Thursday, 4 November 2010


A map of water in Comet Hartley 2, observed by Odin on 29 October
2010. Copyright 2010 Swedish Space Corporation/Centre National
d’Etudes Spatiales/Observatoire de Paris.

The Odin satellite observed Comet Hartley 2 almost continuously from 29 October to 1 November. The water signature (line) was easily detected. Its extension and space distribution is shown on a map (Fig. 1). The production of water derived from the observations ranges from 180 to 300 kg (400 to 660 lb) per second.

This production of water (Fig. 2) is rapidly varying with time. This is in line with variations reported from other means of observation. It may be related to the rotation of the comet's nucleus, for which periods around 17h have been reported.

Comet 103P/Hartley 2 is a Jupiter-family comet orbiting the Sun close to the ecliptic plane, with a period of 6.5 years. Its return this year is exceptional. It passed perihelion on 28 October at 1.059 AU from the Sun (158 million km; 98 million miles) and on 20 October, it came close to the Earth, at only 0.121 AU (18 million km; 11 million miles). Furthermore, it is the target of NASA's mission EPOXI, which is to fly by the comet on 4 November.

At this occasion, this comet is the object of an intense, international campaign of observation which mobilizes all major astronomical resources, including the Herschel Space Observatory. The Odin satellite is participating to this campaign.

The Odin satellite is a small spacecraft, orbiting the Earth, designed and built by Sweden, in collaboration with Canada, Finland and France. It was launched in February 2001. Aimed for studying both the Universe (astronomy) and the terrestrial atmosphere (aeronomy), it allows for the observation of a number of molecular lines, at radio (submillimeter range) wavelengths, otherwise not accessible from the ground: in particular the fundamental water line at 556.9 GHz.

Odin is thus well suited for the study of water, the main constituent of cometary ices, released as water vapor following heating of cometary nuclei by the Sun.

Since its launch, Odin has observed about 15 comets. Now closed for astronomical observations, the Odin satellite concentrates on aeronomical studies, except for special occasions such as the passage of Comet Hartley 2.

Figure 2:

The evolution of the production of water in Comet Hartley 2, as observed with Odin. Copyright 2010 Swedish Space Corporation/Centre National d'Etudes Spatiales/Observatoire de Paris.

Reference:International Astronomical Union electronic telegram No. 2524.

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