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By Emily Lakdawalla

Observations of Uranus system mutual events have now begun

May. 16, 2007 | 19:44 PDT | May. 17 02:44 UTC

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The Armagh Observatory announced today that "an international team of astronomers led by Apostolos Christou at Armagh Observatory has made the first ever observation of one of the satellites of the planet Uranus passing in front of another." On May 2, Earth crossed the Uranian ring plane, which collapsed the Uranian ring and moon system into a line across the sky, making it possible for one moon to appear to pass across another as seen from Earth. The observation was of the satellite Oberon (1,500 kilometers in diameter, similar in size to Rhea and Iapetus) passing in front of Umbriel (1,200 kilometers, similar in size to Charon and Dione), which is simulated in this animation:

Click to enlarge >

First mutual event of Uranus' 2007 ring plane crossing season
This animation shows the geometry of the first mutual event to be observed by astronomers after Earth crossed Uranus' ring plane in early 2007. On May 4, 2007, the moon Oberon passed in front of Umbriel. When the event took place, the total amount of light reflected from both moons dropped by about a third. In this animation, areas not seen by Voyager 2 when it passed by Uranus in 1986 are flat gray in color. Credit: NASA / JPL / Solar System Simulator

One of the striking things about this animation is just how much of Oberon and Umbriel's surfaces have never been seen. Earth-based telescopes cannot resolve details of the surfaces of these moons, but mutual events can reveal broad information about the brightness and darkness of the surfaces of the moons. The Armagh Observatory release states:

As Oberon's disc encroached upon Umbriel's, gradually blocking off Umbriel's light, the combined brightness of the moons dropped by about a third.

Measurements of such changes in brightness, and comparison with models of the satellites' motions, allow astronomers to work out the masses of the moons and the effects of the shape of Uranus on their orbits, and to model their surface features. The current Uranian mutual-event season is expected to lead to some of the greatest advances in the study of the Uranian system since the flyby of the Voyager 2 spacecraft in 1986.

This observation kicks off a campaign extending from now into 2008 to observe the entire mutual event season. It highlights the value of the North and South Faulkes telescopes for recording rare, time-critical events. And because the telescopes have an educational focus, the data will eventually be used not just by astronomers but also by schools and schoolchildren worldwide.

This work will take a long time to produce results; careful photometric observations gathered over the next year and a half by some of the world's largest telescopes will have to be reduced and merged to develop new information about the surfaces of these distant, dark worlds. Still, with no new mission to Uranus in the offing, it's the best we will do for some time. Hopefully, in 42 years, when Uranus reaches its next equinox, an emissary from Earth will be in place to observe it.

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