The task was to construct a physical model of the planetary domain of the Solar System. The Voyage model Solar System was to extend from the Sun to Pluto.

Usually a physical model is constructed at a size that maximizes a person’s ability to manipulate it, and explore it. A reasonable size might be 0.3 meters (1 foot) across, about the size of a typical classroom globe. The Solar System, however, poses a problem for the model builder.

The Solar System is comprised of small objects in a vast space. Even the Sun is dwarfed by the space. You would need over 4,200 Suns side-by-side to span the distance between the Sun and Pluto. The problem is representing the objects and the spaces between them on the same scale.

The smallest object we wanted to depict were moons of at least 1,000 km diameter, like Tethys, Dione, Ariel, and Umbriel. The largest size to be depicted was Pluto’s average distance from the Sun, corresponding to 39.5 Astronomical Units, or 6 billion km.

Consider the two extremes for the model builder. First, consider a model of the Sun-Pluto distance that is constructed about the size of a classroom globe, or equivalently, drawn on a sheet of 8.5” x 11” paper. On the paper you could locate the Sun, all the planets, and Pluto—creating a comfortably sized model of the Solar System. But on this scale the model Sun would be barely visible, with a diameter less than 0.1 mm across, and the planets would be too small to be seen.

On the other extreme, if a model of a 1,000 km diameter moon was constructed about the size of a classroom globe, then the model Sun-Pluto distance would stretch the length of the U.S. pacific coast, and the model Sun would be as tall as the Empire State Building in New York.

The approach to Voyage was to use a scale so that the size of a human being is about mid-way between the smallest object depicted and the largest distance depicted. This is the most effective way a human observer can experience the Solar System as a model. It maximizes the visitor’s ability to directly observe both the sizes of the worlds and the distances between them—at the same time.

A scale of one to 10-billion works perfectly, and as a power of ten, also allows easy calculation of model sizes. It is also conducive to developing lessons that address math skills.

At the one to 10-billion scale everything is reduced in size by a factor of ten billion. A model of a 1,000 km diameter moon is 0.1 mm across and still visible to the human eye. A model of the 6 billion km Sun-Pluto distance is a 600-meter path, or a comfortable 10-minute walk.

For a one to 10-billion scale model Solar System, it turns out that the size of a basketball (0.24 meters in diameter) is mid-way between the 0.1 mm model moon and the 600-meter model Sun-Pluto distance. More precisely, a basketball is about 2,500 times larger than a 0.1 mm diameter model moon, and the 600-meter model Sun-Pluto distance is about 2,500 times larger than a basketball. A basketball, to an order of magnitude, is the same size as a human being. It’s also interesting to note that a basketball is an optimum size for a physical model.

As a scale model Solar System, Voyage was also designed to a minimum pedagogical requirement. Standing in front of any model world, a visitor must be able to easily see the location of the next world. The idea is that a visitor should always be able to directly compare the size of a world to the distance to the next. This provides instant recognition of a tiny world in a vast space, which was the central learning objective of the exhibition. It is in fact the whole reason for the exhibition’s existence.

At the one to 10-billion scale this constraint is easily satisfied even in Voyage’s outer Solar System where the planets are much farther apart. In addition, the Voyage Unit height of 8.5 feet (2.6 m) was obtained from computer modeling meant to ensure that the Unit is visible at a distance even if an intervening crowd of people is close to the visitor.

Voyage actually does far better than the minimum requirement. From the model Uranus, you don’t just see the 8.5-foot tall Sun Unit, but can easily see the model Sun glistening in the distance. From Neptune, it’s harder to see the Sun because of a slight jag in the line of sight, but both Pluto and Uranus Units are visible. From Pluto you don’t have a direct line of sight to the Sun due to the curvature of Jefferson Drive where the exhibition is sited.