This stunning photo was taken by astronaut Dan Burbank as the ISS passed over Australia at 17:40 GMT on December 21, 2011. It was early morning over Australia at the time, and you can see the dark limb of the Earth, the thin green line of airglow (atoms in the upper atmosphere slowly releasing the energy they accumulated over the day), some southern hemisphere stars… and of course, the incredible, ethereal, other-worldly beauty of Comet Lovejoy, its tails sweeping majestically into the sky.
First, the comet was discovered by amateur astronomer Terry Lovejoy in November. It turned out to be a sungrazer, a comet whose orbit plunges it deep into the inner solar system and very close to the Sun’s surface. It screamed past our star last week, on December 15/16, and, amazingly,survived the encounter. Some sungrazers do and some don’t, but Lovejoy is bigger than usual for such a comet, and that may have helped it remain intact as it passed less than 200,000 km over the Sun’s inferno-like surface.
Now the comet is moving back out, away from the Sun and back to the frozen depths of deep space. But the Sun’s heat, even from its greater distance now, is not to be denied. Comets are composed of rock and ice — the ice being what we normally think of as liquid or gas, like ammonia, carbon dioxide, and even good ol’ water. The heat from the Sun turns that ice directly into a gas (in a process called sublimation), which expands around the solid nucleus of the comet, forming what’s called the coma. Pressure from sunlight as well as the solar wind blows this material away from the comet head, resulting in the lovely tail, which can sweep back for millions of kilometers.
Image credit: NASA/Dan Burbank c/o Fragile Oasis