|Matter | Pages 76-77 ||
By comparing celestial objects with everyday things, the unimaginable distances in the cosmos grow slightly more tangible.
For starters, consider our Earth and Moon. Our companion in space has one- eightieth the mass of Earth, making the EarthMoon system verge on being a double planet. If we shrank the system so that Earth was the size of a basketball, the Moon would be the size of a softball about 30 feet away. That distance corresponds to the farthest we have ever sent people into space. On this same scale, Mars (at its closest) is a mile away from Earth. Making the leap to send astronauts to Mars may seem like a simple extension of the Apollo missions, but hopping 30 feet is nothing at all like hopping a mile.
The basketball analogy also is useful for picturing the scale of our solar system. But this time the basketball-sized object will be our Sun. We'll mix our sports analogies just a bit and put the basketball-Sun at home plate on a baseball diamond. On that scale the innermost planet, Mercury, would look like a pellet of birdseed orbiting at a distance of 35 feet--halfway to the pitcher's mound. Earth becomes the size of a sunflower seed about 90 feet away, the distance to first base. Jupiter is a one-inch marble 450 feet away, just over the fence in center field. And tiny Pluto, a grain of pepper, orbits in isolation about 3,500 feet away, far beyond the parking lot. Throw in five more planets and scatter some sand to represent asteroids and you have our model solar system: a sphere more than a mile across containing a basketball at its center, flecks of matter here and there, and lots of empty space.
A vast swarm of trillions of comets, called the Oort Cloud, probably surrounds the solar system as well. The comets drift in cold storage in the depths of space, perhaps as far away as 1,000 times the distance of Neptune from the Sun. In our model that's like dust motes in Baltimore orbiting a basketball-Sun in New York. The bond keeping the comets in orbit around the Sun is exceedingly weak, but it's a testament to the long-range strength of gravity.
The true eye-opener comes when we consider the distances between stars. Our basketball-Sun in New York would have as its nearest neighbor another basketball in Honolulu--5,000 miles away. Except for a few clouds of gas, the space in between is largely devoid of matter. Hollywood movies often show starships cruising through the galaxy, drifting past stars like fireflies at the rate of one or two every second. But the gaps between stars in the galaxy are so great that the vessels would have to travel up (continued)