Friday, April 22, 2011

Comet Elenin: the FAQ for the worried.

Orbit of Comet C/2010 X1 Elenin (red) visualised with Celestia, showing Elenin at closest approach to Earth (click to embiggen)

Breaking News: Comet Elenin appears to be disintegrating. Yep, it's disintegrating. As of October 15 no confirmed sighting of the comet has taken place.

Will the remnants of comet Elenin hit the Earth? No, they will come nowhere near us.

Will it Hit Earth: No, its closest approach is 0.23 AU on Oct 16, 2011, where 1 AU is the distance from the Earth to the Sun. To put this in perspective, this is only a little closer than the closest approach of Venus to Earth, and roughly 100 times the distance from the Earth to the Moon. This distance is from the latest MPEC ephemeris which is based on over 100 observations from multiple observatories that have been continuously tracking the comet, so it won't change appreciably.

But its Earth MOID is only 0.03 AU! MOID stands for Minimal Orbital Interaction Distance. It is how close the orbits are together, not how close the objects are themselves. It's important for objects that have multiple return orbits, like potential asteroid impactors, but not for a one-off visitor like Elenin (well, you could wait for another 10,000 years or so). Earth and Elenin themselves never get closer than 100 times the distance from the Earth to the Moon.

Surely if Elenin Was Going to Hit the Earth NASA/the Government Would Hush it Up? Which government? The Australian Government, the UK Government? The Italian Government? The South African Government? Amateur astronomers world wide are following this comet and continually talking to each other. The have the programs to work out where the comet is going. If the comet was coming anywhere near us, the amateur community would be first to know, and there is no keeping them quite. Consider how wide spread the information is about Apophis, which is a real, if extremely marginal, hazard.

Will it Cause Earthquakes, Abnormally High Tides or Other Disasters: No, Elenin is a mere 3-5 kilometres across and has less than a billionth of the tidal force of the Moon at closest approach (as well as a negligible magnetic field). If the Moon can't cause the poles to tip, cause massive tidal floods or earthquakes, Comet 2010 X1 Elenin won't. We've been closer to other comets before with no ill effect.

But What About Mensur Omerbashich's Paper that Says Elenin is Causing Earthquakes? It shows nothing of the sort, earthquakes are no more common during comet alignments than at any other time (see here for an update, earthquakes still not caused by Elenin).

But it's bigger than Jupiter! No, that's the coma , the thin haze of gas and dust that surrounds the comet nucleus. The nucleus of C/2010 X1 Elenin is roughly 3-4 Km in diameter and Elenin has a coma around 50,000 km wide at the time of writing (which is a third of the diameter of Jupiter). The average density of the coma is about the same as the density of the atmosphere on the Moon. A coma is a feature of all comets that approach the Sun closely, for example comet 81P Wild (nucleus 4 Km diameter) had a coma of 50,000 Km and 103P Hartley had a coma of 150,000 Km. The Great Comet of 1811 had a nucleus of around 30km in diameter and had a coma nearly as big as the Sun. Comet Halley is 6x15 km and had a coma 100,000 km wide when it last approached Earth. We survived them all (and 103P Hartley came nearly twice as close as Elenin will), and we will survive Elenin without incident.

The size of Elenin has been independently confirmed (well, it's probably smaller), by observation of the orbit of the small asteroid 1999 RQ176 when it had a close encounter with Elenin.

But I Can See a Picture of it in WikiSky, it's HUGE! That is the carbon star CW Leonis.

Didn't it Just Cause Saturn's Tilt to Change? No, the author of that article didn't realize that telescopes invert the image.

Is Elenin a Moon of a Brown Dwarf Star? No.

But Brown Dwarf Stars are so Cold, you Can't See Them. No, coldest detected so far is ~370K (about the temperature of a hot cup of tea), the the warmest are around 2200 K, and most range between 500-1000 K. They may not produce much visible light, but they reflect light. Jupiter has a composition similar to those of Drown Dwarf stars. Jupiter's cloud tops are a chilly 128 K and it reflects light just fine. Any Brown Dwarf in the inner solar system would be painfully obvious.

Will Going Through the Comets Tail Affect Us? No, should the rather small tail of Elenin actually pass over us, it's doing a pretty good imitation of a vacuum (about 100 atoms per cm3). We have been through bigger and denser comet tails before with no effect whatsoever (especially the Great Comet of 1861).

Why isn't Comet Elenin in the News? For the same reason that the other 16 comets discovered in 2010 didn't get in the news, or the 5 comets discovered in 2011. They are all dim. The News is only interested in comets that are spectacular, readily visible to the unaided eye or are being visited by spacecraft. Comet 2009 P1 will be as bright, if not brighter than C/2010 X1 Elenin, but that's not in the news either. Amateur and professional astronomers are watching comet Elenin and others avidly, but the news channels don't care about our obsessions with faint fuzzies.

Why Can't I Find Information of Elenin at the NASA Website? Because NASA is not the arbiter of all things astronomical. You won't find information on C/2009 P1, C/2011 C1 or any of the faint comets discovered during 2010 and 2011. NASA does have information of comets that its spacecraft have visited, or are interesting in some other way, but it's not an exhaustive comet site like Cometography or Aerith.

I Saw Comet Elenin Near the Sun in August 2010/Now: In August 2010 only really powerful telescopes could see Elenin. You saw Venus. If you are seeing something bright near the Sun in the morning sky now, it's Venus.

How Can I Tell What IS in the Sky and Avoid The Venus Confusion? For freeware standalone programs there is Cartes du Ciel and Stellarium (my favourite). For Web based solutions Skyview Cafe, Sky-Map and GoogleEarth (KMZ file here) all work.

Where Can I Find Orbital Elements for Celestia or Stellarium? Here.

Where Can I Find Images of Elenin? Here, and here and a nice image of C/2010 X1 near to NGC 3376 is here.

I have Photographed/ Seen a Photograph of a Double Sun, is this Elenin? It's lens flare.

Is this Video from Auckland Elenin? No, it's just a jet contrail.

Other Good Links:

Leonid Elenin calculates what would happen to planets if a Brown Dwarf was in the inner solar system.

The Sceptics on Elenin.

Astronotes, 10 facts you need to know about Elenin.

Astronomy Beat on Elenin (PDF)

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