11. Interplanetary Bodies

Transcript: When we look at the Moon we are looking in a mirror. The Moon has lived for the same length of time as the Earth, 4.6 billion years, and its cratered surface reflects impacts occurring throughout its history. Most of those impacts occurred during the era of heavy bombardment within the first few hundred million years. However, a significant number of large impacts have occurred throughout cosmic time. On the Earth, however, geological activity and erosion has erased most of the evidence of craters. Of the several tens of millions of potentially impacting bodies that reach the upper Earth's atmosphere, only a few land everyday as meteorites, and most of them are very small. So on the Earth evidence of cratering is rare. In northern Arizona, the Barringer crater, in a dry desert region, is one of the best preserved craters dating back twenty-five thousand years. The impactors are traveling fast, 25 to 100 thousand miles per hour. An enormous energy is released when a meteorite is brought to zero velocity, and its kinetic energy must be turned into heat energy. But impacts do happen. In 1994 astronomers watched the comet Shoemaker-Levy break apart into pieces and slam into the atmosphere and eventually the surface of Jupiter using the Hubble Space Telescope.

Transcript: On June 30, 1908, an explosion occurred in the skies above Siberia. Reindeer herdsmen were flung into the air and several were killed. An area of 2000 square kilometers of forest was absolutely leveled to the ground. The shockwave was measured in the United States, and enough debris was thrown into the upper atmosphere that the night sky over Europe was lit up for several months. All the evidence points to a piece of interplanetary debris of size 50 or 60 meters exploding just above the Earth’s surface and so leaving no crater. This type of impact is expected about once a century on the Earth although the actual occurrence rate or time is unpredictable because the events are random.

Transcript: The destructive power of meteorites can be illustrated by considering a few examples of different sizes. A ten meter sized object would have the power of the Hiroshima atomic bomb. They arrive at a rate of a hundred per year and would typically yield a fireball in the upper atmosphere. Hundred meter sized objects would have the power of fifteen millions tons of explosive, a thousand Hiroshima's, arrive at a rate of one per hundred years. Tunguska in 1908 was the last known occurrence. A kilometer sized object has the equivalent explosive energy of the world's arsenals, a billion tons of explosive, arriving at the rate of one per million years, would be equivalent in its devastating effect to a global nuclear war leaving a ten kilometer crater and leveling the area the size of a moderate country. Hundred meter waves would circle the Earth. The largest impactor might be ten kilometers with catastrophic effects for the Earth, but a very rare arrival rate of one per few hundred million years. There's evidence that the death of the dinosaurs and other species 65 million years ago was caused by such a huge impact from space.

Transcript: Interplanetary debris is a negligible component of the mass of the entire solar system, most of which resides in the Sun. The sum of all the planets is about one percent of the mass of the solar system. The sum of all the debris in space, all the interplanetary bodies of every type, is only one thousandth of a percent of the mass of the whole solar system. Nonetheless, when these pieces of debris hit the Earth they can have severe consequences. Because of attrition and disruption of large bodies into small bodies there are many smaller pieces of debris than there are large pieces. Thus, the impact rate depends on the size of the debris. Arriving at the outside of the Earth's atmosphere, the rate varies in the following way: one meter size objects reach the outside of the Earth's atmosphere about once per hour, ten meter size objects about once per year, a hundred meter size objects once every hundred years, kilometer size objects once every million years, and ten kilometer sized objects once every hundred million years. Impactors larger than ten kilometers are exceptionally rare, although asteroids go up in size all the way to a thousand kilometers.

