Scientists of SRON and ESA are now closing in on the cause of the problem with the high resolution spectrometer HIFI on board ESA’s Herschel Space observatory. A series of events which is currently being investigated has most probably led to an overload in some components in a DC/DC converter. This caused a malfunction in the so-called Local Oscillator Control Unit (LCU). The back up system will be switched on when SRON and ESA are confident that all conceivable measures have been taken to prevent this chain of events from happening again. HIFI will then resume the search for the presence of water in remote parts of the universe.
After Herschel was launched on 14 May HIFI functioned perfectly for almost three months. A wealth of highly promising scientific data was obtained during the commissioning phase, until August 3d, when an anomaly occurred in the Local Oscillator Control Unit (LCU), a control module for the self-generated local oscillator signal (see below). HIFI was temporarily switched off to determine the exact cause of the problem. But as the "black box" of the module failed also, this proved difficult and time-consuming. Extensive tests by HIFI and ESA teams – in which scenarios are tested that could reproduce the chain of events – are now underpinning the above mentioned scenario.
HIFI will be fully operational again when the back up system is switched on. However, this will only be done when the most benign environment to the electronics involved (temperature conditioning, preventing radical state changes etc.) is firmly established. This should insure a satisfactory operating lifetime of the back up system. Scientists are also working on a scheme (switching on, re-commission of the redundant branch, and then utilising HIFI in a prioritized science programme before going into routine operations) in order to maximise the scientific impact of the instrument.
Back up system
Dr. Frank Helmich, principal investigator for HIFI: “Although I expect the back up system to function during the complete lifetime of the Herschel mission, a good tradition in space science is that you take every possible risk very seriously. In the case of HIFI-LCU we are going through a very thorough test program on the ground before switch-on, aimed at reproducing the chain of events that led to the malfunction and to test any measure applicable. Furthermore, the science observations are rescheduled in such a way that the most crucial observations will be done as early as possible after the switch-on."
Herschel is still mostly in the performance verification phase, although the first science observations of the two other instruments have been done. Currently switch-on is envisaged over one month from now. HIFI – the Heterodyne Measuring instrument for the Far Infrared – is investigating the composition of interstellar gas clouds and measures, for example, how much carbon and water these contain. The wealth of detail HIFI is expected to deliver will tell us more about the birth and early development of stars and planets. HIFI shall also make measurements of the atmosphere of planets and comets in our solar system.
Mixed signal
The far-infrared light observed by HIFI has a very high frequency, and is very hard to detect and process. Therefore in HIFI the sky signal (terahertz radiation) is mixed with the local oscillator signal that is generated within HIFI . The mixed signal has a much lower frequency but still contains all scientific information and can then be easily processed. HIFI was designed and built by a nationally-funded consortium led by SRON Netherlands Institute for Space Research. The consortium includes institutes from France, Germany, USA, Canada, Ireland, Italy, Poland, Spain, Sweden, Switzerland and Taiwan."
