Radar image of the "Green" Comet 45P / Honda-Mrkos-Pajdušáková on
2017 February 12, by the huge Arecibo Radio Telescope in Puerto Rico. This
display combines 13 images of the comet received over two hours. This is only
the seventh comet to come close enough to be imaged by radar. On 2011 August
19 and 20, it became only the fifteenth comet to be detected by a ground radar
telescope.
(Image Sources: Arecibo Observatory, NASA, National Science Foundation, Sky and Telescope
Magazine )
By Glenn A. Walsh
Reporting for SpaceWatchtower
At the present time, two dim comets may
be visible in a telescope (and, possibly binoculars), if you know
where to look. While the “Green” Comet was in the early morning
sky earlier this month, now both the Green Comet and Encke's Comet
are in the evening sky.
American astronomer Fred Whipple
described a comet as a “dirty snowball.” Comets are a combination
of rocks, dust, water ice, and other frozen gases, from the early
days of our Solar System.
The solid core of a comet is known as
the nucleus. Streams of dust and gas released from the comet, as it
nears the Sun, form a thin atmosphere around the comet nucleus called
the coma. The coma is composed mostly (90 per-cent) of water, with
dust making-up the rest of the coma.
Most, but not all, comets have one or
more visible tails. The tail(s), which are usually not visible in the
Outer Solar System, are caused by solar radiation as the comet comes
closer to the Sun; this radiation usually is too weak to create tails
in the Outer Solar System. Normally, a comet's tail(s) points away
from the Sun, no matter the direction of movement of the comet;
hence, a comet leaving the Inner Solar System often has a tail
pointing in the direction of the comet's motion.
Comets usually have a highly-eccentric,
elliptical orbit around the Sun, which brings a comet into the Inner
Solar System for a short time, while it spends most of its time in
the Outer Solar System. Short-period comets originate in the Kuiper
Belt, just beyond the orbit of the Planet Neptune, while long-period
comets are thought to originate in the Oort Cloud, a spherical cloud
of icy bodies beyond the Kuiper Belt.
The “Green” Comet
The first one, nick-named the “Green”
Comet due to its coloration, is actually Comet 45P /
Honda-Mrkos-Pajdušáková. This comet passed closest to the Earth on
February 11. At that time it was 7.4 million miles / 12 million
kilometers from Earth. This distance can also be expressed as 0.08
Astronomical Units (1 A.U. or Astronomical Unit is the mean distance
between the Sun and the Earth) and 30 times the mean distance between
the Earth and the Moon.
At this distance, the Green Comet
cannot be seen with the naked-eye; the astronomical Visual Magnitude
at its closest point to Earth was expected to be around +6.5 to +7,
but in reality it never was brighter than +8. It might be seen with
strong binoculars, but a telescope is best for trying to find this
comet. Unlike stars which shine their own light and appear as
pin-points of light in the sky, comets only reflect sunlight. Hence,
they appear as diffuse, fuzzy objects, which make them even more
difficult to find.
The Green Comet may still be visible
until the end of February. However, it will be a challenge to find.
You will need a dark sky, away from city lights. Until a few days
ago, this was even more complicated as the Moon was in the early
morning sky. But, now the comet is rising in the late evening sky as
the Moon rises even later. However, as the end of February gets
closer, the comet will be fading in brightness.
The best time to view this comet is
around 10:00 to 11:00 p.m. local time. As the comet fades, it will be
moving through the constellations Corona Borealis, Boötes, Canes
Venatici, Ursa Major and into Leo the Lion. Internet links to
additional news articles, which include star charts to help find the
comet, are located at the end of this blog-post.
The Green Comet was discovered on 1948
December 3 by Japanese astronomer Minoru Honda. It is named for
Minoru Honda, Czech astronomer Antonin Mrkos, and Slovak astronomer
Ludmila Pajdusakova. The comet appears green because it emits
diatomic carbon, which glows green in the near-vacuum of Outer Space.
This apparition of the Green Comet is
the second-closest to Earth between the years 1900 and 2043. The
comet's closest approach to Earth came in August of 2011, when it
came as close as 0.06 A.U or 5.6 million miles / 9.01233 million kilometers.
