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Date: Tue, 16 Aug 2011 20:08:00
From: NASA Jet Propulsion Laboratory <>
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Subject: Comet Elenin Poses No Threat to Earth


         Aug. 16, 2011 

Comet Elenin Poses No Threat to Earth

The full version of this story with accompanying images is at:

Often, comets are portrayed as harbingers of gloom and doom in movies and on
television, but most pose no threat to Earth. Comet Elenin, the latest comet
to visit our inner solar system, is no exception. Elenin will pass about 22
million miles (35 million kilometers) from Earth during its closest approach
on Oct. 16, 2011.

Also known by its astronomical name C/2010 X1, the comet was first detected
on Dec. 10, 2010 by Leonid Elenin, an observer in Lyubertsy, Russia, who
made the discovery "remotely" using an observatory in New Mexico. At that
time, Elenin was about 401 million miles (647 million kilometers) from
Earth. Since its discovery, Comet Elenin has ? as all comets do ? closed the
distance to Earth's vicinity as it makes its way closer to perihelion, its
closest point to the sun.

NASA scientists have taken time over the last several months to answer your
questions. Compiled below are the some of the most popular questions, with
answers from Don Yeomans of NASA's Near-Earth Object Program Office at
NASA's Jet Propulsion Laboratory in Pasadena, Calif., and David Morrison of
the NASA Astrobiology Institute at the NASA Ames Research Center in Moffett
Field, Calif.

Most Popular Questions About Comet Elenin

     When will Comet Elenin come closest to the Earth and appear the

Comet Elenin should be at its brightest shortly before the time of its
closest approach to Earth on Oct. 16, 2011. At its closest point, it will be
22 million miles (35 million kilometers) from us.

     Will Comet Elenin come close to the Earth or between the Earth
     and the moon?

Comet Elenin will not come closer to Earth than 22 million miles (35 million
kilometers). That's more than 90 times the distance to the moon.

     Can this comet influence us from where it is, or where it will
     be in the future? Can this celestial object cause shifting of
     the tides or even tectonic plates here on Earth?

There have been incorrect speculations on the Internet that alignments of
comet Elenin with other celestial bodies could cause consequences for Earth
and external forces could cause comet Elenin to come closer. "Any
approximate alignments of comet Elenin with other celestial bodies are
meaningless, and the comet will not encounter any dark bodies that could
perturb its orbit, nor will it influence us in any way here on Earth," said
Don Yeomans, a scientist at NASA JPL.

"Comet Elenin will not only be far away, it is also on the small side for
comets," said Yeomans. "And comets are not the most densely-packed objects
out there. They usually have the density of something akin to loosely packed
icy dirt.

"So you've got a modest-sized icy dirtball that is getting no closer than 35
million kilometers [about 22 million miles)," said Yeomans. "It will have an
immeasurably miniscule influence on our planet. By comparison, my subcompact
automobile exerts a greater influence on the ocean's tides than comet Elenin
ever will."

     I've heard about three days of darkness because of Comet Elenin.
     Will Elenin block out the sun for three days?

As seen from the Earth, comet Elenin will not cross the sun's face," says

But even if it could cross the sun, which it can't, astrobiologist David
Morrison notes that comet Elenin is about 2-3 miles (3-5 kilometers) wide,
while the sun is roughly 865,000 miles (1,392,082 kilometers) across. How
could such a small object block the sun, which is such a large object?

Let's think about an eclipse of the sun, which happens when the moon appears
between the Earth and the sun. The moon is about 2,500 miles (4,000
kilometers) in diameter, and has the same apparent size as the sun when it
is about 250,000 miles (400,000 kilometers) away -- roughly 100 times its
own diameter. For a comet with a diameter of about 2-3 miles (3-5
kilometers) to cover the sun it would have to be within 250 miles (400
kilometers), roughly the orbital altitude of the International Space
Station. However, as stated above, this comet will come no closer to Earth
than 22 million miles.

I've heard there is a "brown dwarf" theory about Comet Elenin. Would its
mass be enough to pull Comet Honda's trajectory a significant amount? Could
this be used to determine the mass of Elenin?

