C/2010 X1 (Elenin)（2）

前回、彗星C/2010 X1 (Elenin)について、素人の筆者と素人と思われるブロガーの見方を紹介しました。当事者であるべきNASAの公式見解は、まだ見つかっていません。そこで、NAI – NASA Astrobiology Instituto のホームページにあるコラム“Ask an Astrobiologist”で、David MorrisonがQ&A形式で、この彗星に関する専門家の意見を述べているので、それを聞きましょう。引用は、彗星C/2010 X1 (Elenin)のものだけを集めましたが、既に抹消されているコメントも幾つかありました。

NAI home page “Astrobiology: life in the universe” / “Ask an Astrobiologist”

1 Question
So what the deal with comet elenin, there does not seem to be too much credible information. Everyone seems to be freaking out about it, but no one even knows how big it is. Does this comet pose a threat and what is its size and nature? AND I have read about comet Elenin and how some nasa people want to keep it a secret. what are the chances of it coming to earth and what threat does it bring with it, and has this comet got anything to do with 2012 hoax? AND Elinin is only .117 au from earth on mar 11,2011. From march 8 to mar 15, 2011, what are the chances that Elinin either impacts us, or gets close enough to cause a major catastrophy. Please be honest.

I have been receiving many questions about Comet Elenin. Many of them refer to the outrageous lies on some conspiracy theory websites, which are amplified by people who seem to think this comet is highly unusual. It is not unusual, and it is not a threat to Earth. See the Wikipedia article on this comet http://en.wikipedia.org/wiki/C/2010_X1 for more information. C2010 X1 Elenin (to give its full name) is a long-period comet, which takes about 10,000 years to complete one orbit around the Sun. It was discovered with a robotic telescope in New Mexico on 10 December 2010 by Russian amateur astronomer Leonid Elenin. When comets or asteroids are first discovered, their orbits are quite uncertain; usually it takes several months of observations to firmly establish the orbit. Elenin's perihelion (closest to the Sun) is in early September 2011 at a distance from the Sun of 40-45 million miles. It will be closest to Earth on about 16 October, at a distance of about 21 million miles, which is nearly a hundred times farther than the Moon. The comet never comes close to the Earth, but it is expected to be visible in binoculars during August and October. Part of the Internet chatter concerns its size. Comets are exceedingly small and enveloped in a tenuous cloud of gas and dust, so the only way to be sure of their actual dimensions is to visit with a spacecraft. Half a dozen comets have been the target of spacecraft missions, and all of them (even Comet Halley) are less than 10 km in diameter. There is no reason to think Elenin is any different. This means its mass is less than one billionth the mass of the Earth. Needless to say, we will not be aware of the tiny gravitational pull from Elenin. In spite of these simple facts, some websites (such as godlikeproductions) are making wild claims that Elenin will hit the Earth, or disturb our orbit, or cause tides, or interact with our magnetic field. Such claims are pure fiction. One of the worst examples is a video that someone posted on the NASAbuzzroom website that claims that the magnetic field of the comet will cause a large shift in the rotation axis of the Earth and produce mega-earthquakes on March 15, 2011. It is hard to imagine anyone would take this seriously. Comets don't have magnetic fields, and magnetic fields can't change the rotation axis or cause earthquakes no matter how large they are. Adding to the craziness is a claim that this same comet caused the Earth's axis to shift by 3 degrees in February 2010 and caused the Chile Earthquake. These are simply lies; the comet was far away a year ago, and there was no change whatever in Earth's rotation axis. The Chile earthquake was a normal slippage in an active fault that has produced many previous earthquakes, including the one witnessed by Charles Darwin in 1835. Please remember that just because someone says he or she is from NASA doesn't make it true. Many people who post YouTube videos lie about their affiliation. Incidentally, only NASA is allowed to use the NASA logo, the blue circle with the word NASA and a satellite orbiting around it.

David Morrison
Astrobiology Senior Scientist
March 1, 2011

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2 Question
Why was C2010 X1 Elenin discovered so late? If an object was approaching to earth, you should have found it several month ago, right? I found in your posted answer that you use many "about". So why can you say it would pass earth at a distance of about 21 million miles?

Please don't confuse the warning time for asteroids with that for comets. In several previous answers I have discussed the Spaceguard Survey and the fact that it is designed to provide many years of warning for any near earth asteroid (NEA) that might be on a collision course with Earth. We can carry out such a comprehensive survey for asteroids because their orbits bring them frequently within range of our telescopes. NEAs typically have orbital periods between 6 and 24 months. Comets, in contrast, come from deep space. The orbital period for Comet Elenin is about 10,000 years, so obviously we have had no previous opportunity to observe it or to calculate its orbit. Hence it was not discovered until December 2010, and there is still some uncertainty in its orbit; in this case, the closest distance to Earth is 21 +/- 1 million miles. That still tells you that it will miss our planet by a very large margin. There is zero chance that it will come close to Earth, let alone hit us. Non-scientists often have trouble understanding scientific calculations. Usually there is some uncertainty, but this in no way should make you suspicious of the result. Most scientific calculations are presented with an uncertainty or margin of error. This is just being honest. It is nonscientists who sometimes pretend absolute accuracy and don't seem to recognize the meaning of scientific uncertainty.

David Morrison
Astrobiology Senior Scientist
March 23, 2011

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3 Question
This is again about comet Elenin. The photo that is usually shown is a fake image and it is actually comet 81P/Wild http://www.suite101.com/view_image_articles.cfm/2683306. NASA supposedly has limited info and no pictures. Oh, really? Not from the WISE satellite, designed specifically to look at low light objects (like dwarf stars) with high sensitivity infrared? What about the South Pole telescope which has excellent IR detection capabilities? How about Hubble? Nothing! Is this really the dwarf star that would end all life on earth? Why doesnt NASA ever talk anything about it?