While this comet comes toward the Sun fairly often (it is a
short-period comet with an elliptical orbit of 5.25 years), the next
time it is bright from Earth's vantage-point will be in October of
2032 when it is expected to reach a Visual Magnitude of +7.
Encke's Comet
Comet Encke or
Encke's Comet is now visible in the evening sky, but will only be
visible through early March. Like the Green Comet, Encke's Comet is
not bright enough to be seen with the naked-eye (it has a very low
albedo, reflecting only 4.6 per-cent of the light it receives), but
it may be visible in binoculars or a telescope.
Enke's Comet is
currently visible in the western sky about 90 minutes after local
sunset. It is to the left of the Great Square of Pegasus, near the
planets Mars and Venus. At the end of this blog-post is an Internet
link to pages that show a map of how to find Comet Encke.
Encke's Comet was
discovered in 1786 by French astronomer and surveyor Pierre Mechain.
However, it was not immediately recognized to be a comet. It was not
understood to be a periodic comet until 1819, when its orbit was
calculated by German astronomer Johann Franz Encke.
Hence, as with the
more famous Halley's Comet, Encke's Comet is one of the few comets
named after the person who computed the comet's orbit, instead of the
comet's discoverer. In fact, Encke's Comet was the first periodic
comet discovered after Halley's Comet.
Encke's Comet has
the shortest orbital period, 3.3 years, of any reasonably bright
comet. A fainter comet, 311P / PANSTARRS, has an orbital period of
3.2 years.
In 1978, Slovak
astronomer Lubor Kresak proposed that a fragment of Comet Encke may
be the cometary body that caused the 1908 Tunguska Event. On the
morning of 1908 June 30, a huge explosion occurred over a
sparsely-settled area of Siberia in Russia known as the Stony
Tunguska River Valley, which flattened 1,200 square-miles / 2,000
square-kilometers of forest, but caused no human casualties. The
cause of the Tunguska Event, which was not observed by any living
person, is thought to be an air-burst explosion of the atmospheric
entry of a meteoroid, asteroid, or comet.
Internet Links to Additional Information ---
Information & Maps On How to View the 2 Comets -
"Green" Comet:
Link 1 >>> http://www.skyandtelescope.com/observing/green-comet-zooms-moonless-skies/
Link 2 >>> http://astrobob.areavoices.com/2017/02/14/goodbye-moon-but-will-the-green-comet-still-show/
Link 3 >>> http://earthsky.org/astronomy-essentials/comet-45phonda-mrkos-pajdusakova-new-years-eve
Encke's Comet:
Link 1 >>> https://theskylive.com/encke-tracker
Link 2 >>> https://stardate.org/astro-guide/gallery/evening-comet
Comet: Link >>> https://en.wikipedia.org/wiki/Comet
The "Green" Comet - Comet 45P / Honda-Mrkos-Pajdušákov:
Encke's Comet - Comet Encke: Link >>> https://en.wikipedia.org/wiki/Comet_Encke
Tunguska Event: Link >>> https://en.wikipedia.org/wiki/Tunguska_event
Related Blog Posts ---
"Comet of 1491: Self-Correction of Science." 2016 Feb. 20.
Link >>> http://spacewatchtower.blogspot.com/2016/02/comet-of-1491-self-correction-of-science.html
"Comet Lovejoy: Best View Next 2 Weeks." 2015 Jan. 7.
Link >>> https://spacewatchtower.blogspot.com/2015/01/comet-lovejoy-best-view-next-2-weeks.html
"Comet ISON vs. the Solar Storm." 2013 Nov. 26.
Link >>> http://spacewatchtower.blogspot.com/2013/11/comet-ison-vs-solar-storm.html
"Comet: Source of Mysterious Water on Jupiter." 2013 May 4.
Link >>> http://buhlplanetarium2.tripod.com/weblog/spacewatchtower/pearcee/pe-jupiterwater.html
"Comet of the Century?" 2013 Jan. 19.
Link >>> https://spacewatchtower.blogspot.com/2013/01/comet-of-century.html
Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
2017 February 19.
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