The HIFI instrument onboard ESA's Herschel observatory has been successfully switched on, after being inactive for more than 160 days due to an unexpected anomaly in the electronic system. This achievement brings Herschel back to its full observing capacity.

HIFI was built specifically to observe water in a variety of celestial objects. Over the next three years HIFI spectra will be used to probe stellar environments and to study the role of gas and dust in the formation of stars and planets and the evolution of galaxies.

The quality of the first spectra obtain with HIFI on 22 June 2009 effectively demonstrated the versatility of the instrument and showed that its accuracy and resolution exceeded specifications.

On 3 August 2009, early in the performance verification phase, it was discovered that HIFI was no longer operating normally. This was later traced to an unexpected voltage peak in the electronics of HIFI's Local Oscillator Control Unit (LCU) having shut it down.

In case of such events HIFI has a redundant set of electronics including the LCU which provides full functionality, however before it was utilised, ESA and the HIFI consortium wanted to fully understand the cause of the problem.

Following the shut down of HIFI, the consortium, joined by ESA, carried out a detailed and thorough investigation before making the necessary adaptation to software that monitors the LCU to prevent the problem from happening again.

All tests have confirmed that the LCU back-up system is functioning perfectly after switch-on last week. In addition, the HIFI sensors are performing with the same high quality as they did when they captured the spectra of the warm molecular gas in the DR21 star-forming region in Cygnus.

This spectra showed clear evidence for water, as well as for singly ionized carbon (a key diagnostic for molecular cloud material), and carbon monoxide.

HIFI was built specifically to observe water in a number of different celestial targets as part of the investigation into the role of gas and dust in the formation of stars and planets, and the evolution of galaxies. The successful activation of HIFI comes as an important target, the star forming regions of the Orion Nebula or M42, approaches the instruments field of view.

HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the USA under the leadership of the Netherlands Institute for Space Research (SRON), Groningen, The Netherlands, and with major contributions from Germany, France and the USA. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: OAN(IGN), CAB(CSIC-INTA); Sweden: Chalmers University of Technology - MC2, RSS and GARD; OSO; SNSB, Stockholm University - SO; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC.