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A “MESSENGER” to Mercury
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From Celestial North, this is IT’S OVER YOUR HEAD for the week of July
28th, 2004, a look at what’s up in the sky over Puget Sound.
In December last year, the European Space Agency’s Mars Express
entered orbit around Mars. In January of this year, the Stardust mission
collected samples from a comet to return to Earth, and the twin Spirit and
Opportunity rovers began to explore the history of water on the Martian surface.
During late June and early July, the Cassini/Huygens spacecraft flew by Saturn’s
outer icy moon Phoebe, and then went into orbit around our solar system’s “Lord
of the Rings.” In September, the Genesis spacecraft will return samples of the
Sun’s solar wind to Earth.
But wait, there’s more! Sometime in early August, yet another spacecraft is
set to break the bonds of Earth and start a new and exciting mission of discovery.
The name of this new spacecraft? MESSENGER. Its target? The planet
Mercury.
Mercury is the closest planet to our Sun, and the least explored of the socalled terrestrial planets, which are the four rocky inner planets that also include
Venus, Earth, and Mars. Other than 3 flybys by Mariner 10 in 1974 and ’75, no
other spacecraft have yet explored the first rock from the Sun.
Why study Mercury? Though Mercury is only somewhat bigger than our
own Moon – around 3,000 miles in diameter compared to Earth’s 8,000 miles – it
is extremely dense. So dense, in fact, that two-thirds of the planet is believed to
be iron metal. Since the four terrestrial planets formed at the same time, how did
Mercury come to have such a high percentage of metal?
Copyright © 2004 Celestial North, Inc. All rights reserved.
040728.doc
A “MESSENGER” to Mercury
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As MESSENGER’s Principal Investigator Sean Solomon of the Carnegie
Institution of Washington puts it, “The inner planets are all litter-mates, if you will,
the products of a single early stage in the evolution of a star, and yet the siblings
turned out very differently. In order to understand what processes most control
the differences in outcomes, we really have to study and learn about the most
extreme of those outcomes - and that's Mercury."
Earth orbits the Sun at an average of about 93 million miles, while
Mercury’s average orbital distance is only 36 million miles. At Mercury’s close
distance, the Sun looks almost 3 times larger and 11 times brighter than on
Earth. To withstand the intense heat that close to the Sun, MESSENGER has a
ceramic sunshade. "The outside of the sunshade will get as hot as a pizza oven,
while the rest of the spacecraft will remain at nearly room temperature," says
mission systems engineer James Leary.
Since Mercury is the closest planet to the Sun, it would be natural to
assume that it is also the hottest. But actually Venus is hotter due to its thick,
carbon-dioxide-rich atmosphere – a testament to the devastating potential of the
greenhouse effect. But Mercury is indeed the second hottest planet, with surface
temperatures on the sunlit side exceeding 450 degrees Celsius, or 840 degrees
Fahrenheit, easily hot enough to melt softer metals such as tin, lead, and zinc.
Copyright © 2004 Celestial North, Inc. All rights reserved.
040728.doc
A “MESSENGER” to Mercury
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Surprisingly, temperatures on the night side of Mercury can plunge to –185
degrees Celsius, or 300 degrees below zero Fahrenheit – about the temperature
of liquid oxygen. How can Mercury be so close to the Sun, and yet have such a
huge temperature swing? First, Mercury rotates very slowly. The length of its
solar day – the time from one sunrise to the next – is 176 Earth days, so there is
ample time for its surface to heat up during the long day and cool off during the
equally long night. Second, Mercury has nearly no atmosphere to transfer and
distribute heat more evenly across the planet.
Ground-based images of Mercury’s polar regions taken by the giant Arecibo
radio telescope show that the crater floors there are highly reflective at radar
wavelengths. A layer of frozen water is a likely explanation for this high
reflectivity. Earth’s spin axis is tilted some 23-½ degrees, so even its polar
regions receive some sunlight for some portion of the year. But Mercury’s axial
tilt is nearly zero, so crater floors at the poles are in perpetual shadow. So,
strange as it may sound, the planet closest to the Sun may in fact harbor frozen
water!
But there are other possibilities for the high reflectivity in polar crater floors:
any volatile with a freezing point above –180 degrees Celsius could also be
responsible, as could elemental sulfur or frozen silicates. Hopefully
MESSENGER can help settle this question once and for all.
Copyright © 2004 Celestial North, Inc. All rights reserved.
040728.doc
A “MESSENGER” to Mercury
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Though the Earth and Mercury can sometimes pass within 60 million miles
of each other, MESSENGER’s journey will require it to travel more than 80 times
that distance before going into orbit around Mercury. As a spacecraft gets closer
to the Sun, it requires more and more energy to change its orbit. MESSENGER
will cover almost 5 billion miles and require 6 gravity-assist flybys of Earth,
Venus, and Mercury before finally settling into a one-year orbit around its target
in the year 2011.
We’re on the web at CelestialNorth.org. Until next time, this is ____
_______ and ____ _______, with a reminder that the night is very large, and full
of wonders!
Copyright © 2004 Celestial North, Inc. All rights reserved.
040728.doc
A “MESSENGER” to Mercury
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REFERENCES:
http://www.astrobio.net/news/article1099.html
http://messenger.jhuapl.edu/faq/index.html
http://messenger.jhuapl.edu/faq/faq_mission.html
http://messenger.jhuapl.edu/faq/faq_journey.html
http://messenger.jhuapl.edu/faq/faq_planet.html
http://messenger.jhuapl.edu/faq/faq_science.html
http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html
http://www.vcs.ethz.ch/chemglobe/ptoe/
http://site.ifrance.com/okapi/chemistry.htm
Copyright © 2004 Celestial North, Inc. All rights reserved.