A bright star-like point of light stands to the right of the Moon in the twilight this evening: the planet Mercury. It's one of four planets that're being watched by orbiting spacecraft.
Scientists use telescopes here on Earth to complement the findings of those probes. In the case of Mercury, for example, an international team is using two radio antennas to measure a slight "wobble" in the planet's axis. When combined with observations by the orbiting Messenger spacecraft, that information will help them probe Mercury's interior.
We already know that Mercury has a core made of molten iron and nickel. It takes up about three-quarters of the planet's diameter. But many of the details remain elusive. We don't know, for example, if the wobble is caused by processes inside Mercury, or by the feeble gravitational tug of Jupiter.
One way to find out is to measure the wobble.
To do that, the team of scientists uses two large radio dishes -- the Green Bank Telescope in West Virginia, and a deep-space tracking station in California. The Green Bank Telescope sends out strong pulses of radio waves, which reflect off the surface of Mercury. Both dishes receive the faint echo.
Tiny differences in the frequency at the two receivers reveal just how Mercury is rotating on its axis, which reveals its wobble. The exact characteristics of the wobble reveal its cause -- allowing scientists to look deep into the heart of the little planet.
Script by Damond Benningfield, Copyright 2011
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