The Herschel Space Observatory – Lost in Space?

Thanks to observations made this September using the 2.0m Faulkes Telescope South (FTS) in Australia and 1.0m LCOGT-B scope in South Africa, the heliocentric orbit of this far-infrared observatory is now known with sufficient accuracy that at its next pass of the Earth in 2027 its position in the sky should be known to within a couple of degrees. This should allow it to be easily recovered, despite it being extremely faint (around 22nd magnitude – just about the absolute limit of the FTs!)  at that time.

ESA’s Herschel observatory was originally placed at the L2 region of space, some 1.5 million km from Earth, from where it carried out its science programme during 2009-2013 after which time all of its liquid helium coolant had been used up and its mission very successfully accomplished. Orbits in the L2 region are unstable and ESA decided to place Herschel on a known trajectory in orbit around the Sun where it would not encounter the Earth for many centuries. On June 17th a burn was carried out, its fuel tanks were then depleted and the onboard computer instructed to cease communications. Unfortunately, not enough time was left for the ground station to make accurate measurements of the new heliocentric orbit.

Orbital mechanics expert Bill Gray alerted observers to the need for astrometry but, in response, positions were forthcoming only during the interval from June 26th to July 1st; the last measures being obtained by Ernesto Guido and Nick Howes using the FTS. A new orbit was determined based on an arc some 14 days in length and an ephemeris was made available via JPL HORIZONS. The problem however was that the uncertainty in the exact nature of the orbit was large such that at its next return in 2027, it could be many tens of degrees away from its predicted position. An article posted here on ESA’s blog mistakenly gave the impression that a precise orbit had been determined and all was well.

 Bill Gray sent out another alert to the community of observers on July 16th asking “Could somebody get another night on it before it vanishes into Outer Darkness”. As it happened, Herschel project astronomer Mark Kidger gave a talk on the subject of “Living the Herschel Mission Dream” at a British Astronomical Association meeting in Bristol on September 7th during which he made an appeal for someone to try to recover Herschel and report astrometry of it.

Richard Miles was in the audience at the time and made a note to try and do just that. Two hours of telescope time on the 9th were fruitless owing to cloud at Siding Spring. The next night (10th) was clear and the spacecraft was detected as a 23rd magnitude object 0.7 arcminutes from its expected position; this is equivalent to a 2 m/s difference in velocity from that expected following the final burn. Aided by the revised orbit solution, Tim Lister used the 1.0-m LCOGT-B scope at Sutherland to successfully image Herschel and report astrometry the next night (11th). Two weeks later on the 24th, Richard Miles devoted 2 hours of telescope time on the FTS under clear, moonless conditions to capture the rapidly fading Herschel for one last time. By then it was almost 10 million km distant and 24th magnitude but was just detectable in sets of 17 x 150s exposures. Here’s an image depicting Herschel at that time:

FTS images of Herschel

Note the burnt-out images of two stars on the right of the picture. These 9th-10th magnitude stars are about 1 million times brighter than the spacecraft and almost swamped the feeble amount of sunlight reflected from Herschel in their glare! Astrometric positions were reported and these and other measures are available here on the Web along with details of Herschel’s new heliocentric orbit.

 Richard Miles

British Astronomical Association