Comet Hartley 2

In the period 24 October to 17 November 2010 Herschel used its complement of state-of-the-art instruments to observe the far-infrared and submillimetre spectrum and to image the thermal dust radiation of comet 103P/Hartley 2, covering the 55-671 μm spectral range. Herschel is taking part in an extraordinary worldwide astronomical campaign to study comet Hartley 2 before, during, and after the flyby the NASA's Deep Impact spacecraft on 4 November 2010 as part of its EPOXI mission.

This return of comet Hartley 2 in 2010 was exceptional. The comet was at "perihelion", meaning closest to the Sun, on 28th October and passed just 18 million km from the Earth on 20th October, making this closest approach to the Earth since its discovery in 1986 and by far its closest approach in the next century. Comet Hartley 2 passed 16.4 million km from Herschel on October 20, providing an opportunity to acquire sensitive measurements, complementary to the observations from EPOXI and other telescopes and spacecraft. The early Herschel observations of comet Hartley 2 demonstrated the observatory’s unique capabilities: sensitive far-infrared images constrained the size of the large dust particles, and spectra reveal the distribution of water molecules released from the nucleus as the ices evaporate when the comet approaches the Sun.  From these measurements, it is calculated that the comet is losing about 230 kg of water every second. The emission from water is constrained using observations of high-excitation rotational water transitions using the three Herschel instruments: HIFI, SPIRE and PACS. All three instruments detect the presence of water, and HIFI is sensitive enough to measure its velocity. PACS also provides images of the dust and water emission at the time of the EPOXI flyby.

Comet Hartley2 as seen by Herschel's three instruments. In all cases, the sun is down and to the right, though the images were taken on different days (24th, 25th and 30th October for SPIRE, PACS and HIFI respectively).  In the HIFI image, the superimposed line shows the emission by water at a wavelength of 538 microns.  Larger versions of these image are available (PACS, SPIRE, HIFI).  Image credit: ESA / SPIRE / PACS / HIFI / HSSO