Astronomy Pluto satellite find a fortunate accident

Press, 2 July 1979, Page 14

Astronomy Pluto satellite find a fortunate accident

The discover,’ last year of a satellite to the distant planet, Pluto, was a fortunate accident. The United States Naval Observatory was trying to get improved data on the Orbit of Pluto by positional observations with its 155 cm astrometric telescope at Flagstaff, Arizona. When the 18 plates were examined the image of the planet had a very faint extension. . This extension was obviously not an image flaw, because it appeared on all plates. Some shelved a southerly extension and some a northern. They were not caused by images of faint and distant stars because no stars were in the right positions.

This left only two. possible explanations. The. first concerned the actual brightness of Pluto itself, which was known to vary by about 20 per cent in a period of slightly more than six days nine hours. This possibility was ruled out because computer experiments proved it was impossible for such variation to produce an elongation of the planet’s image. The second explanation was that ; the image extensions were caused by a satellite. An examinaition of earlier plates, taken in 1965 and 1970, showed slight image extensions. These agreed with the preliminary orbit that had been obtained. Confirmation came from Observations with the 4m telescope in Chile and from additional photographs taken at Flagstaff and elsewhere. It appeared that the satellite moved around Pluto in an almost circular orbit, at a mean distance of about 20,000 km. As the satellite is too faint to account for the observed light variations of Pluto, it is probable that the period of these is the same as the rotation period of the planet.

That the satellite has the same period means that it hangs motionless over the same spot on Pluto, The satellite has been officially called 1978P1 pending approval

of the suggested rihrne of Charon, after the mytical boatman who ferried souls across the River Styx to Hades, the domain of Pluto. ~

The discovery of Charon enables the combined mass of the system to be found. This is much smaller than the mass Pluto was previously believed to have, and is about 0.0026 times the mass of Earth. It is believed that Pluto is covered with highly reflective methane ice, and based on this it is estimated that because it is so faint. Pluto has a diameter of about 3000 km. Charon presumably has a similar surface coating, and is much fainter again, and so its diameter is estaimated at 1400 kilometres, and a mass about 2 per cent of the Moon’.. Pluto’s mass is 20 per cent of Earth’s Moon, suggesting that its density is comparable to that of water. If correct, then Pluto can be best thought

By

F.M. BATESON

of as a methane snowball. If the discovery of Charon was an accident, that of Pluto itself was the result of a very painstaking and tedious search. Its discovery came after several thousand hours had been spent examining two million images on photographic plates.

The famous French astronomer, Camille Flammarion, suggested 100 years ago that a planet existed beyond Neptune. This planet, he wrote in 1879, would move very slowly and be fainter than twelfth magnitude. Uranus and Neptune were both out of step with their predicted orbits by 1905. The logical explanation was that they were disturbed by the gravitational pull of another planet. Percival Lowell calculated the position of what was then termed “Planet X” from the inequalities in the motion of Uranus that were only partly explained by the perturbations caused by Neptune. Another astonomer, Wil-

liam Pickering, calculated the position of planet X and asked the Mount Wilson. Observatory to photograph the area of the sky in which he predicted this trans-Neptunian planet would be found. He was right, as its image appeared on four of the pates taken, looking like a fifteenth-magnitude star. Unfortunately no-one recognised it, as the. images lay outside the area that was examined. At the Lowell Observatory, in Arizona, searches had been made without success from 1906 to 1916. The search was then discontinued, to be resumed in 1929 when a better photographic instrument became available. It was naturally assumed that the planet would lie along the zodiac, but the constellation, Gemini, was the preferred area. The method used was to take three plates of each area to be examined within the same week. The hundreds of thousands of star images on each plate were examined using a blink microscope. As the name of this instrument implies, when two plates are examined the images of objects that have varied in position or brightness blink. It was by this method that Clyde Tombaugh examined plates centred on the star Delta Geminorum taken on January 21, 23, and 29, 1930.

After examining 25 per cent of the images he found that of Pluto. Its image had shifted slightly between January 23 and 29. Confirmation was obtained by additional plates taken in February, all of which showed that the object had moved position by the right amount. It was a disappointing object, because it appeared as a star-like image of fifteenth magnitude. Tombaugh continued his search after the discovery qf Pluto, in the hopes that another planet might be discovered. That was not to be. Between 1929 and 1945 he spent 7000 hours at the blink microscope, during which time he examined the images of about 90 million stars.