Morrison says that there is no 'brown dwarf theory' of this comet. "A comet
is nothing like a brown dwarf. You are correct that the way astronomers
measure the mass of one object is by its gravitational effect on another,
but comets are far too small to have a measureable influence on anything."

If we had a black or brown dwarf in our outer solar system, I guess no one
could see it, right?

"No, that's not correct," says Morrison. "If we had a brown dwarf star in
the outer solar system, we could see it, detect its infrared energy and
measure its perturbing effect on other objects. There is no brown dwarf in
the solar system, otherwise we would have detected it. And there is no such
thing as a black dwarf."

Will Comet Elenin be visible to the naked eye when it's closer to us? I
missed Hale-Bopp's passing, so I want to know if we'll actually be able to
see something in the sky when Elenin passes.

We don't know yet if Comet Elenin will be visible to the naked eye. Morrison
says, "At the rate it is going, seeing the comet at its best in early
October will require binoculars and a very dark sky. Unfortunately, Elenin
is no substitute for seeing comet Hale-Bopp, which was the brightest comet
of the past several decades."

"This comet may not put on a great show. Just as certainly, it will not
cause any disruptions here on Earth. But, there is a cause to marvel," said
Yeomans. "This intrepid little traveler will offer astronomers a chance to
study a relatively young comet that came here from well beyond our solar
system's planetary region. After a short while, it will be headed back out
again, and we will not see or hear from Elenin for thousands of years.
That's pretty cool."

This comet has been called 'wimpy' by NASA scientists. Why?

"We're talking about how a comet looks as it safely flies past us," said
Yeomans of NASA's Near-Earth Object Program Office. "Some cometary visitors
arriving from beyond the planetary region ? like Hale-Bopp in 1997 -- have
really lit up the night sky where you can see them easily with the naked eye
as they safely transit the inner-solar system. But Elenin is trending toward
the other end of the spectrum. You'll probably need a good pair of
binoculars, clear skies and a dark, secluded location to see it even on its
brightest night."

     Why aren't you talking more about Comet Elenin? If these things
     are small and nothing to worry about, why has there been no
     public info on Comet Elenin?

Comet Elenin hasn't received much press precisely because it is small and
faint. Several new comets are discovered each year, and you don't normally
hear about them either. The truth is that Elenin has received much more
attention than it deserves due to a variety of Internet postings that are
untrue. The information NASA has on Elenin is readily available on the
Internet. (See ) If
this comet were any danger to anyone, you would certainly know about it. For
more information, visit NASA's AsteroidWatch site at

     I've heard NASA has observed Elenin many times more than other
     comets. Is this true, and is NASA playing this comet down?

NASA regularly detects, tracks and characterizes asteroids and comets
passing relatively close to Earth using both ground- and space-based
telescopes. The Near-Earth Object Observations Program, commonly called
"Spaceguard," discovers these objects, characterizes a subset of them and
predicts their paths to determine if any could be potentially hazardous to
our planet. For more information, visit the NASA-JPL Near Earth objects site
at .

However, neither NASA nor JPL is in the business of actively observing
Elenin or any other comet. Most of the posted observations are made by
amateur astronomers around the world. Since Elenin has had so much
publicity, it naturally has attracted more observers.

     I was looking at the orbital diagram of Comet Elenin on the JPL
     website, and I was wondering why the orbit shows some angles
     when zooming? If you pick any other comet, you can see that
     there are no angles or bends.

Many people are trying to plot the orbit of the comet with the routine on
the JPL website, without realizing that this is just a simple visualization
tool. While the tool has been recently improved to show smoother
trajectories near the sun, it is not a scientific program to generate an
accurate orbit. Yeomans explains that the orbit plotter on the Near-Earth
Object website is not meant to accurately depict the true motion of objects
over long time intervals, nor is it accurate during close planetary
encounters. For more accurate long-term plotting, Yeomans suggests using the
JPL Horizons system instead:
20X1 .


DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.

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