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Look up

Comet 252P/LINEAR swings by Earth, much brighter than expected

The greenish glow of Comet 252P/LINEAR seen to the left of the Large Magellanic Cloud (Swan Hill, Victoria.March 19, 5am). M. Mattiazzo, CC BY-NC-ND

In a surprise to comet watchers, Comet 252P/LINEAR is rapidly brightening as it flies past Earth. The comet will make its closest approach around 12:15am on March 22 (AEDT), and it’ll be a close one for a comet, bringing it about 5.3 million km from Earth (or 14 times further away than the moon). It’s the fifth closest cometary approach on record.

But what’s causing a stir is that the comet is around 100 times brighter than expected. It’s even beginning to push the limits of being visible to the naked eye from a dark location. It won’t be a show-stopper but it’s certainly a lovely target for binoculars and astro-photographers.

A southern comet

The comet is perfect for viewing from the southern hemisphere being well placed in our southern sky. It can be found in the evening sky for the next few nights, and recently travelled past the Large Magellanic Cloud, which sits opposite the Southern Cross.

The one disappointment, is that the full moon occurs on Wednesday, March 23, so the faint comet will be battling against the bright moonlight.

The path of Comet 252P/LINEAR in the evening sky, around 10pm (AEDT) each night. Museum Victoria/Stellarium

Each evening the comet rises a little later and by the end of the month, best viewing will occur in the early hours of the morning. The comet is moving very quickly, so you might like to try making such image plots for yourself, personalising it for your location and observing time.

This can be done by using the free open source program Stellarium and following this handy guide for adding in comets.

The path of Comet 252P/LINEAR in the morning sky, around 4am (AEDT). Museum Victoria/Stellarium

As seen in photographs, the comet is a lovely shade of green. This is due to the release of diatomic carbon (C2), a gas that glows green when its molecules are ionised or excited. Ionisation causes electrons within the molecules to gain energy and when the electrons drop back to their normal state, they emit light of a particular wavelength. The gas is a very strong emitter, so its green colour dominates the comet.

A close-up of the comet’s eerie glow. (Swan Hill, Victoria. March 19, 5:30am). M. Mattiazzo

Chip off the old block

In fact this is not the only comet flying by Earth at this time. Comet P/2016 BA14 (Pan-STARRS) is hot on its heels.

The second comet, which is smaller and much fainter, will come even closer to Earth, flying by at about 3.5 million km (9 times as far as the moon) around 2:31am (AEDT) on Wednesday, March 23. It’s a telescope target for amateur astronomers in the Northern Hemisphere.

With orbits that similar it suggests that Comet P/2016 BA14, which is roughly half the size of Comet 252P/LINEAR, could well be a fragment that has broken away from the larger comet some time ago. Be sure to watch the movie above to get a great view of how the comets are travelling together through space and appear to pass so close to Earth.

Comet P/2016 BA was discovered in January this year and it was originally designated a potentially hazardous asteroid due to its close approach to Earth. But when researchers put two and two together, realising that an asteroid and a comet were due to fly by Earth at a similar time, it prompted follow up investigations.

Sure enough, the asteroid was found to have a tail – it wasn’t a dead chunk of rock but an active mix of dust and ice.

The asteroid has a tail! Michael S. P. Kelley and Silvia Protopapa (UMD)

The comets are so close, that astronomers are grabbing the oportunity to study them in more detail. Astronomer, Michael Kelley and collaborators have secured time with the Hubble Space Telescope to image Comet 252P/LINEAR during its close approach. While radar observations of Comet P/2016 BA14 will be made by NASA’s Deep Space Network.

Comet dust falling to Earth

There’s also the chance that a weak meteor shower, of perhaps 5 to 10 meteors per hour, could occur on March 28, as Earth crosses the comet’s orbit. The meteor’s radiant is expected to be within the constellation of Lepus, which sits above Orion in the western sky.

Meteors may emerge from the eye of Lepus, seen here at 9pm (AEDT). Museum Victoria/Stellarium

Comets leave a trail of dust in their wake, and what meteor modellers do, is model the potential dust ejected by the comet each time it passes by Earth and track how the dust spreads out over time within the vicinity of Earth’s orbit. For Comet P252/LINEAR, which orbits the sun every 5.33 years, the model traces the orbits of the comet since 1850.

Earth will not pass through the densest part of the dust trail, but it is expected to collided with a diffuse cloud of meteroids, dust that was ejected over the years 1894 through to 1926.

The meteor radiant can be seen from across Australia, in the western sky from sunset until midnight, local time.