Dear readers: I wrote on March 1 that I did not intend to answer any more questions about Comet Elenin. I am changing my mind because I am worried about the effects of the fear campaign that is being waged on the Internet. We all remember the Heaven's Gate cult that became so obsessed with Comet Hale-Bopp in 1997 that they committed mass suicide http://en.wikipedia.org/wiki/Heaven's_Gate_(religious_group). I don't want anything like that to happen again, so I will answer this and the following two questions about Comet Elenin. Perhaps I can also promote some critical thinking about these claims. (1) Yes I know that this photo (which is not faked, just misidentified) is not of Comet Elenin. In fact, most of the images that come up when you Google "Elenin images" are not of Elenin. I don't know how this particular photo of Comet Wild became associated with Elenin, except that Elenin is so faint that perhaps some people wanted to substitute a picture of a brighter comet. (2) The WISE infrared telescope ceased operations at about the time Elenin was discovered. This little comet, then in the asteroid belt, was probably too faint to be picked up by WISE. (3) I don't know much about the NSF telescopes at the South Pole, but in the 5 months since the discovery of Elenin, the South Pole has been in continuous daylight. The South Pole would be one of the worst places to go to study this comet. (4) The Hubble Space Telescope will undoubtedly study Elenin when it is close to Earth, but I doubt that anyone has used it yet. Most astronomers will wait until the comet is closer and brighter. (5) Do I need to remind my readers that a brown dwarf is billions (with a B) of times more massive than a comet? I suspect this brown dwarf rumor started when someone who is not familiar with the sky turned a small telescope on Jupiter with its four moons. (6) There is no reason that NASA (or the media) would be commenting on this small faint comet. About the only people observing it now are amateur astronomers, who are tracking its orbit and its increasing brightness as it approaches the inner solar system. I am answering questions only because of the craziness that is circulating on the Internet, from people who either don't know much about astronomy or are intentionally making up stories to frighten gullible people. Shame on them!

David Morrison
Astrobiology Senior Scientist
April 25, 2011

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4 Question
What is the size of comet Elenin? I have heard it is twice the diameter of Earth. Some websites say that Elenin is 80,000km in diameter including coma. AND Can you put something out there about what NASA knows about Comet Elenin, just to smooth things over before people get crazy? Some are claiming that the comet has disappeared or that there is a government plot to hide it.

I will be glad to tell you what I know about Comet Elenin. Most of what is known comes from amateur astronomers, who have made hundreds of observations; see http://www.aerith.net/comet/catalog/2010X1/2010X1.html. Our knowledge of its orbit is constantly improving, with updates available from the Minor Planet Center http://ubasti.cfa.harvard.edu/~cgi/ReturnPrepEph?d=c&o=CK10X010. Elenin is still estimated to have an orbital period of about 10,000 years. There are many photos posted on the web, but be careful: the great majority of the images that come up in a Google search are not of Comet Elenin but of other comets or artist impressions (and some are fakes). For one good example see photos from Bernhard Hausler taken in early March http://thewatchers.adorraeli.com/2011/03/11/new-approach-of-a-comet-with-an-asteroid-elenin/. The comet is too faint to be seen or photographed with a small telescope, but people who say the comet has disappeared or photos are being suppressed are lying. To understand questions about the size of the comet, you must distinguish between the small solid nucleus of rock and ice and the large atmosphere (coma) and tail that develop as a comet approaches the Sun. The nucleus has not been measured but is probably about 4 km across. The coma was reported to be about 80,000 km across in early April, with a tail estimated at ten times that length. These are not unusual values for a comet, and both coma and tail are expected to grow as the comet approaches the Sun (it is still beyond the orbit of Mars at this writing on April 15). Some comets have developed atmospheres that are more than a million km across, but please remember that the atmosphere (coma and tail) is extremely tenuous, far less dense than the best vacuum that can be produced in the lab. This thin gas and dust can have no effect on the Earth or anything else.

David Morrison
Astrobiology Senior Scientist
April 26, 2011

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5 Question
Up until now, I've been trying not to pay too much attention to this doomsday stuff. However, release today of this video to NASA employees http://www.nasa.gov/centers/hq/emergency/personalPreparedness/index.html and the association people are making on the Internet with comet Elenin and the terrible impacts it would have on our planet have me worried. AND The preparedness video sent to NASA employees seems to prove that you are worried about Comet Elenin. All the conspiracy theorist are speculating about the comet impacting the Earth in October, and they say this is the first notification to be published.

I am amazed to learn that the conspiracy websites are linking this video sent to NASA employees with Comet Elenin. I am sure that the people who made the NASA video have never heard of Comet Elenin. And indeed, why should they -- a wimpy little comet that never comes anywhere near the Earth? You need to understand that the Internet ho-ha about Elenin is crazy talk. Ever since the 9-11 attack people have been talking about "connecting the dots". That is fine if the "dots" are related to each other. But the conspiracy nuts love to look for correlations and relationships where there are none, as in this example. Science, in contrast, deals with actual cause and effect. Most of my replies discussing Comet Elenin are collected together by the Astronomical Society of the Pacific at http://www.astrosociety.org/elenin/. The other NASA official comment on Elenin is from JPL at http://www.jpl.nasa.gov/news/news.cfm?release=2011-135. Here is also a good story at Universe today http://www.universetoday.com/86421/comet-elenin-just-passing-by/. Hints for observers (and some photos) are at AstroBob http://astrobob.areavoices.com/2011/04/04/comet-elenin-will-not-destroy-the-earth/. I hope these links help.

David Morrison
Astrobiology Senior Scientist
June 14, 2011