Posts Tagged ‘ICE International Comet Explorer’

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Versatile spacecraft took on rendezvous job

November 30, 1985

Winnipeg Free Press, Saturday, November 30, 1985 page 63

FOR AMATEUR astronomers, this is a wonderful year for seeing comets. If it were not for Comet Halley, another comet would have been the most memorable astronomical object of the year. Comet Giacobini-Zinner was an easy object to see in a telescope, and it was the first comet ever to be visited by a spacecraft.

This August, as those of us in Manitoba were trying in vain to have our first glimpse of Comet Halley, the way to end a frustrating observing session was to look at Comet Giacobini-Zinner. It was an easy object in medium-size telescopes, it moved noticeably across the sky from night to night, and there was the knowledge that soon it was to be visited by a space probe.

Comet Giacobini-Zinner stays much closer to us, and the sun, than Comet Halley, and it returns every six and a half years. It has been observed 11 times since it was discovered by Michel Giacobini in 1900. At its closest point to the sun, it is just slightly beyond the orbit of the earth. This is ideal for an approach by a space vehicle.

Fleet of spacecraft

Although there is a small fleet of space craft heading for Comet Halley, none is from the United States. For those interested in the U.S. space program, this is a major disappointment. On Sept. 11, the U.S. probe ICE penetrated the head of comet Giacobini-Zinner, and beat all the other missions to a comet.

ICE began its existence as International Sun-Earth Explorer 3 (ISEE-3), a little spacecraft that was designed to park itself in orbit around the earth, at an altitude of 120 times the diameter of the earth, in the direction of the sun. (The moon is only 30 earth diameters away.) It was supposed to quietly monitor the solar wind; its designers never thought about comets.

Fortunately, to stay in orbit, it required the use of onboard maneuvering thruster rockets. After three years ISEE-3 proved it was a versatile craft when it carried out a second mission. Using the thrusters, it journeyed to the other side of the earth to investigate activity in the other regions of the earth’s magnetic field. That was in 1982, and U.S. space planners had heard the final word that they would not have the budget for the Comet Halley mission. Now they had an alternate plan.

For the low cost of three million U.S. dollars, ISEE-3 became International Comet Explorer (ICE) and, on Feb. 8, 1983, It was on its way to rendezvous with Comet Giacobini-Zinner.

Success story

While It is a wonderful success story, those of us who work at a planetarium tend not to be as enthusiastic about ICE as the mission merits. Since it is designed to measure the activity of the solar wind, it contained no cameras. We have trouble showing people who visit the planetarium a lot of data — we prefer to show pictures. ICE is blind. Instead, ICE can measure magnetic fields, detect sub-atomic particles, and record the energy of radio waves. This instrumentation was well suited for a mission to a comet, and it produced some new information about comets, and the changes in space caused by their passage.

ICE roared into the tail of the comet at the meteor-like speed of 21 kilometres per second. Although the comet looked like a tiny wisp of light in our telescopes, and in spite of the fact that the lump of ice at the centre of the comet — the nucleus is only a kilometre in diameter, the atmosphere around the comet is vast. ICE spent two hours and 42 minutes traversing 204,000 kilometres of the interior of the comet. Scarcely a second passed when a particle of dust did not strike the antenna. There was some concern that, if any larger particles were encountered, the craft might be disabled.

Scientists are gloating over the fact that most of the new information confirms the current theories about how comets behave. They discovered that the most common molecule sublimating from the comet’s nucleus is water. This is consistent with our picture of a comet; that is, 85 per cent frozen water with ices of carbon monoxide, methane and ammonia, along with fine dust and perhaps some rocks.

Some of the surprises include the discovery that the interior of the comet’s tail is writhing with magnetic fields. It was thought that the interaction between the magnetic field of the comet and the solar wind would be similar to that which occurs around the earth and other planets. Here, the solar wind compresses the magnetic field of the earth, which can then effectively deflect much of the wind. ICE produced evidence of more complex processes.

Soon, the European Space Agencies’ space craft, Giotto, will plunge into the head of Comet Halley. It will fly so close to the nucleus of the comet that it will be destroyed by the density of material streaming away from the comet. It has a huge battery of scientific equipment on board, including a camera. Pictures!

With all of Giotto’s discoveries, it will be hard to remember this little craft that was the first to fly by a comet.

Valuable service

The ICE mission to Comet Giacobini-Zinner performed a very valuable service. In addition to providing information about the environment of the comet that will be useful in determining the best trajectory for Giotto, it gives us two comet examples to study. Although the U.S. did not manage a mission to Comet Halley, their clever use of ICE will contribute to our understanding of Halley, and all comets.

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While posting this article to the blog, The Return of Comet Halley, I came across this account of the recent contacts with the spacecraft ICE called “IT’S ALIVE!” << Follow the link to read that article posted at The Planetary Society. If ICE is to have another comet mission after it returns to the Earth-Moon in 2014, the person who will likely make the proposal is Jim Slavin.

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

Added to this blog on 2010 April 18

Sky Watch - Robert Ballantyne | Winnipeg Free Press

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