Rosetta: Extra-terrestrial Observation

Full-frame NAVCAM image taken on 9 August 2014 from a distance of about 99 km from comet 67P/Churyumov-Gerasimenko. Image: ESA/Rosetta/NAVCAM

Full-frame NAVCAM image taken on 9 August 2014 from a distance of about 99 km from comet 67P/Churyumov-Gerasimenko. Image: ESA/Rosetta/NAVCAM

Most people will have seen last week’s news about ESA’s Rosetta spacecraft arriving at comet 67P/Churyumov-Gerasimenko and the animated images of the ‘rubber-duck’ shaped object taken from Navigation Camera (NavCam), part of Rosetta’s Attitude and Orbital Control System. The arrival generated many headlines, from the 10 years it took to catch the comet, through the history making first rendezvous and comet orbit, to the final part of the mission and the intention to land on the comet. However there was little detail about the remote sensing aspect of the mission, which we feel is a missed opportunity as it’s using many of the techniques and methodologies employed in Earth observation (EO).

The orbiter part of Rosetta carries eleven different remote sensing experiments with a wide variety of sensors gathering data about the comet before the lander touches down. Amongst the instruments on-board are three separate spectrometers; a visible and infrared thermal imaging spectrometer (VIRTIS) focussing on temperature and geography; an ultraviolet imaging spectrometer (ALICE) looking at gases and the production of water and carbon dioxide/monoxide; and finally ROSINA has sensors for measuring the composition of the comet’s atmosphere and ionosphere.

The VIRTIS instrument has two channels; the VIRTIS-H channel is a high spectral resolution mapper operating from 2 to 5µm, whereas the VIRTIS-M is the mapper operates at a coarser spectral resolution and one of its main products will be a global spectral map of the comet’s nucleus. This instrument has already been used to undertake measurements of Earth. In November 2009, on Rosetta’s third Earth fly-by, VIRTIS measurements were compared to existing EO instruments from ENVISAT/AATSR, SCIAMACHY and MODIS. Overall, there was a strong correlation with the EO data, but differences were also seen – especially in the 1.4µm water absorption feature.

VIRTIS has a key role in supporting the selection of the November’ landing site, a task that has become more difficult now the comet has been imaged in detail and is seen to have a complex shape. In addition, recent VIRTIS measurements have shown the comet’s average surface temperature to be around minus seventy degrees centigrade, which means the comet is likely to be too warm to be ice covered and instead must have a dark, dusty crust.

Remote sensing is playing a huge part in the Rosetta mission and it should be celebrated that these instruments will gather data over the next eighteen months to help scientists determine the role comets play in the evolution of planets. It will be amazing if remote sensing techniques developed to explore, monitor and analyse our planet, will be the same techniques that help determine if the water on Earth originally came from comets.