Transcript: The odds of being hit by a meteorite are incredibly small. In pre-history we don't know what happened, but in the last few hundred years some interesting events have occurred. A monk in Milan, Italy, was killed while praying in 1650. In 1911 a dog was killed in Nakhla, Egypt, by a meteorite that was probably a Martian meteorite. In 1838 an Illinois woman heard a crash in her garage and found a meteorite lying on the cushion of her car seat. Mrs. Hodges, a woman living in Alabama, was injured in 1954 on the hip by a meteorite that came through her roof and ricocheted on to her. In 1971 a house in Wethersfield, Connecticut, was hit by a meteorite, and eleven years later another house just a mile away was hit. In 1991 two boys in Noblesville, Indiana, heard whistling and a thud and found a small crater on the sidewalk with a meteorite still warm to the touch. In 1992 Michelle Knapp’s 1980 Malibu was destroyed by a meteorite that melted into the car. She was offered $69,000 for the bolide, far more than the car was worth.

Transcript: Stony and carbonaceous meteorites are the most primitive kind because they form unmelted material that gives information from the earliest phases of the solar system. The technique of radioactive dating has been used to show that these primitive meteorites formed over a period of only 20 million years about 4.6 billion years ago. Thus, the entire process in the solar nebula of aggregation of material from microscopic dust grains to solid rock-like objects took a very short time. If the history of the solar system were compressed to a year this process took only about a day.

Transcript: Meteorites come in three basic types: stony, metallic, and carbonaceous. These types mirror the basic types of asteroids, and in fact, there is good evidence that meteorites are fragments of asteroids. A particular type of meteorite is a brecciated meteorite which occurs when two different rocks of different types have been smashed together by pressure or collision into one stone, indicating that asteroids collide, and meteorites can be fragments of collided asteroids. Of all the meteorite finds and falls on the Earth, 80 percent are the stony variety that have never melted which means their parent bodies were never hot enough or close enough to the Sun to have melted. About 10 percent are lava-like basaltic material that has melted and then solidified, and a small percentage, only four or five percent, are almost pure iron/nickel. These of course, are the easiest to find because they differ from the surrounding rocks of the terrestrial landscape.

Transcript: Objects that fall from the sky have long been a source of awe and wonder. The Winona stony meteorite was buried in a crypt by prehistoric Native Americans in northern Arizona. A stony meteorite was venerated thousands of years ago in the temple of Diana at Ephesus, one of the seven wonders of the ancient world. A black stony meteorite was placed in the Muslim shrine at the Kaaba in Mecca. In 1492 French emperor Maximilian started a crusade after a meteorite fell. The meteorite sits in the Ensisheim church in France.

Transcript: When a rocky or metallic interplanetary body hits the Earth we call it a meteorite. Meteorites are free samples of debris left over from the formation of the solar system. Planetary scientists use them in a form of archeology to speculate meaningfully about the processes that led to the formation of the Earth and the other planets. The best evidence suggests that meteorites are the fragments of Earth crossing asteroids ejected from the main asteroid belt.

Transcript: The composition of asteroids tells us about their history over the age of the solar system. The fact that composition depends on distance from the Sun means asteroids are not all mixed up; they lie at their original distances of formation. Those closest to the Sun have melted and got iron cores. Those farthest from the Sun contain carbonaceous materials, water, and even ice. Asteroids have a complex geological history. Some asteroids have melted and re-solidified to have iron and nickel cores, mantels of dense rock, and crusts of basaltic lava, just like the Earth. Others have had more quiescent histories and are essentially commentary material.

Transcript: The main asteroid belt runs from about 2.1 astronomical units to 3.5 astronomical units from the Sun. There are two major dividing lines that affect the composition of asteroids. At about 2.7 astronomical units is the soot line. Beyond this distance the composition of asteroids is mostly dark, carbon-rich material, or soot. A less distinct dividing line is the frost line at a distance of 3 to 4 astronomical units. Beyond this distance asteroids can contain a significant amount of frozen water and other volatiles. Beyond a distance of 5 astronomical units most interplanetary bodies have the composition like a comet nucleus.

