Astronomy Picture of the Day 03-05-04

Explanation: Variable star V838 Monocerotis lies near the edge of our Milky Way Galaxy, about 20,000 light-years from the Sun. Still, ever since a sudden outburst was detected in January 2002, this enigmatic star has taken the center of an astronomical stage while researchers try to understand where it fits into the picture of stellar evolution. As light from the stellar flash echoes across pre-existing dust shells around V838 Mon, its appearance changes dramatically. Revealed in a sharp snapshot recorded in February by the Hubble's Advanced Camera for Surveys, this portion of the dust shell is about six light-years in diameter. But because light reflected from the dust follows only a slightly indirect path compared to the direct line-of-sight to the star, the light echoes visible now are only lagging about two years behind the outburst itself. Astronomers expect the expanding echoes to continue to light up the dusty environs of V838 Mon for at least the rest of the current decade.

Rosetta's comet target seen from Earth telescope
EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE
Posted: March 3, 2004

In the morning of March 2, the Rosetta spacecraft was launched on board an Ariane 5 launcher from the European Spaceport in Kourou, French Guiana. The European Space Agency (ESA) spacecraft will be the first to land on a comet.

Before the launch, and as a salute to their colleagues at ESA, astronomers used the New Technology Telescope at the European Southern Observatory (ESO) of La Silla in Chile to image Rosetta's target, Comet 67P/Churyumov-Gerasimenko, an approximately 4 kilometre size "dirty snowball" that orbits the Sun once every 6.6 years.

A composite image of the nucleus of Comet 67P/Churyumov-Gerasimenko (the point of light at the center), recorded on February 26, 2004, at 6:15 UT with the SUSI-2 camera on the 3.5-m New Technology Telescope. It is based on fifteen series of exposures seen in three different wavebands and since the images were aligned on the comet, the images of stars in the field are trailed. The fact that the image of the comet's 'dirty snowball' nucleus is almost star-like indicates that it is surrounded by a very small amount of gas or dust. The distance to the comet from the Earth was approx. 600 million km. Credit: ESO

These new images show the object at a distance of approximately 670 million kilometres from the Sun - 4.5 times the distance from the Earth to the Sun.

These observations provide further confirmation that at this distance the activity on 67P/Churyumov-Gerasimenko is very low.

This is very good news for the mission, because it means that when Rosetta will meet in 2014 its target comet, at 790 million kilometres from the Sun, there will not be so much dust near the nucleus to hinder the landing.

Originally timed to begin about a year ago, Rosetta's journey had to be postponed. This delay meant that the original mission's target, Comet Wirtanen, which was observed two years ago by astronomers using ESO's Very Large Telescope could no longer be reached. Instead, a new target has been selected, Comet Churyumov-Gerasimenko.

The observations presented here are part of a continuous effort by astronomers to monitor Rosetta's target and provide the spacecraft controllers and the astronomers with very useful, regular updates, e.g., about the 'cometary weather' at the time of arrival.

The exposures have been combined to show the background objects in real colours. Because of its motion, the comet now appears as a trail. Credit: ESO

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