Image Credit: NASA/SDO/Genna Duberstein
May 15, 2016 - In Case You Missed It: On May 9, 2016, Mercury passed directly between the sun and Earth. This event – which happens about 13 times each century – is called a transit. Our Solar Dynamics Observatory, or SDO, studies the sun 24/7 and captured the entire seven-and-a-half-hour event. This composite image of Mercury’s journey across the sun was created with visible-light images from the Helioseismic and Magnetic Imager on SDO.
Transits provide a great opportunity to study the way planets and stars move in space– information that has been used throughout the ages to better understand the solar system and which still helps scientists today calibrate their instruments.
Although Mercury whips around the sun every 88 days – over four times faster than Earth – the three bodies rarely align. Because Mercury orbits in a plane 7 degrees tilted from Earth’s orbit, it usually darts above or below our line of sight to the sun. As a result, a Mercury transit happens only about 13 times a century. The last one was in 2006, and the next one isn’t until 2019.
“Astronomers get excited when any two things come close to each other in the heavens,” said Louis Mayo, program manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is a big deal for us.”
Mercury transits have been key to helping astronomers throughout history: In 1631, astronomers first observed a Mercury transit. Those observations allowed astronomers to measure the apparent size of Mercury’s disk, as well as help them estimate the distance from Earth to the sun.
“Back in 1631, astronomers were only doing visual observations on very small telescopes by today’s standards,” said Mayo.
Since then, technological advancements have allowed us to study the sun and planetary transits in greater detail. In return, transits allow us to test our spacecraft and instruments.
Scientists for the Solar and Heliospheric Observatory, or SOHO (jointly operated by NASA and ESA, the European Space Agency), and NASA’s Solar Dynamics Observatory, or SDO, worked in tandem to study the May 9 transit. The Hinode solar mission also observed the event. Hinode is a collaboration between the space agencies of Japan, the United States, the United Kingdom and Europe led by the Japan Aerospace Exploration Agency.
SOHO launched in December 1995 with 12 instruments to study the sun from the deep solar core all the way out to the sun's effects on the rest of the solar system. Two of these instruments — the Extreme ultraviolet Imaging Telescope and the Michelson Doppler Imager — were brought back into full operation to take measurements during the transit after five years of quiescence.
The SOHO measured the sun’s rotation axis using images captured by the spacecraft.
“Instruments on board SDO and SOHO use different spectral lines, different wavelengths and they have slightly different optical properties to study solar oscillations,” said SOHO Project Scientist Joseph Gurman. “Transit measurements helped us better determine the solar rotation axis.”
Such data is another piece of a long line of observations, which together help us understand how the sun changes over hours, days, years and decades.
“It used to be hard to observe transits,” Gurman said. “If you were in a place that had bad weather, for example, you missed your chance and had to wait for the next one. These instruments help us make our observations, despite any earthly obstacles.”
SDO will be able to use the transit to help with instrument alignment. Because scientists know so precisely where Mercury should be in relationship to the sun, they can use it as a marker to fine tune exactly how their instruments should be pointed.
The transit can also be used to help calibrate space instruments. The utter darkness of the planet provides an opportunity to study effects on the observations of stray light within the instrument. The backside of Mercury should appear black as it moves across the face of the sun. But because instruments scatter some light, Mercury will look slightly illuminated.
“It’s like getting a cataract — you see stars or halos around bright lights as though you are looking through a misty windshield,” said SDO Project Scientist Dean Pesnell. “We have the same problem with our instruments.”
Scientists run software on the images to try and mitigate the effect and check whether it can remove all of the scattered light.
Source: NASA
