As for what will happen to Phobos, the Wikipedia article rests in part on a NASA Web Archive. As stated above: it could break up or hit the surface. We will just have to wait and see.
Friday, December 28, 2018
Problems with Pop Sci from Sky & Telescope (Part 2)
Similar to my critique of Steven Hawking’s A Brief History of Time, the source of the many problems may be that the editors at Sky & Telescope are trying to condense complicated truths into a few lines of common English.
February 22: “Small meteorites aren’t hot when they hit the ground. Earth’s atmosphere heats and removes a very thing surface layer, but the rest of the meteorite is still ice-cold from its time in space.”
August 13: “Occasionally, meteor-watchers hear a hiss or crackle accompany particularly bright fireballs. Astronomers still debate the source of the sound.”
Those two resulted in an interesting search: “are meteorites hot or cold when they hit earth?” Apparently the answer may be “yes.” The consensus is that they are cold. They quickly lose their outer layers by ablation, the same phenomenon used to protect spacecraft: the outer layers burn away carrying the heat. That said, accounts of meteorites being at least warm are not totally discounted. At the same time, also accepted are reports of newly-fallen objects being covered with frost. That meteorites are hot when they fall was commonly accepted until recently even by educated people. An article in Popular Astronomy for February 1934 takes the Smithsonian to task for perpetuating the error. (See Astrophysics Data Site archive of Harvard online here.)
December 27: “In 50 million years or so, Phobos will spiral into Mars, crashing on its surface or breaking up in pieces. In the meantime, though, the little moon has quite a view: Mars fills much of its sky because of the close orbit.” The spiral is not a possible orbit. That was a teaching point from a Heinlein juvenile novel. Living in a spacefaring culture, our young hero takes an aptitude test to be trained as a pilot. “What would you do if you suddenly found that you were spiraling in to a planet?” It is a trick question because the spiral is not a possible orbit. Considering the problem again, the easiest general statement is that successive elliptical orbits decay by atmospheric drag.
Mathematical Association of America
Re-entry of Orbital Debris
Video Tutorials on Mechanics and Orbital Motion
Physics Department at the University of New South Wales
October 31: “As it sails beyond the solar system, Voyager I hears the lonely radio whistles of plasma waves passing through interstellar space. Listen here: . This was interesting, but it is a transduction. Note that the video (YouTube here: ) also presents color blobs representing those waves. We can make them sounds in any octave, lines or shapes in any color, depending on our choices of coordinates. And, as we all know, you really cannot hear radio waves traveling through interstellar space.
December 5: “You can cry in space, but your tears won’t fall—due to water tension (and a lack of gravity) they form a floating, liquid sphere.” “There is no gravity in space” is one of those many easy sayings spoken by people who watch television shows about science, but who never take a formal class in astronomy or physics. If you lived in a spacefaring culture you might be told that if you were trapped in a gravity well, your tears would be pulled away from your face as the tremendous inertial acceleration overpowers the molecular adhesion that holds them to your face. If you were “in space” as a passenger of an accelerating vehicle, you would experience an inertial force indistinguishable from “gravity.”
(Some tests could reveal the larger context of your condition. You might be on a rocket accelerating; you might be in a spinning torus like the classic space station; you might be on a large body such as a moon or planet in space. But within the reach of a human—a fathom—it would be difficult to find any differences. The differences were explained to me by a friend of mine from high school who went to MIT. I am not smart: I learn well.)
March 28: “Thanks to its solar wind, the Sun is losing roughly an Earth’s worth of mass every 150 million years.”
March 29: “Because the Sun is (slowly) losing mass, Earth’s orbit gets about an inch bigger every other year.”
April 15: “The Moon is drifting away from Earth at a rate of 1.5 inches per year.”
July 7: The expansion of space means that the solar system is expanding, too, but only at an infinitesimal amount: one part in septillion over its lifespan.
1. A so-called “solar wind” is an essential characteristic of every star. The phrase is just a way that we conveniently think of the energy of the star. The loss of mass is integral to the nature of the star and we know how stars age.
2. What is interesting is that Earth is losing mass, also. That must mean that all of the planets and, in fact, all other material bodies do so as well. Net loss to Earth is 50,000 tonnes per year (BBC News Magazine online here). That is the difference between the meteoric dust which falls in and the hydrogen and helium gasses which escape into space. Other effects come from the core’s heat being lost, volcanoes and other relatively minor events. (See this homework problem from Weber State University.)
3. Finally, as for the expansion of “space” (or space-time, or the stuff “in” it or “in” which space-time exists), while not quite lying with statistics, casually tossing out large numbers obfuscates the question. If the universe is 13.8 billion years old and expanding one part in a septillion over 10 billion years, the numbers just do not explain the facts we believe from observation and measurement – unless the expansion rate is highly variable, greatly slowing down or expected to greatly increase. Deceleration seems to be the accepted theory of the day. But it is more complicated than can be explained in five lines of a calendar page.
· Universe's Expansion Rate Is Different Depending on Where You Look By Elizabeth Howell, Space.com Contributor July 13, 2018 04:03 pm ET
· The Expanding Universe: From Slowdown to Speed Up; Distant supernovae are revealing the crucial time when the expansion of the universe changed from decelerating to accelerating By Adam G. Riess, Michael S. Turner on September 23, 2008
4. Just to note about the actual age of our planet and its sisters: “The inner edge of the Sun’s habitable zone is moving outwards at a rate of about 1 metre per year. The latest model predicts a total habitable zone lifetime for Earth of 6.3 billion–7.8 billion years, suggesting that life on the planet is already about 70% of the way through its run. Other planets — especially those that form near the outer boundary of a star’s habitable zone or orbit long-lived, low-mass stars — may have habitable-zone lifetimes of 42 billion years or longer.” (Daily news blurb from Nature online here.)
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