Aussie amateur spots Mercury’s tail

The planet Mercury

The planet Mercury. Astronomers have known that the planet has a tenuous gas tail stretching into space, but now an Australian amateur astronomer has spotted it in data from a Sun-monitoring satellite.

Scientists from Boston University’s Centre for Space Physics reported today that NASA satellites designed to view the escaping atmosphere of the Sun have also recorded evidence of escaping gas from the planet Mercury.

The STEREO mission has two satellites placed in the same orbit around the Sun that the Earth has, but at locations ahead and behind it. This configuration offers multi-directional views of the electrons and ions that make up the escaping solar wind.

Movie of Mercury's tail

This movie covers a 4-day period and shows Mercury's tail as seen by the STEREO A spacecraft on 6-9 February 2008. The faint tail can be seen streaming off to the top left of the planet.

On occasion, the planet Mercury appears in the field of view of one or both satellites. In addition to its appearance as a bright disc of reflected sunlight, a ‘tail’ of emission can be seen in some of the images.

Of special interest is the way the tail feature was spotted in the STEREO data. It was not found by the Boston University team, but by Dr Ian Musgrave, a medical researcher in Adelaide, South Australia, who has a strong interest in astronomy.

Viewing the on-line database of STEREO images and movies, Dr Musgrave recognised the tail and sent news of it to Boston asking the BU team to compare it with their observations.

“A joint study was started and now we have found several cases, with detections by both STEREO satellites,” explained Jeffrey Baumgardner, Senior Research Associate in the Centre for Space Physics at Boston University, and the designer of the optical instruments that discovered the exceptionally long sodium tail.

Schematic representation of the viewing geometry that allows the STEREO camera systems to make observations of Mercury’s tail

One of the twin STEREO spacecraft was in the right position to see the tail sweeping away from Mercury in a direction opposite to the Sun.

Tail blown by sunlight

This new method of observing Mercury and efforts to try to understand the nature of the gases that might make up this tail feature were topics presented at the European Planetary Science Congress meeting in Rome today.

It has been known that Mercury exhibits comet-like features, with a coma, or cloud, of tenuous gas surrounding the planet and a very long tail extending in the anti-sunward direction.

From Earth, observations of both of these features can be done using light from sodium gas sputtered off the surface of Mercury. The Sun’s radiation pressure then pushes many of the sodium atoms in the direction away from the Sun, creating a tail that extends many hundreds of times the physical size of Mercury.

“We have observed this extended sodium tail to great distances using our telescope at the McDonald Observatory in Texas,” Boston University graduate student Carl Schmidt explained, “and now the tail can also be seen from satellites near Earth.”

Much closer to Mercury, several smaller tails composed of other gases, both neutral and ionised, have been found by NASA’s MESSENGER satellite as it flew by Mercury in its long approach to entering into a stable orbit there.

Adapted from information issued by Europlanet / Boston University / NASA.

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