While the planets move in elliptical orbits around the Sun, their orbits are not very elliptical. At first glance they are, by and large, indistinguishable from circles. It is the comets – especially the long-period comets – that have dramatically elliptical orbits. The planets are the old-timers in the inner solar system; the comets are the newcomers. Why are the planetary orbits nearly circular and neatly separated one from the other? Because if planets had very elliptical orbits, so that their paths intersected, sooner or later there would be a collision. In the early history of the solar system, there were probably many planets in the process of formation. Those with elliptical crossing orbits tended to collide and destroy themselves. Those with circular orbits tended to grow and survive. The orbits of the present planets are the orbits of the survivors of this collisional natural selection, the stable middle age of a solar system dominated by early catastrophic impacts.
In the outermost solar system, in the gloom far beyond the planets, there is a vast spherical cloud of a trillion cometary nuclei, orbiting the Sun no faster than a racing car at the Indianapolis 500.* A fairly typical comet would look like a giant tumbling snowball about 1 kilometer across. Most never penetrate the border marked by the orbit of Pluto. But occasionally a passing star makes a gravitational flurry and commotion in the cometary cloud, and a group of comets finds itself in highly elliptical orbits, plunging toward the Sun. After its path is further changed by gravitational encounters with Jupiter or Saturn, it tends to find itself, once every century or so, careering toward the inner solar system. Somewhere between the orbits of Jupiter and Mars it would begin heating and evaporating. Matter blown outwards from the Sun’s atmosphere, the solar wind, carries fragments of dust and ice back behind the comet, making an incipient tail. If Jupiter were a meter across, our comet would be smaller than a speck of dust, but when fully developed, its tail would be as great as the distances between the worlds. When within sight of the Earth on each of its orbits, it would stimulate outpourings of superstitious fervor among the Earthlings. But eventually they would understand that it lived not in their atmosphere, but out among the planets. They would calculate its orbit. And perhaps one day soon they would launch a small space vehicle devoted to exploring this visitor from the realm of the stars.
* The Earth is r = 1 astronomical unit = 150,000,000 kilometers from the Sun. Its roughly circular orbit then has a circumference of 2pr »109 km. Our planet circulates once along this path every year. One year = 3 x 107 seconds. So the Earth’s orbital speed is 109 km/3 x 107 sec »30 km/sec. Now consider the spherical shell of orbiting comets that many astronomers believe surrounds the solar system at a distance »100,000 astronomical units, almost halfway to the nearest star. From Kepler’s third law it immediately follows that the orbital period about the Sun of any one of them is about (105)3/2 = 1075 »3 x 107 or 30 million years. Once around the Sun is a long time if you live in the outer reaches of the solar system. The cometary orbit is 2pa = 2p x 105 x 1.5 x 108 km »1014 km around, and its speed is therefore only 1014 km/ 1015 sec = 0.1 km/sec »220 miles per hour.
Sooner or later comets will collide with planets. The Earth and its companion the Moon must be bombarded by comets and small asteroids, debris left over from the formation of the solar system. Since there are more small objects than large ones, there should be more impacts by small objects than by large ones. An impact of a small cometary fragment with the Earth, as at Tunguska, should occur about once every thousand years. But an impact with a large comet, such as Halley’s Comet, whose nucleus is perhaps twenty kilometers across, should occur only about once every billion years.
When a small, icy object collides with a planet or a moon, it may not produce a very major scar. But if the impacting object is larger or made primarily of rock, there is an explosion on impact that carves out a hemispherical bowl called an impact crater. And if no process rubs out or fills in the crater, it may last for billions of years. Almost no erosion occurs on the Moon and when we examine its surface, we find it covered with impact craters, many more than can be accounted for by the rather sparse population of cometary and asteroidal debris that now fills the inner solar system. The lunar surface offers eloquent testimony of a previous age of the destruction of worlds, now billions of years gone.
Impact craters are not restricted to the Moon. We find them throughout the inner solar system – from Mercury, closest to the Sun, to cloud-covered Venus to Mars and its tiny moons, Phobos and Deimos. These are the terrestrial planets, our family of worlds, the planets more or less like the Earth. They have solid surfaces, interiors made of rock and iron, and atmospheres ranging from near-vacuum to pressures ninety times higher than the Earth’s. They huddle around the Sun, the source of light and heat, like campers around a fire. The planets are all about 4.6 billion years old. Like the Moon, they all bear witness to an age of impact catastrophism in the early history of the solar system.
As we move out past Mars we enter a very different regime – the realm of Jupiter and the other giant or Jovian planets. These are great worlds, composed largely of hydrogen and helium, with smaller amounts of hydrogen-rich gases such as methane, ammonia and water. We do not see solid surfaces here, only the atmosphere and the multicolored clouds. These are serious planets, not fragmentary worldlets like the Earth. A thousand Earths could fit inside Jupiter. If a comet or an asteroid dropped into the atmosphere of Jupiter, we would not expect a visible crater, only a momentary break in the clouds. Nevertheless, we know there has been a many-billion-year history of collisions in the outer solar system as well – because Jupiter has a great system of more than a dozen moons, five of which were examined close up by the Voyager spacecraft. Here again we find evidence of past catastrophes. When the solar system is all explored, we will probably have evidence for impact catastrophism on all nine worlds, from Mercury to Pluto, and on all the smaller moons, comets and asteroids.
There are about 10,000 craters on the near side of the Moon, visible to telescopes on Earth. Most of them are in the ancient lunar highlands and date from the time of the final accretion of the Moon from interplanetary debris. There are about a thousand craters larger than a kilometer across in the maria (Latin for ‘seas’), the lowland regions that were flooded, perhaps by lava, shortly after the formation of the Moon, covering over the pre-existing craters. Thus, very roughly, craters on the Moon should be formed today at the rate of about 109 years/104 craters, = 105 years/crater, a hundred thousand years between cratering events. Since there may have been more interplanetary debris a few billion years ago than there is today, we might have to wait even longer than a hundred thousand years to see a crater form on the Moon. Because the Earth has a larger area than the Moon, we might have to wait something like ten thousand years between collisions that would make craters as big as a kilometer across on our planet. And since Meteor Crater, Arizona, an impact crater about a kilometer across, has been found to be twenty or thirty thousand years old, the observations on the Earth are in agreement with such crude calculations.
The actual impact of a small comet or asteroid with the Moon might make a momentary explosion sufficiently bright to be visible from the Earth. We can imagine our ancestors gazing idly up on some night a hundred thousand years ago and noting a strange cloud arising from the unilluminated part of the Moon, suddenly struck by the Sun’s rays. But we would not expect such an event to have happened in historical times. The odds against it must be something like a hundred to one. Nevertheless, there is an historical account which may in fact describe an impact on the Moon seen from Earth with the naked eye: On the evening of June 25, 1178, five British monks reported something extraordinary, which was later recorded in the chronicle of Gervase of Canterbury, generally considered a reliable reporter on the political and cultural events of his time, after he had interviewed the eyewitnesses who asserted, under oath, the truth of their story. The chronicle reads:
There was a bright New Moon, and as usual in that phase its horns were tilted towards the east. Suddenly, the upper horn split in two. From the midpoint of the division, a flaming torch sprang up, spewing out fire, hot coals, and sparks.