Comet Elenin Poses No Threat to Earth

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DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
agle@jpl.nasa.gov

News Release: 2011-255 Aug. 16, 2011

Comet Elenin Poses No Threat to Earth

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-255&cid=release_2011-255

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 brightest?

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 Yeomans.

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
http://www.jpl.nasa.gov/news/news.cfm?release=2011-135 ) If this comet were any
danger to anyone, you would certainly know about it. For more information, visit
NASA's AsteroidWatch site at http://www.jpl.nasa.gov/asteroidwatch/.

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 http://neo.jpl.nasa.gov/ .

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:
http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=sb&sstr=C/2010%20X1 .

#2011-255

DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
agle@jpl.nasa.gov
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