Using light and heat, scientists measure distant moons

While the Voyager 2 spacecraft drifts toward its rendezvous with Uranus in 1986, ground-based astronomers have already measured the size of four of that planet's moons.

They also have set new upper limits on the size of Triton, Neptune's largest moon, and of the remote planet Pluto.

This is a tour de force for Earth-based observers, since these objects appear as little more than points of light in their telescopes. However, by making clever use of data on the amount of sunlight the objects reflect and the amount of heat they radiate, R. Hamilton Brown, Dale P. Cruikshank, and David Morrison of the University of Hawaii have been able to infer their size.

Thus Voyager scientists now have a better concept of these moons to aid in planning their observations three years hence.

The Hawaii scientists find the Uranian moons, Ariel, Umbriel, Titania, and Oberon, to be somewhat larger than earlier, less reliable estimates had anticipated. With uncertainties of less than 10 percent, the new diameters are: Ariel, 1,330 km. (826 mi.); Umbriel, 1,110 km. (690 mi.); Titania, 1,600 km. ( 994 mi.); and Oberon, 1,630 km. (1,013 mi.). They are similar in size to Saturn's icy moons.

Reporting their work in a pair of papers in the journal Nature, the three scientists urge that priority now be given to determining the masses of these moons with comparable accuracy. This would enable planetologists to compute the average density of the satellites - facts crucial to understanding the origin and development of the Uranian system and helpful in Voyager planning.

The measurements for the Neptune moon Triton and for Pluto are less certain. The researchers put a likely upper limit of 2,100 km. (1,300 mi.) on Triton's diameter, with a best guess at the actual size of 1,600 km. (994 mi.), give or take 200 km. Drs. Brown, Cruikshank, and Morrison note that this is a size range comparable with that of Jupiter's moons Io and Europa and well under that of the Jovian giants Ganymede and Callisto or Saturn's Titan. This rules out the speculation of some scientists that Triton might be the biggest moon in the solar system.

Pluto also appears to be quite small, although the scientists point out that its actual size is unknown. They give 1,700 km. (1,060 mi.) as a probable upper limit.

To make their estimates, the scientists measure a satellite's brightness in terms of reflected light and its infrared (IR) or heat radiation. Since the satellites are in thermal balance, neither heating up or cooling off, they radiate as much energy as they absorb. Thus their IR radiation is a measure of how much incoming solar energy they absorb.

Since sunlight falling on a satellite is either reflected or absorbed, the two kinds of measurements give scientists full information on a satellite's energy budget. From this, they can infer the satellite's albedo (how light or dark colored it is) and its size. The Uranian satellites appear to be darker than Saturn's icy moons. Astronomers would say they have lower albedos.

The albedo can be calculated from the mathematical ratio between the measurement of the reflected sunlight and the measurement of the IR radiation. There is a mathematical formula that relates size, albedo, the visual magnitude of an object, and its distance from Earth and the Sun.

However, the IR signals are faint and are heavily absorbed by the atmosphere. The Hawaii scientists picked up the Uranian moons at IR wavelengths, but could detect nothing for Triton or Pluto. That's why they can only give upper limits on the size of those two bodies. However, they say that merely doubling the sensitivity of their IR detector should enable them to pick up the objects.

For astronomers, the new measurements have meaning beyond their purely scientific value. They underscore the importance of this kind of work at a time when budget cutters have threatened to curtail it.

The data were gathered at the National Aeronautics and Space Administration's Infrared Telescope Facility on Mauna Kea. The clear, thin air above the 14,000 -foot-high mountain makes that location ideal for IR astronomy. NASA had considered stopping funding for its facility but later reconsidered.

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