Crescent comet

This European Space Agency image was the Astronomy Picture of the Day (APOD) last week (April 29, 2015). Isn’t it beautiful? It’s a crescent comet, in this case Comet 67P/Churyumov–Gerasimenko. We could not see the comet in this way if it weren’t for the fact that the Rosetta spacecraft has been orbiting it since last July. The spacecraft will follow the comet through its August, 2015 perihelion, or closest point to the sun. APOD wrote:

As the 3-km wide comet moves closer to the sun, heat causes the nucleus to expel gas and dust. The Rosetta spacecraft arrived at the comet’s craggily double nucleus last July and now is co-orbiting the sun with the giant dark iceberg. Recent analysis of data beamed back to Earth from the robotic Rosetta spacecraft has shown that water being expelled by 67P has a significant difference with water on Earth, indicating that Earth’s water could not have originated from ancient collisions with comets like 67P. Additionally, neither Rosetta nor its Philae lander detected a magnetic field around the comet nucleus, indicating that magnetism might have been unimportant in the evolution of the early solar system. Comet 67P, shown in a crescent phase in false color, should increase its evaporation rate as it nears its closest approach to the sun in August, 2015, when it reaches a sun distance just a bit further out than the Earth.