New discoveries help Planetarium design comet model

ASTRONOMERS LOVE TO DESCRIBE nature in terms of numbers and mathematical formulae. At the planetarium, we look at all of this and say, “Yes, that is all very interesting, but what does it look like?” A planetarium has to be able to show it. The question is simple, but it can produce some interesting results.

Because it is small and well concealed inside the atmosphere, or coma, of the comet, the nucleus of Comet Halley has been the invisible author of the great celestial show that we see every 75 years.

When it returns

In the new planetarium program, The Comet Adventure, producer Bill Peters wanted the show to illustrate what it might be like as the first humans land on the icy heart of Comet Halley, when it next returns in the year 2061. Artist Ed Barker had to find the answer to “what does it look like?” Everyone imagined that, because the comet is icy, the surface of the comet would present a miniature Arctic scene.

The first clue that this concept might be misleading came from some of the new research that was done on Comet Halley early this year. Astronomers made a series of infrared measurements with the three-meter NASA infrared telescope facility, and the 2.2-meter University of Hawaii telescope on Mauna Kea, in Hawaii. Surprisingly, they were able to announce the approximate color of the comet’s nucleus.

At the beginning of 1985, the comet was close enough to the sun that its heat was starting to evaporate some of the surface ices to produce the coma. There was a short period between the time when the nucleus was close enough, and bright enough to observe, and the point when the nucleus vanished inside the growing coma.

The astronomers measured light from the nucleus through filters at different wave-lengths, and were able to deduce that they were looking at a very dark reddish object. The astronomers said “No comet observed by us has a color consistent with clean ice.” This was exciting new information.

For dramatic reasons, we wanted the surface of the comet to be an active place. The next piece of startling evidence was collected during the 1910 passage of Comet Halley, although the researchers at the time did not know what they had found.

Photographs

In Arizona, planetary astronomer Stephen Larson has been re-working 1910 photographs of Halley using a computer to enhance some of the features. As a result, Larson has been able to derive the speed of rotation of the Comet, as well as the orientation of its spin axis.

For our show, we discovered the surface of comets could be very dynamic. Warmed by the sun, the gases do not simply evaporate from the surface of the comet like snow on a sunny day. Larson’s pictures show violent jets of material being spewed into space.

Recent discoveries aid Planetarium staff in making comet model

Barker decided to illustrate the changes that take place on the surface on the comet by building a model of the surface. We use full-circle photography of the model to put the audience right in the scene.

He began with Styrofoam on a doughnut-shaped table that was 5-1/2 feet in diameter. It had a two-foot hole in the centre for the camera. For days Baker worked with a hot-wire Styrofoam knife to sculpt the fantastic surface of the comet as it was when it was as far from the sun as planet Jupiter. This is the point when the sun is just beginning to transform the sleeping comet from the little ice world that has remained unchanged since it headed out of the solar system in 1986, to the fuming giant that will shine in earthly skies In 2061.

Acrid smell

When the acrid smell of burning plastic faded, Barker was spraying, dabbing, and brushing the surface with a palette of colors, especially reds. Carborundum and barnsite (grit) were worked into the surface for texture. And finally, he added a dusting of stucco sparkle to provide reflections from the comet’s crystals.

The Fred Whipple, a comet-lander spacecraft, was added to the scene and photographed. Immediately, Barker attacked the surface of the comet with the hot-wire knife, lopping off icy spires, and opening up fissures where special-effects projectors will allow great jets of gasses to vent into space. This is the comet at its closest approach to the sun. Four scenes use this model. When we last see it, there is a light frosting of ice crystals caused by some of the coma freezing to the surface of the comet as it begins another 75 year Journey away from the sun.

At the planetarium, we will be interested in all of the new information that will be discovered about Comet Halley in the coming year. We particularly will be intrigued to see if there is anything that will indicate how accurate we are with our images of the comet’s nucleus.

In 1985 Robert Ballantyne was director of the Manitoba Planetarium. His weekly column in the Winnipeg Free Press, called Sky Watch, focused on the return of Comet Halley.

© 1985 Robert Ballantyne and The Winnipeg Free Press