photometric Imaging Receiver
Website	herschel.esac.esa.int

The Herschel Space Observatory is a space observatory sensitive to the far infrared and submillimetre wavebands. It was carried into orbit in May 2009, reaching the second Lagrangian point (L2) of the Earth-Sun system, 1,500,000 kilometres (930,000 mi) from the Earth, about two months later. It is currently taking data and reported to be functioning well.

Herschel was originally proposed in 1982 by a consortium of European scientists, and was named after Sir William Herschel, the discoverer of the infrared spectrum and planet Uranus, and his sister and collaborator Caroline.^[1]

The Herschel Observatory has the largest single mirror ever launched in a space telescope^[2] and is capable of seeing the coldest and dustiest objects in space; for example, cool cocoons where stars form and dusty galaxies just starting to bulk up with new stars. The observatory will sift through star-forming clouds—the "slow cookers" of star ingredients—to trace the path by which potentially life-forming molecules, such as water, form.

Herschel is a mission of the European Space Agency (ESA) with significant contributions and funding from NASA.^[3] It is the fourth 'cornerstone' mission in the ESA science program, along with Rosetta, Planck, and the Gaia mission.

Science

Herschel will specialise in collecting light from objects in our Solar System as well as the Milky Way and even extragalactic objects billions of light-years away, such as newborn galaxies, and is charged with four primary areas of investigation:^[4]

• 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;
• Molecular chemistry across the universe.

Instrumentation

The mission, formerly titled the Far Infrared and Sub-millimetre Telescope (FIRST), involves the first space observatory to cover the full far infrared and submillimetre waveband.^[4] At 3.5 meters wide, its telescope incorporates the largest mirror (made not from glass but from sintered silicon carbide) ever deployed in space.^[5] The light is focused onto three instruments with detectors kept at temperatures below 2 K (−271 °C). The instruments are cooled with liquid helium, boiling away in a near vacuum at a temperature of approximately 1.4 K (−272 °C). The 2,000-litre supply of helium on board the spacecraft will limit its operational lifetime, nonetheless it is expected to be operational for at least 3 years.^[6]

Herschel carries three detectors:^[7]

PACS (Photodetecting Array Camera and Spectrometer)

An imaging camera and low-resolution spectrometer covering wavelengths from 55 to 210 micrometres. The spectrometer has a spectral resolution between R=1000 and R=5000 and is able to detect signals as weak as −63 dB. It operates as an integral field spectrograph, combining spatial and spectral resolution. The imaging camera can image simultaneously in two bands (either 60–85/85–130 micrometres and 130–210 micrometres) with a detection limit of a few millijanskys.^[8][9]

Herschel in a clean room

SPIRE (Spectral and Photometric Imaging Receiver)

An imaging camera and low-resolution spectrometer covering 194 to 672 micrometre wavelength. The spectrometer has a resolution between R=40 and R=1000 at a wavelength of 250 micrometres and is able to image point sources with brightnesses around 100 millijanskys (mJy) and extended sources with brightnesses of around 500 mJy.^[10] The imaging camera has three bands, centered at 250, 350 and 500 micrometres, each with 139, 88 and 43 pixels respectively. It should be able to detect point sources with brightness above 2 mJy and between 4 and 9 mJy for extended sources. A prototype of the SPIRE imaging camera flew on the BLAST high-altitude balloon. NASA's Jet Propulsion Laboratory in Pasadena, Calif., developed and built the "spider web" bolometers for this instrument, which is 40 times more sensitive than previous versions.

HIFI (Heterodyne Instrument for the Far Infrared)

A heterodyne detector which is able to electronically separate radiation of different wavelengths, giving a spectral resolution as high as R=10⁷.^[11] The spectrometer can be operated within two wavelength bands, from 157 to 212 micrometres and from 240 to 625 micrometres. NASA developed and built the mixers, local oscillator chains and power amplifiers for this instrument.^[12]

Service module

A common service module (SVM) was designed and built by Thales Alenia Space in its Turin plant, for the Herschel and Planck missions combined into one single program.^[13]

Structurally, the Herschel and Planck SVM's are very similar. Both SVM's are of octagonal shape and for both, each panel is dedicated to accommodate a designated set of warm units, while taking into account the dissipation requirements of the different warm units, of the instruments as well as the spacecraft.

