Sun spews CME toward Mercury

On Sept 8th around 2300 UT, the SOHO and STEREO spacecraft detected a significant CME emerging from the farside of the sun. Earth is not in the line of fire, but the planet Mercury is. Analysts at the Goddard Space Weather Lab estimate that the cloud will reach the innermost planet on Sept. 9th at 12:00 UT (plus minus 7 hours). Click to view a movie of their CME model:

NASA's MESSENGER spacecraft is in orbit around Mercury, so it will have a front row seat for the impact. Researchers are keen to learn how Mercury's magnetosphere responds to CMEs. In particular, they wonder if CMEs can overpower Mercury's magnetic field and sputter atoms right off the planet's surface. Thanks to the Goddard forecast, MESSENGER's controllers know the CME is coming, and they can prepare to observe the impact.

Interplanetary space weather forecasting is a new thing. It became possible in 2010-2011 when NASA and ESA spacecraft surrounded the sun. Working together, SOHO, the Solar Dynamics Observatory, STEREO-A and STEREO-B now have the entire star under surveillance. CMEs can be tracked no matter where they go, which means space weather isn't just for Earth anymore. (SpaceWeather)

Mariner 10 showed that Mercury has a magnetic field that is 1% as strong as Earth's. This magnet field is inclined 7 degrees to Mercury's axis of rotation and produces a magnetosphere around the planet. Magnetic fields are generated from the rotation of a conductive molten core and is known as the "dynamo effect."

Friday, September 9, 2011 
Mercury passes 0.8 degrees to the north of the star Regulus. Mercury at -0.9 magnitude is brighter than Regulus at +1.4 magnitude. Find the pair 36 degrees to the lower left of Mars. Look to the ENE at dawn.

Mercury's orbit is so close to the Sun that it is difficult to see. This explains why some early astronomers didn’t see it. Viewed from Earth, Mercury is never far from the Sun in the sky. Because the Sun is so bright, Mercury can only be seen in twilight. The side of Mercury facing the Sun can reach temperatures of 800ºF while the side facing away from the Sun can get as low as -280ºF at the same time.

Along with the planet Venus, Mercury is one of only two inferior planets, those with orbits encompassed by Earth's. It is the planet that moves closet to the sun, coming within 46 million kilometers of it, less than a third of the distance between the Earth and the Sun. This accounts for the extreme temperatures on its surface. Mercury is also the third closest planet to the Earth, located roughly midway between it and the Sun.

Mercury has an extremely eccentric orbit, 0.21 from the circular. That is, there is a twenty-four million kilometer difference between the closest and farthest points from the sun, respectively its perihelion and aphelion. More, it moves so quickly around the sun that it rotates exactly three times around its axis for every two orbits around the Sun--there are three days in two years! This may sound incredible, but some scientists speculate that in the past the rotation was even faster, even as rapid as 8 hours (as compared to its current period of 58.6 hours).

Though Mercury is not tidally locked to the Sun, its rotational period is tidally coupled to its orbital period. Mercury rotates one and a half times during each orbit. Because of this 3:2 resonance, a day on Mercury (sunrise to sunrise) is 176 Earth days. A strange effect of Mercury's highly elliptical orbit and rotational period is that the Sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the Sun appears to set, rise briefly, and then set again. A day on Mercury has three sunrises and three sunsets!

Timocharis made the first recorded observation of Mercury in 265 BC.


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