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Transcript
040728.doc A “MESSENGER” to Mercury Page 1 of 5 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 Page 2 of 5 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 Page 3 of 5 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 Page 4 of 5 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 Page 5 of 5 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.