Thursday, July 23, 2015

WHT Observes Pluto in Support of NASA's New Horizons Mission

The William Herschel Telescope (WHT) has participated in 2014 and 2015 in a worldwide campaign to spectroscopically follow up Pluto from the ground in support of the encounter of NASA's New Horizons spacecraft with Pluto.

Constant monitoring of the surface of Pluto is necessary because it is known to be spectrally and photometrically variable from season to season, and probably during the whole secular calendar. By gathering data at different wavelengths astronomers are able to characterize the distribution of the materials which make up the surface and atmosphere in different ways, from the layers of volatile ices (bright, whitish areas made up of methane, nitrogen, and carbon monoxide) to the more complex organic residues, which are reddish.

Last year Pluto was already observed for six nights using the WHT. The spectra, obtained using ACAM and planned as a series of overrides, showed two principal characteristics of the surface of Pluto, the clearest being the absorption bands due to methane ice. The second characteristic is the continuum slope of the spectrum which is an indicator of the colour of the surface. This colouring agent is not uniformly distributed over Pluto's surface, but changes significantly during its rotation period, which is 6.4 Earth days. 

Images of Pluto taken from the New Horizons probe. Below, spectra from the observing campaign at the WHT in 2014. The difference between the two spectra indicates differences in the composition of the surface of the planet. The spectrum printed in yellow (dark zone) has a larger slope, which is associated with the presence of very dark complexes of organic materials, which seem to be abundant in the dark region to the left of the map. The spectrum printed in red (bright zone) has somewhat deeper absorption bands, which indicate that there is more methane ice in the bright heart-shaped zone. Credits: NASA-JHUAPL-SWRI & ORM team [ JPG ].

This year, the observations were planned in a similar way and for a period of 11 nights, from 3rd to 14th July, coinciding with the closest approach of New Horizons spacecraft with Pluto. The new spectra will provide an important independent calibration of the MVIC (Multispectral Visible Imaging Camera on board New Horizons).

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