Thank you and goodbye to the Herschel Observatory

2013 November 29

Richard Miles

In 2009, the European Space Agency launched the Herschel Space Observatory, the most advanced far-infrared space telescope to date. This ambitious project successfully placed the observatory with its 3.5m diameter main mirror (the largest yet to go into space) near the so-called second Lagrangian (L2) point: a region of space some 1.5 million km behind the Earth in its orbit as seen from the Sun, where the gravitational forces exerted by the Sun, Earth and Moon roughly balance each other out. In this semi-stable orbit, Herschel dutifully carried out its observing programme, exceeding its planned mission life by several months until 2013 April 29 when its liquid helium coolant finally evaporated.

If Herschel were left near the L2 point, its orbit would be unstable and it would gradually wander off into a somewhat chaotic orbit that could present dangers to future spacecraft. ESA managers therefore decided to use most of its remaining fuel to inject it into an orbit around the Sun where it would not encounter Earth for many centuries. On June 17, Herschel’s thrusters were activated and a burn was completed putting it into a different heliocentric orbit. Any remaining unused fuel was then jettisoned and the onboard computer instructed to cease communications. Unfortunately, not enough time was left for the ESA ground station to make accurate measurements of the new orbit.

Orbital mechanics expert Bill Gray realised that the spacecraft would effectively be lost, so he alerted observers pointing out the need for follow-up astrometry via a message on the Minor Planet Mailing List. In response, eight astrometric positions were forthcoming for June 26 to July 1, the last measures being obtained by Ernesto Guido & Nick Howes using the remotely operated 2.0m Faulkes Telescope South (FTS) at Siding Spring, Australia. A new orbit was determined based on an observing arc of only 14 days, and an ephemeris was made available via JPL Horizons. However the then uncertainty in Herschel’s orbit was so large that at its next return in 2027, it was likely to be situated many tens of degrees away from its predicted position and difficult to find given it would be very faint, around magnitude 22 or 23 at that time.

Bill Gray sent out another alert to the community of observers on July 16 asking ‘Could somebody get another night on it before it vanishes into Outer Darkness?’. Unfortunately no-one was able to achieve any further follow-up observations and that seemed to be that.

Two months later, Herschel project astronomer Mark Kidger gave a fascinating talk on the subject of ‘Living the Herschel Mission Dream’ at the BAA’s Autumn Weekend meeting held in Bristol on September 6–8 2013, during which he made an urgent appeal for someone to try to recover Herschel and report astrometry of it. This author was in the audience at the time and made a note to try and do just that.

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