Transcript: Although most asteroids in the main belt are found at distances between two and three astronomical units from the Sun, their distribution with radius is neither random nor uniform. Structure in the asteroid belt radially was discovered by Daniel Kirkwood the astronomer in 1866. In particular he found regions where there were gaps in the distribution. These gaps correspond to gravitational resonances with the orbit of Jupiter where the orbital period of the asteroid is a fixed fractional ratio to the orbit of Jupiter. Thus Jupiter clears out gaps in the asteroid belt much as Mimas clears out gaps in the rings of Saturn. The most prominent gaps are at fractional orbit ratios of one half, two-thirds, two-fifths, and three-sevenths.

Transcript: Trojan asteroids are a subgroup located outside the region of the main belt. There are two swarms in the orbit of Jupiter located sixty degrees ahead and sixty degrees behind the planet as it orbits the Sun. These particular positions are called Lagrangian points, places where the asteroids are held by a combination of the gravity force of the Sun and Jupiter. A hundred or more are known. Most are 50 kilometers or larger in size. They’re all named after Homer’s epic poem about the Trojan War. The largest is 624 Hector which is 100 by 300 kilometers in size.

Transcript: A close up of the asteroid 243 Ida by the Galileo space craft revealed a surprise, a one kilometer moon orbiting the 50 kilometer main body. A number of asteroids have been found that have moons orbiting them by direct observation or imaging. This fact explains an older mystery, the fact that on the Moon, Earth, and Mars it's not unusual to see pairs of craters with exactly the same age. This is now explained as an asteroid and its surrounding moonlet impacting at the same time. Based on these statistics, ten to twenty percent of asteroids have sizable moonlets. In the 1980s and 19990s the technique of radar imaging was used to reveal another surprise, the presence of compound asteroids, asteroids with dumbbell shapes or where two or three rounded, mountain-sized rocks were in close proximity held together and nudging and bumping each other by gravity.

Transcript: Asteroids are rocky and metallic objects that are found throughout the solar system, but most of them are concentrated between the orbits of Mars and Jupiter in the main asteroid belt. Asteroids are in a position in the solar system predicted by Bode’s rule, and in fact they form their debris of a planet which failed to form. The sum of the mass of all the planets in the asteroid belt is only about five-hundredths of a percent of the mass of Earth, and there are over one hundred thousand known. The largest asteroid is 1,000 kilometers across. Many are more than 100 kilometers in size and they range down in size to a few meters.

Transcript: Interplanetary debris becomes visible when it hits the Earth’s atmosphere due to the conservation of energy. The large kinetic energy of the incoming projectile is turned into heat and light as it slows down in the thicker Earth's atmosphere. The smallest pieces of debris create meteors and meteor showers. Much more occasional and large pieces of debris create spectacular events called fireballs. Fireballs usually explode in the upper atmosphere rather than hitting the ground. They have been mistaken for UFO's and even in the 1960s by both the Russians and the Americans as violations of the nuclear test ban treaty. Chinese fireball records date back 2000 years, and there is a clear association of many fireballs with the exact incidence of meteor showers which indicates that fireballs themselves are also due to commentary debris.

Transcript: The smallest interplanetary particles are microscopic dust grains concentrated towards the Sun and spread out in the plane of the solar system. If you look west in a clear rural sky far from city lights as the last glow of the sunset disappears, you can see a diffuse glow of light reflecting off these tiny particles; it’s called the zodiacal light. The zodiacal light is best visible about an hour and a half after sunset or the same amount before sunrise. It moves up the horizon, following the zodiac and along the ecliptic, and is one of the most impressive but subtle features of the night sky.

Transcript: Meteors arriving directly into the Earth's atmosphere or from above burn up the fastest and leave the most spectacular and brightest trails, so we are more likely to see meteors that arrive head-on into the Earth's atmosphere than those that come in at shallow or glancing angles. This creates a perspective illusion where the meteor shower appears to diverge from a fixed point in the sky. For a particular meteor shower, the point where this center appears is named after the constellation that gives the meteor shower its name.

