Part 11: Minor debris and worldlets that permeate the solar system.
These short videos were created in August 2007 by Dr. Christopher D. Impey, Professor of Astronomy at the University of Arizona, for his students. They cover a broad range of terms, concepts, and princples in astronomy and astrobiology. Dr. Impey is a University Distinguished Professor and Deputy Head of the Astonomy Department. All videos are intended solely for educational purposes and are licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. The full list of collections follows below:
01. Fundamentals of Science and Astronomy
02. Ancient Astronomy and Celestial Phenomena
03. Concepts and History of Astronomy and Physics
04. Chemistry and Physics
05. Quantum Theory and Radiation
06. Optics and Quantum Theory
07. Geology and Physics
08. Solar Neighborhood and Space Exploration
09. Outer Planets and Planetary Atmospheres
10. The Solar System
11. Interplanetary Bodies
12. Formation and Nature of Planetary Systems
13. Particle Physics and the Sun
14. Stars 1
15. Stars 2
16. Stars 3
17. Galactic Mass Distribtuion and Galaxy Structure
19. Galaxies 2
20. Galaxy Interaction and Motion
21. Deep Space and High-Energy Phenomena
22. The Big Bang, Inflation, and General Cosmology
23. The Big Bang, Inflation, and General Cosmology 2
24. Chemistry and Context for Life
25. Early Earth and Life Processes
26. Life on Earth
27. Life in the Universe
28. Interstellar Travel, SETI, and the Rarity of Life
29. Prospects of Nonhuman Intelligences
Orbits of Interplanetary Bodies
Transcript: The different types of interplanetary bodies have different orbits. Asteroids are mostly found between the orbits of Mars and Jupiter, on nearly circular orbits; this is the region of the main asteroid belt. Most meteorites have orbits that are elliptical whose most distant point is somewhere in or near the asteroid belt, which is good evidence that they originate in the asteroid belt and are broken asteroids. Many comets and meteors have elliptical orbits whose outer distance is much further, far beyond the orbit of Pluto. The different orbits of these different types of bodies relate to their compositions. Objects like comets which travel to the distant outer reaches of the solar system contain substantial amounts of icy material, whereas meteors and asteroids are mostly rocky.
Transcript: The astronomical unit is defined as the mean distance between the Earth and the Sun. It has to be defined as the mean because the orbit is elliptical by a couple of percent. The astronomical unit is 150 million kilometers or 98 million miles. This sets the scale of the typical distances between planets. The entire solar system is about 100 astronomical units across, and the distance from the Sun to the nearest stars is of order 100,000 astronomical units. Thus, the distance humans have traveled in space, just to the Moon, only a quarter of a million miles, is a tiny fraction of the size of the solar system and an even tinier fraction of the distance to the nearest stars.
Setting the Scale of the Solar System
Transcript: Kepler’s Laws give the relative distance of the planets in the solar system, but setting the absolute scale requires the measurement of the distance to at least one planet. This technique was first attempted in the seventeenth century using the idea of triangulation and the measurement of a parallax angle. Two simultaneous observations of a planet against the backdrop of the fixed stars are made from points well separated on the Earth’s surface. The technique thus requires accurate transcontinental maps, the skill of cartography, and observations made at exactly the same time, the technique of chronometry. In practical terms, it’s easier to do the technique with Venus rather than Mars because the transit of Venus across the face of the Sun can be used as a fixed backdrop.
Longitude and Latitude
Transcript: Measurement of the distance to a nearby planet from a parallax angle on the Earth's surface requires accurate time keeping. In two positions on the Earth's surface, measuring their difference in latitude is easy from the elevation or altitude of the pole star. Measuring their difference in longitude requires accurate time keeping because the Earth spins, 24 hours orbit corresponding to 360 degrees of rotation. Thus, a clock which loses five or ten minutes per day, typical in the seventeenth century, would translate to an error in longitude of 500 miles after ten days at sea. This uncertainty limited the measurements of the parallax angle until the uneducated carpenter John Harrison submitted a spectacular design for a watch, a mechanical device, that kept time accurate to five seconds in eighty days. This accurate device for the first time allowed the definition of longitude as well as latitude on the Earth's surface and not incidentally led to the dominance of England as a sea power.
Transcript: The best opportunity to measure the scale of the solar system by parallax of a nearby planet occurs for transit of Venus, when Venus crosses the surface of the Sun as seen from the Earth. There were twin transits in 1631 and 1639 which were not successfully observed, and in any case the timekeeping pieces of the time were very inaccurate. Venus transits are rare; astronomers had to wait over a century for the next pair of transits, 1761 and 1769. Mason and Dixon tried to measure the 1761 transit observing in North America, but their clocks were inaccurate. Success came to the intrepid explorer James Cook; in 1769, from the South Pacific, he made a successful measurement of the transit at the same as time someone was making the measurement in England. The result was a measurement of the scale of the solar system accurate to ten percent.
Distance to the Planets
Transcript: The distance to nearby planets is now set with exquisite accuracy using radar techniques. Rather than using parallax and geometric measurement, astronomers bounce radar signals off the moons and nearby planets and receive the dim radar signals in return. Timing coupled with knowledge of the exact speed of light, measured in laboratories, gives an accurate measurement of the distance. This accuracy is sufficient to see tiny orbital perturbations in the nearby planets.