Introduction

In 350 b.c.e., Aristotle, the great Greek philosopher, published De

Caelo (On the heavens), one of the most influential books in the history of science. Aristotle wrote that the Earth is the center of the solar system and that all the other bodies in the solar system orbit the Earth while set into a complex series of spheres. In his philosophy, the Earth was the only site of change, of birth, life, death, and decay, while the heavenly spheres were perfect and eternally changeless. Aristotle's system of spheres could not be reconciled with the apparent motions of the planets, which include speeding up, slowing down, and even moving backward (known as retrograde motion). About 500 years later, Ptolemy, a second Greek philosopher, published his masterwork He Megale Syntaxis (Almagest, or The mathematical compilation). In the Almagest, Ptolemy develops a complex mathematical explanation for the apparent movement of the planets and thus supports the Aristotelian theory sufficiently for it to live on as the primary theory for the structure of the solar system for almost 2,000 years. The Aristotelian and Ptolemaic theory of an Earth-centered solar system thus may be one of the longest-lived theories in the history of science.

In 1514 c.e., Nicolaus Copernicus, a Prussian scientist and canon in the catholic church, published a tiny unsigned pamphlet called the Commentariolus (Little commentary). In this little handwritten book, he posits that the center of the universe is not the Earth but near the Sun; that the distance from the Earth to the Sun is imperceptible compared to the distance to the stars; and that the apparent retrograde motion of the planets is due to observing them from the orbiting Earth. These axioms, now all known to be absolutely true, were earthshaking at the time (Copernicus delayed a full publication of his theories until just before his death in 1543, when De Revolutionibus Orbium [On the revolutions of the celestial spheres] was published with the help of an assistant). Galileo Galilei, the great Italian astronomer who lived from 1564 to 1642, was a great advocate of the Copernican system, for which he xix was condemned by the Inquisition of heresy and lived the remainder of his life under house arrest. Galileo was fortunate not to have been executed for his belief that the universe did not orbit the Earth. Galileo's trial, held in 1633, further drove any Copernicans in Catholic countries into secrecy, and the theory was not held by a majority of natural philosophers until the late 17th century. The development of physics and in particular the theory of gravitation showed that only the Sun could be the center of the solar system, and slowly the world of science was won over to a Sun-centered system.

The Sun's huge mass (greater than 99 percent of the total mass of the solar system) controls the orbits of all the planets and smaller bodies of the solar system, and its radiation and magnetic field dominate the space environments of the inner planets and define the size of the solar system itself. The Sun, therefore, takes its deserved place at the beginning of this six-book set, which covers the state of scientific knowledge of the solar system's planets and smaller bodies, starting from the center, the Sun, and moving outward, as shown in the depiction of the solar system on page xxi.

Had Jupiter been 15 times more massive, it also would have begun nuclear fusion reactions and become a small star itself. Instead, the Sun is the sole star of this solar system. The intense heat of fusion and its complex and powerful magnetic fields are just beginning to be understood, though the importance of the Sun's energy has been recognized by mankind since prehistory. A description of the current thoughts on solar system formation is included in this part, as well as the changing effect of the Sun's energy on the planets as the Sun evolves.

Mercury, the topic of the second part of this volume, is the subject of intense scientific attention this year because of the launch of the Messenger mission. MESSENGER is only the second spacecraft to visit the planet (Mariner 10, which flew by the planet three times in 1974 and 1975, was the first), and its advanced instruments should produce invaluable information about the planet.

Though Mercury is far closer to the Earth than Jupiter or Saturn, more is known about those distant gas-giant planets than about Mercury. Mercury is the least visited of terrestrial planets, as shown in the figure on page xxii. Mercury, Uranus, and Neptune have each been visited one time by space missions, and only Pluto has never been visited. Since space missions deliver the bulk of information about planets, Mercury is little-known.

All Orbits: Asteroid Belt, Kuiper Belt, Oort Cloud

Asteroid belt Mercury Venus Earth Mars

Asteroid belt Mercury Venus Earth Mars

This book covers the innermost solar system, including the Sun, Mercury, and Venus, whose orbits are highlighted here. All the orbits are far closer to circular than shown in this oblique view, which was chosen to show the inclination of Pluto's orbit to the ecliptic.

The approximate number of successful space missions from all nations shows that the Moon is by far the most visited body, only Pluto has had no missions, and Mercury is as neglected as Uranus and Neptune. The definition of a successful mission is arguable, so totals for Mars and the Moon in particular may be disputed.

Successful Space Missions to Solar System Bodies

J Moon

.Venus ^ ^Jupiter

>Mercury ^Saturn Uranus

I Neptune Pluto

20 30

AU from the Sun

Mercury is exceptionally difficult to see because, from the point of view of the Earth, it is always close to the Sun. The Sun's brightness either damages instruments that attempt to view Mercury or simply makes Mercury a dim and ill-resolved speck in comparison. Mercury's orbit shows that is it unusually heavy, compared to the other terrestrial planets (Venus, Earth, and Mars), probably due to iron enrichment. Mercury also has a magnetic field, which is unexpected for a planet without internal movements manifested by volcanic activity. (Mercury appears to have had no volcanic activity for billions of years.) Mercury's density and its magnetic field are mysteries scientists hope to solve with the help of data from Messenger and the planned European mission BepiColombo.

The last part of this volume discusses Venus, the sister planet to the Earth in size and distance to the Sun, though the two planets could not be more different in other ways. Lacking surface water and plate tectonics, Venus's surface is characterized by immense, roughly circular features sometimes hundreds of kilometers across volcanic and tectonic centers bounded by faults, some depressed and others raised in their centers.Venus's surface temperature is a blazing 855°F (457°C), hotter even than Mercury. The surface is dominated by dense clouds of acid that create a pressure about 90 times that of the Earth's.Venus thus forms a perfect laboratory for studying the long-term effects of small differences in climate. It is the product of runaway greenhouse heating, where the Sun's energy is trapped within the atmosphere rather than reflected or radiated back into space. This same greenhouse effect is occurring on Earth, with the alterations in the atmosphere caused by humankind. Though humans may eventually drive the climate to dangerously high temperatures, it will not reach the extremes of Venus, where a human visitor would be simultaneously crushed by the atmospheric pressure, burned by its heat, and asphyxiated by its acids. The differences between Earth and Venus are exceptional, considering their similar positions in the solar system.Venus is also the subject of intense scientific interest and will receive more spacecraft visits in upcoming decades, adding to the 16 missions that have thus far taken images or sent probes into the planet's atmosphere.

The conditions on Mercury and Venus today are the direct consequences of orbiting so close to the immensely powerful Sun, which baked and stripped Mercury of its atmosphere and which created the intense greenhouse heat on Venus. As the Sun's energy output changes over the coming hundreds of millions of years, the climates on the Earth and Mars will change as well.

Part One: The Sun

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