Transcript: On a moonless night from a dark observing site you can typically see several meteors per hour if you look carefully. Just before dawn and in the early morning the frequency goes up to about 10 to 15 meteors per hour. What causes this? The meteor impacts arrive randomly at the outer Earth's atmosphere. But the Earth is spinning and orbiting the Sun, and there's an increased likelihood that an object will create a visible light event in the upper atmosphere if that object is on a head-on collision with the Earth as it spins and moves in its orbit. This corresponds to the time between midnight and dawn. Thus, the frequency of most meteor showers increases in the dawn hours as well. On a typical meteor shower, the frequency is about 50 to 100 meteors per hour, but occasionally when the comet debris is large in that part of the orbit, a spectacular increase can bee seen. The most famous meteor shower of all was the Leonid event of November 17, 1966, when a blizzard of meteors arriving at a rate of 2000 per minute was visible for over an hour.

Transcript: In 1866, Schiaparelli, the man who also made observations of canals on Mars, discovered that the Perseid meteor shower occurred whenever Earth crossed the debris trail of a particular comet. Since then it has become clear that meteor showers are caused by situations where the Earth crosses the path of a comet. Comets leave debris along the trails of their elliptical orbits, and when the Earth passes through this debris it increases the frequency of impacts of small interplanetary bodies in the upper atmosphere. Because the debris is not uniformly strewn through the comet orbit, the intensity of meteor showers will vary from year to year. In 1983 the Infrared Astronomical Satellite showed clearly the debris tail associated with the Temple 2 comet. Other famous meteor showers are associated with famous comets. For example, the Taurid shower is associated with comet Encke, and the Orionid shower is associated with comet Halley.

Transcript: On a typical night you can see about one or two meteors per hour. Certain days of the year, however, the frequency goes up and you can see several per minute. These famous meteor showers are identified by the constellation in which they appear to come from. The most prominent meteor showers around the calendar year are the Lyrids which occur in the morning of April 21, the Perseids on the morning of August 12, the Draconids on the evening of October 10, the Orionids on the morning of October 21, the Taurids around midnight on November 7, the Leonids in the morning of November 16, and the Geminids in the morning of December 12.

Transcript: On a typical night you can see about one or two meteors per hour. Certain days of the year, however, the frequency goes up and you can see several per minute. These famous meteor showers are identified by the constellation in which they appear to come from. The most prominent meteor showers around the calendar year are the Lyrids which occur in the morning of April 21, the Perseids on the morning of August 12, the Draconids on the evening of October 10, the Orionids on the morning of October 21, the Taurids around midnight on November 7, the Leonids in the morning of November 16, and the Geminids in the morning of December 12.

Transcript: Newton's law of gravity is written for the idealized situation of two bodies in space with no other gravitational influences, but in the solar system, as in elsewhere in astronomy and in the universe, there are often more than two objects involved. Typically we talk about the sphere of gravitational influence of an object which is the region around the Sun, say, or a planet where the planet or the Sun totally dominates the motions of smaller objects, but in general, when we calculate an orbit we have to be aware of gravitational perturbations. A perturbation is a small amount of gravitational force added by a third, fourth, or additional object. The perturbation can be a small kick or extra force on a planet orbit, but it can add up to a substantial effect. One of the reasons that comet orbits are hard to predict and chaotic is because they suffer perturbations or small extra gravitational forces occasionally through their lifetimes.

Transcript: Most orbits in the solar system are regular and repeatable. The orbits of the planets, for example, change very little over periods of hundreds of millions of years. Comet orbits, however, can be quite different. Comets are examples of solar system objects where the orbits can be chaotic. A chaotic orbit is an orbit where a slight change in the initial conditions or a chance encounter leads to a vastly different outcome. For example, if a comet heading in from the Oort cloud happens to chance near one of the giant outer planets, if its far from the outer planet its orbit will not be noticeably affected. But, if it has a near grazing incidence with an outer planet, its orbit may be deflected and entirely changed, and the whole course of the future history of that comet will be different, perhaps leading to an encounter with the Earth for example. This chaotic situation where there's no predictability of the future of the orbit is a typical situation for small bodies in the solar system.