Furthermore, on both spacecraft a common design for the avionics, the attitude control and measurement system (ACMS) and the command and data management system (CDMS), and power subsystem and the tracking, telemetry and command subsystem (TT&C) has been achieved.

All spacecraft units on the SVM are redundant.

Power subsystem

On each spacecraft, the power subsystem consists of the solar array, employing triple-junction solar cells, a battery and the power control unit (PCU). It is designed to interface with the 30 sections of each solar array, provide a regulated 28 V bus, distribute this power via protected outputs and to handle the battery charging and discharging.

For Herschel, the solar array is fixed on the bottom part of the baffle designed to protect the cryostat from the sun. The three-axis attitude control system maintains this baffle in direction of the sun. The top part of this baffle is covered with Optical solar reflector (OSR) mirrors reflecting 98% of the sun energy, avoiding heating of the cryostat.

Attitude and orbit control

This function is performed by the attitude control computer (ACC) which is the platform for the ACMS. It is designed to fulfil the pointing and slewing requirements of the Herschel and Planck payload.

The Herschel spacecraft is three-axis stabilized, the absolute pointing error needs to be less than 3.7 arc sec.

The main sensor of the line of sight in both spacecraft is the star tracker.

Launch and orbit

The spacecraft, built in the Cannes Mandelieu Space Center, under Thales Alenia Space Contractorship, was successfully launched from the Guiana Space Centre in French Guiana at 13:12:02 UTC on 14 May 2009, aboard an Ariane 5 rocket, along with the Planck spacecraft, and placed on a very elliptical orbit (perigee: 270.0 km (intended 270.0±4.5), apogee: 1,197,080 km (intended 1,193,622±151,800), inclination 5.99 deg (intended 6.00±0.06)^[14]), on its way towards the second Lagrangian point.^[15][16][17]

On June 14, 2009, ESA successfully sent the command for the cryocover to open which will allow the PACS system to see the sky and transmit images in a few weeks. The lid had to remain closed until the telescope was well into space to prevent contamination. Herschel is reported to have completed 90% of the distance to its orbit 1.5 million km away from Earth.^[18]

Five days later the first set of test photos, depicting M51 Group, was published by ESA.^[19]

In mid-July 2009, approximately sixty days after launch, it entered a Lissajous orbit of 800,000 km average radius around the second Lagrangian point (L2) of the Earth-Sun system, 1.5 million kilometres from the Earth.^[17][20]

Operational mission

On 21 July 2009, Herschel commissioning was declared successful, allowing the start of the operational phase. A formal handover of the overall responsibility of Herschel was declared from the programme manager Thomas Passvogel to the mission manager Johannes Riedinger.^[17]

Discoveries

Herschel was instrumental in the discovery of an unknown and unexpected step in the star forming process. The initial confirmation and later verification via help from ground based telescopes of a vast hole of empty space, previously believed to be a dark nebula, in the area NGC 1999 shed new light in the way newly forming star regions discard the material which surrounds it.^[21]

On July 16, 2010, a special issue of Astronomy and Astrophysics was published with 152 papers on initial results from the observatory^{[22] [23]}.

See also

References

Further reading

• Harwit M. (2004). "The Herschel Mission". Advances in Space Research 34 (3): 568–572. doi:10.1016/j.asr.2003.03.026. 
• Thorsten Dambeck (May 2009). "One Launch, Two New Explorers: Planck Readies to Dissect the Big Bang". Sky and Telescope: pp. 24–28. 
• News related to ESA launches Herschel Space Observatory and Planck Satellite at Wikinews

External links

• ESA Herschel Science Centre
• ESA Information on Herschel
• NASA Herschel website
• ESA Herschel operations page
• ESA video on YouTube