Science & Technology

Comets Seen From Space Are Spellbinding

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In between doing awesome science experiments, astronaut crews in space keep an eye on the sky. And over the decades, they've captured some celestial visitors: comets! These collections of ice and dust shine brightly in several photos on a NASA Flickr page. It's an amazing reminder that our universe is a big place that's constantly changing.

What's a Comet?

A comet is a tiny world packed with ices — often including water, carbon dioxide, carbon monoxide, and methane. Comets originate in a region of the solar system known as the Oort Cloud: an area of icy objects beyond the orbit of Neptune. From time to time, a disturbance of some sort — maybe a star passing a little close to our solar system — knocks a few of these comets into orbits that head for the sun.

That's when some magic happens. As the comet approaches the sun, the ice begins to melt due to the sun's heat and pressure from particles streaming off the sun's surface. The ice melts off the comet in a spectacular way, creating a tail — sometimes two tails. The tricky thing, however, is that it's hard to predict how much ice will melt and how bright the comet will be. Many astronomers still remember Comet ISON in 2013, which was supposed to be one of the brightest comets in decades. The comet instead ended up falling apart after it swept around the sun.

So astronomers, where possible, try to do close-up observations of comets to learn more about them. A spectacular example was the European Rosetta mission, which visited Comet 67P/Churyumov–Gerasimenko between 2014 and 2016. 67P swept closer to the sun as Rosetta launched. Rosetta took thousands of pictures of the surface as the comet changed from a dormant world to an active one. It also deposited a lander, called Philae, on to the surface.

Scientists are still analyzing the results today, but we have learned some things, such as that 67P's water has different ratios of hydrogen isotopes (element types) than the water on Earth. This implies that 67P-like comets were not a source of Earth's water.

What the Pictures Show

The pictures on the NASA Flickr page show several comets that astronauts photographed over the years. The oldest in the set is Comet Kohoutek, which astronauts photographed on the old Skylab space station in 1973. It was an early indication that astronauts in space stations could do useful astronomy — in between their other duties, of course. Today, it's routine for crews on the International Space Station to turn their eyes to the sky.

S73-38731 (December 1973): Photograph taken of the Comet Kohoutek from the Skylab space station in Earth orbit by a Skylab 4 crew member.

A couple of decades later, astronauts on a space shuttle photographed the spectacular Comet Hale-Bopp, which passed by Earth in 1996. The comet was visible for several weeks from the Northern Hemisphere with even the naked eye and received a lot of press coverage. (Too bad there was no social media back then, and digital cameras were in their infancy!)

STS083-507-023 (4-8 April 1997): A 35mm camera was used to record this time-exposed image of Comet Hale-Bopp at sunset. As a spin-off of the more lengthy time exposure, city lights and petroleum fires are seen as distorted streaks.

Another spectacular cometary example is Comet Lovejoy, which the Expedition 30 ISS crew photographed in 2011. This comet was best visible from the Southern Hemisphere, especially in regions like Australia. And if you peer closely, in this 2013 image, you can see Comet ISON before it broke up.

ISS030-E-014406 (21 Dec. 2011): Comet Lovejoy is visible near Earth’s horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 21, 2011.

Comets are interesting worlds to study because they show us what our solar system used to look like before the planets and moons were all formed. In a way, studying these little worlds is like a cosmic archaeology expedition. Today, there are two missions that are examining asteroids — a NASA mission called OSIRIS-REx and a Japanese mission called Hayabusa2. Both missions aim to bring back pieces of these space rocks to Earth, so we can learn even more about our ancient history.

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Written by Elizabeth Howell June 5, 2019

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