Transcript: The life story of comets begins 4.6 billion years ago in the outer regions of the solar system as ice crystal and carbon rich dust grains aggregate into the nuclei of comets. Comets mostly exist beyond the orbit of Pluto and formed in the plane of the solar system. Many comets, however, were flung into the Oort cloud due to interactions with giant planets. Comets therefore are stored in the Oort cloud spending most of there time at huge distances from the orbits of the planets in a spherical swarm around the mostly planar orientation of the inner solar system. They are in complex interaction with planets as they come in the inner solar system, and many of these orbits modify to form short period comets. Halley's Comet is one example. In general, the life of a comet is a complex gravitational ballet involving interactions with planets in the solar system. The other thing that happens for comets that penetrate to the inner solar system is the gradual removal of icy materials or volatiles due to interaction with the radiation of the Sun, turning a comet from an icy ball of rock into a situation more like pure rock.

Transcript: The comets we occasionally see as spectacular apparitions in the night sky are occasional visitors from the outer reaches of the solar system. In the 1950s the Dutch astronomer Jan Oort speculated about comets orbits and hypothesized the existence of a huge spherical cloud of comets spanning a distance of 50 to 100,000 astronomical units on highly elliptical orbits that took ten to sixty million years. Evidence for this was the fact that most comets are one time visitors to the inner solar system, that they arrive from any direction in the sky, and that by Kepler’s law they must spend most of their elliptical orbits at large distances from the Earth. The Oort cloud is therefore a hundred billion strong repository in a spherical halo of dead frozen comets awaiting their quick journeys into the inner solar system.

Transcripts: In 1866, Schiaparelli, the man who also made observations of canals on Mars, discovered that the Perseid meteor shower occurred whenever Earth crossed the debris trail of a particular comet. Since then it has become clear that meteor showers are caused by situations where the Earth crosses the path of a comet. Comets leave debris along the trails of their elliptical orbits, and when the Earth passes through this debris it increases the frequency of impacts of small interplanetary bodies in the upper atmosphere. Because the debris is not uniformly strewn through the comet orbit, the intensity of meteor showers will vary from year to year. In 1983 the Infrared Astronomical Satellite showed clearly the debris tail associated with the Temple 2 comet. Other famous meteor showers are associated with famous comets. For example, the Taurid shower is associated with comet Encke, and the Orionid shower is associated with comet Halley.

Transcript: Comets travel on highly elliptical orbits. From Kepler's laws we know that the comet must travel faster when it's close to the Sun. Kepler's third law also relates the orbital period to the semi-major axis of the orbit. For Halley's Comet, with an orbital period of about 75 years, its most distant point is at 36 AU from the Sun, near the orbit of Uranus. However, for a long period comet with a maximum distance of perhaps 100,000 astronomical units, its period is millions of years. The consequence of Kepler's laws is that when the comet is close to the Earth or in the inner solar system it is traveling very fast, perhaps 100,000 miles or greater, but in the outer part of its orbit where it spends the vast majority of its time it's traveling slowly, one a few hundred miles per hour. As a result, for every comet that we see in the inner solar system there must be a vastly larger number in the reservoir of comets in deep outer space at a distance far too large to see.

Transcript: The current model of a cometary nucleus is called the dirty iceberg model, first proposed 50 years ago by Fred Whipple. The icy nucleus is a composite with rock, and comets can break up under gravitational forces or under the pressure of expanding frozen gases. Shoemaker-Levy 9 near Jupiter was an example of a comet breaking up under the gravity force of the giant planet. The icy material in a comet nucleus is composed primarily of frozen water, methane, ammonia, and carbon dioxide. In 1986 a fleet of spacecraft got very close to comet Halley, and Giotto in particular was able to take beautiful pictures of the comet nucleus showing that it was in fact very dark and covered with dust as dark as black velvet reflecting only four percent of the light from the Sun. Different comet nuclei may be darker, caused by dust material, or brighter, caused by frozen gases.

