## Distance in the Solar System and Beyond

Unfortunately, we quickly lose our ability to understand, let alone comprehend, distances relevant in describing the scale of our solar system. The Earth is approximately 93,000,000 miles from the Sun. It we could take the Space Station and send it sunward, still traveling at 17,000 miles per hour, it would take 227 days to reach its destination. And sending it on a direct trajectory to the Sun is not possible due to the fact that everything in the solar system, including the Earth, is in orbit around the Sun, thereby making any craft follow a curved path, or trajectory, as it spirals to its destinationâ€”which can dramatically increase the total distance to be traversed. Since it is arduous to calculate a trajectory, and hence the actual distance to be traveled, and since this only makes the distance problem worse, we will ignore it for illustrative purposes without compromising the intent of our discussion.

Mars is about 1.5 times as far from the Sun as is the Earth. Since Earth and Mars have different orbital periods (the time it takes to complete one revolution around the Sun), the distance between the two planets varies between approximately 40,000,000 miles and 235,600,000 miles. A spacecraft launched from the Earth to explore Mars takes approximately 9 months to cover this distance on an optimal trajectory. Now we have a realistic interplanetary distance and a realistic timeframe based on actual flight mission history on which to base our discussion. Nine months is a long time, but not so long as to be discouraging. Our continental ancestors took this period of time to get from the East to the West coasts, and they did it readily. Surely we are now on the way to making the solar system our backyard! Not so fast...

Jupiter, the largest of all the planets, is 482,500,000 miles from the Earth, or 3.5 times farther away than Mars (a straight-line distance, not in terms of a required trajectory). The recently completed Galileo mission, following an optimized trajectory, took 5 years to reach its destination! Pluto, the most distant planet in the solar system, is 39.5 times the Earth-Sun distance from the Sun. (A straight-line distance of approximately 3,666,000,000 miles.) The New Horizons mission to Pluto launched in 2006 will not arrive at Pluto until 2015!

It is at this point that most people's eyes begin to fog over and they lose comprehension and interest in understanding the distances involved. After all, who can experientially understand the difference between 3.6 billon miles (the Pluto-Sun distance) and a mere 93 million miles (the Earth-Sun distance)? We then throw into the mix the knowledge that the distance to the nearest star, in the Alpha Centauri system, is 24,790,771,495,138 miles awayâ€”and that is the distance to the nearest star! To put that in units of millions of miles, it is approximately 24,000 million million miles away. Now we've all lost our understanding of the distance and confusion and despair seem a likely outcome.

Fortunately, there are ways to comprehend this. One way is to use the mathematical construct of logarithms. On a logarithmic scale, each incremental unit of distance is 10 times longer than the previous one. Using this notation, the distance from the Sun to the nearest star system

Heliosphere

Interstellar Medium

WiAS/1 / Interstellar Probe Science A Technology Definition Teem

FIGURE 8.1 A logarithmic representation of nearby interstellar space. Each increment of distance represents 10 times more total distance than the previous one.

can be seen fairly easily (Figure 8.1). An alternative method to visualize these immense distances is to create our own version of the solar system, using units of measure that we can readily comprehend. First, we will conveniently "shrink" the Earth-Sun distance to 1 foot. If the Sun is at the center, then the Earth is 1 foot away from it. Mars is conveniently located just 1.5 feet away, Jupiter 5.2 feet, and distant Pluto 39.5 feet. Okay, surely we can now comprehend the distance across the gulf between the stars to our nearest stellar neighbor? On this scale, Proxima Centauri is 50 miles away!!! Astronomers long ago were challenged by understanding these distances and created a new unit, called the "Astronomical Unit," to represent the Earth-Sun distance. To convert our "39 foot" solar system into these terms is trivial: replace feet with Astronomical Units, or "AUs," and you have it: the Earth is 1 AU from the Sun, Jupiter 5.2 AU, and Pluto 39.5 AU. Proxima Centauri is then approximately 250,000 AU from the Sun. Now we finally realize that the distance between stars is HUGE. The question becomes: "Can we, the descendants of those Roman soldiers who marched across Europe, cross this vast distance?" In 1999, NASA asked this very question.

NASA's Interstellar Initiative and the

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