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Shooting star, seen from above

ISS image of a meteor

The bright streak of a Perseid meteor as it flashes into Earth's upper atmosphere. The image was snapped by an astronaut aboard the International Space Station.

THIS ASTRONAUT PHOTOGRAPH, taken from the International Space Station (ISS) while over China (approximately 400 kilometres to the northwest of Beijing), provides the unusual perspective of looking down on a meteor as it passed through the atmosphere.

Many people have spent time outdoors under a dark sky, watching for “shooting stars” to streak across the firmament. In some cultures, this event is an occasion to make a wish; in others it is viewed as a herald of important events, such as the birth of a future ruler.

While not actual stars, “shooting stars” do come from outer space, in the form of meteoroids entering the Earth’s atmosphere.

Meteor or meteorite?

Meteoroids are small objects moving through the Solar System that are attracted to the Earth by its gravitational pull.

These small objects—typically fragments of asteroids or comets, though they can also originate from the Moon or Mars—begin to heat and burn up as they collide with air molecules in Earth’s atmosphere, creating a bright vapour trail or streak.

At this point, the object is known as a meteor. If any remnant of the object survives to impact the Earth’s surface, it becomes known as a meteorite.

While most meteorites are natural in origin, on occasion manmade space debris can re-enter the atmosphere and also become a meteor or even a meteorite!

Comes from a comet

The image was taken on August 13, 2011, during the Perseid Meteor Shower that occurs every August. The Perseid meteors are particles that originate from Comet Swift-Tuttle; the comet’s orbit is close enough for these particles to be swept up by the Earth’s gravitational field every year—leading to one of the most dependable meteor shower displays.

Green and yellow airglow appears in thin layers above the limb of the Earth, extending from image left to the upper right. Atoms and molecules above 50 kilometres in the atmosphere are excited by sunlight during the day, and then release this energy at night, producing primarily green light that is observable from orbit.

Part of a ISS solar panel is visible at upper right; behind the panel.

Astronaut photograph provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Centre. Text adapted from information issued by William L. Stefanov, Jacobs/ESCG at NASA-JSC.

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Fire in the sky

  • Perseid meteor shower to put on a show
  • Best times to see it, August 12-13

The Perseids meteor shower is due to put on a sky show for Northern Hemisphere stargazers over the next couple of nights.

The best time to see the Perseids will be from the hours of 10pm to dawn, on the nights of August 12 and 13.

Adding to the spectacle will be the sight of the planets Venus, Mars and Saturn, along with a crescent Moon, in the western sky. These four will have dipped below the horizon by the time the meteor shower begins.

A meteor is the flash of light seen in the sky when a meteoroid—which in size can be anything from smaller than a grain of sand, all the way up to metres across—enters the Earth’s atmosphere.

The tremendous speed of entry means that the thin air gets compressed around the meteoroid, and briefly glows. The meteoroid itself usually disintegrates during the process.

If a meteoroid is large enough and solid enough, it might—or pieces of it might—survive and fall to the ground. When this happens, those pieces are known as meteorites.

Most meteoroids, particularly those involved in showers like the Perseids, are tiny in size.

Sky diagram showing the Perseid meteor shower

To see the Perseids, look to the western sky between 10pm and dawn, August 12 and 13, 2010.

Meter showers occur when the Earth’s orbit intersects the dusty trail left behind by a comet.

To avoid disappointment, it’s important to know, however, that meteor showers do not look like continuous fireworks displays. Most showers, including the Perseids, have a maximum of tens of meteors per hour—so working that out, you might see one every couple of minutes.

The Perseids will appear to “radiate” from one region of the sky, namely, the constellation Perseus. Perseus is a far northern constellation, which means that the best views of the shower are to be had by those who live in the Northern Hemisphere.

Southern Hemisphere stargazers are not favoured by the Perseids. Perhaps a handful of particular strong meteors might be seen zipping over the northern horizon, but generally they will not be visible.

Story by Jonathan Nally, editor Video courtesy NASA. Images by Pete Lawrence of Selsey, UK, and NASA.

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