Transcript: English astronomer William Huggins was the first to provide evidence on a physical nature of comets. Using a spectrograph in 1869, he showed that the composition included gaseous carbon compounds. More modern observations have cataloged many molecules in the nucleus of a comet including many different combinations of the basic elements, hydrogen, oxygen, nitrogen, and carbon. Among the molecules or ions that have been detected are CN, CH, OH, H2O, CN, CH, OH, N2, CO, and CO2. More recently complex organics such as CH2CN have been discovered. This is exciting because some of these are the basic building blocks of life.

Transcript: English astronomer Edmond Halley was the first person to show that comets are repeatable astronomical phenomena. Halley was a friend of Newton, the man to persuaded Newton to publish his master work on gravity, The Principia. Halley paid the publication costs himself and personally sent copies of the book to scientists around Europe. Halley made careful observations of comets himself, and he identified four consecutive sightings with one single visitor and predicted the return for the comet that would bare his name. In 1758 it appeared on Christmas day as predicted. Halley's Comet has appeared most recently in 1910 in a spectacular visit when we passed through its tail and less spectacularly in 1986.

Transcript: In ancient times, before science explained the phenomenon, comets were interpreted as evil omens. The appearance of Halley's Comet in 66 AD heralded the destruction of Jerusalem. Five orbits later it was said to mark the defeat of Attila the Hun in 451. In 1066 it presided over the Norman conquest of England. And in 1456 its appearance coincided with the threat of invasion of Europe by the Turks, and Pope Callixtus III prayed for deliverance from the devil, the Turk, and the comet.

Transcript: Two important points in any elliptical orbit in the solar system or beyond are perihelion and aphelion. They come from the Greek roots “peri” meaning close, “ap” meaning far, “helion” related to Helios, the word for the Sun. Perihelion is the closest approach of an object to the Sun, and aphelion is its most distant place. The planets in the solar system have perihelions and aphelions that are close to each other because their orbits are slightly elliptical. In the case of a comet, perihelion is a much smaller distance than aphelion, and a comet is most active when it is closest to the Sun.

Transcript: At the heart of a comet is its nucleus, a tiny world of rock and ice somewhere between one and twenty kilometers across, about the size of a small city. Gas and dust emitted from the nucleus form a halo around the nucleus called the comets head or coma. The size of the coma can be larger then the largest planet, but the gas is very diffuse in this region. The most spectacular part of a comet is its tail, a faint extended glowing region of gas that goes away from the nucleus and points away from the direction of the Sun. The tail of a comet can be more than one AU across, larger than the distance from the Earth to the Sun, and on the sky a nearby comet can have a tail that extends ten or twenty degrees across the night sky.

Transcript: Comets have highly elliptical orbits around the Sun, and their activity depends on their distance from the Sun. At its most distant point, comets are usually far beyond the orbit of Pluto, and they're inactive. Their surfaces may be covered with dark dust from interplanetary space. As a comet speed up in its orbit and approaches the orbit of Jupiter, distance of four to six astronomical units, it starts to become active the volatile ices start to boil off. A comet will reach its peak activity around the orbit of the Earth, about one astronomical unit from the Sun, when volatile gases boil off and create the extensive tail that can extend a distance of one or two astronomical units. At its closest point to the Sun, perhaps less than an astronomical unit, the comet is active and in fact may even break up. As it passes by the Sun and begins to slow down, it becomes less active. The tail, still pointing away from the Sun, recedes until once again beyond the orbit of Jupiter the comet becomes inactive for the bulk of its orbit spent far beyond the distance of Pluto.

Transcript: Comets are spectacular visitors to the inner solar system. They reach here on highly elliptical orbits from the outer reaches of the solar system far beyond the orbit of Pluto. A bright comet appears visible from the Earth without the benefit of binoculars about once per decade. The comets appear to drift slowly from night to night among the fixed stars. Brahe used observations of a comet to show that its orbit must pass between the planets’ orbits, thus destroying the Greek idea of crystalline spheres.

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.

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.

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: 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.

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.

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.