The tiny spot of Mercury can be seen crossing the face of the sun in this observation of the transit on May 9 from Boyertown, Pennsylvania.
Mercury, the smallest planet in our solar system and the closest to the sun, took on a number of personas to ancient civilizations. To the Egyptians it was Thoth, the god of knowledge, speech, writing and arithmetic. To the Greeks it was alternatively known as Apollo (the god of truth, arts, archery, plagues, and divination) and Hermes (the god of writing and messenger to the other gods), depending on whether it rose in the morning or evening. But details of the planet beyond its orbit were unknown until we visited it with the Mariner 10 spacecraft 40 years ago.
The Mariner program was born during the Apollo era to explore the inner planets of the solar system. It was a compliment to the Pioneer missions that aimed at the outer giant planets, and the final two spacecraft, Mariners 11 and 12, were eventually repurposed into the Voyager 1 and 2 spacecraft. There were ten Mariner spacecraft built and launched. Three were lost to launch vehicle failures, four went to Mars, two were aimed squarely at Venus, and one was designed to explore Mercury. It was Mariner 10, the last of the series.
Mariner 10, like all the Mariner spacecraft, was octagonal in shape and made of magnesium and aluminum. It was squat and long, measuring about 18 inches high and 4.5 feet in diameter. With its solar panels deployed -- flying toward the sun, it used solar power for electricity -- it measured a substantial 26 feet across. Its science payload was also fairly common for the Mariner family. Its suite of instruments included cameras, a magnetometer, a plasma science experiment, a charged particle telescope, an ultraviolet spectrometer and an infrared radiometer. Mariner 10 launched on an Atlas Centaur rocket on Nov. 3, 1973. Not long after leaving the Earth, it turned around and looked back at our planet, snapping a stunning picture of our home planet and the moon together in one frame.
Its first planetary encounter was with Venus. Mariner 10 took some 4,000 images of the second planet from the sun, revealing little beyond the thick clouds enveloping the planet. Other instruments revealed Hadley-type circulation in Venus' atmosphere, as well as confirmed the existence of a weak magnetic field surrounding the planet and an ionosphere that interacted with the solar wind to form a bow shock. Mariner 10 used its encounter with Venus to slingshot itself on a path to Mercury, the first time a gravity-assist maneuver was done on a multi-planet mission. The spacecraft reached Mercury and made its first flyby on March 29, 1974.
Photos taken as the spacecraft approached its target showed scientists a moon-like surface: the planet was heavily cratered and barren. The spacecraft's magnetometers found only a weak magnetic field. Radiometer readings found temperatures vary significantly on the small world, from -297°F in the dark to 368°F in the sunlight. This first came on March 29, 1974 at a range of 436 miles from Mercury’s surface. A second flyby happened on Sept. 21, 1974; the spacecraft passed 29,868 miles from the surface. The third and final flyby came on March 16, 1975 and it was the closest pass. Mariner 10 flew within 203 miles of the planet’s surface. NASA’s last contact with the spacecraft came less than two weeks later on March 24.
Photographs and scientific measurements taken during the Mariner 10’s three passes revealed a wealth of information about the planet. The mission confirmed that Mercury has no appreciable atmosphere, a small magnetic field, and a relatively large iron-rich core. Just under half the planet was imaged by the spacecraft, and though it was far from uniform in appearance, the planet was found to be uniformly cratered and ragged. Like so many early survey missions, Mariner 10 raised more questions about Mercury than it answered, like, what is the planet’s internal structure? Is there volcanic activity? What is the story behind Mercury’s atmosphere and magnetosphere? Ground-based observations after the mission ended shed some light on the remaining mysteries, as hasNASA’s MESSENGER mission
, the only spacecraft dedicated to exploring Mercury since Mariner 10 launched.
;Lunar and Planetary Institute
On Monday, the transit of Mercury wasn't just an opportunity for (most of) the planet to share a common astronomical phenomenon; it was a chance to carry out some cutting-edge science as well as a special moment to do classical astronomical measurements that underpin our modern understanding of the universe.
As part of the inaugural #MeetESO social media meet-up, Discovery News was lucky to be perfectly-placed to see the Mercury Transit at the driest region of the planet, high in the Atacama Desert in Chile, visiting the European Southern Observatory's Paranal Observatory.
As part of an effort to live-stream the Mercury transit in conjunction with another event in Tenerife, ESA astronomers also made the journey to the observatory to watch the solar system's smallest planet make its big entrance.
When a planet passes in front of its star, the event is known as a transit. This applies to our solar system as well as exoplanets orbiting other stars. In the case of the solar system, from our planet's perspective, we will only see Mercury or Venus make their several-hour dash across the sun's disk as they are the only 2 planets that orbit closer to the sun than Earth.
When this happens, an opportunity presents itself.
“Many exoplanets have been discovered orbiting other stars, but by watching the transit here, we can compare this with exoplanets passing in front of other stars," said Miguel Pérez Ayucar, of ESA's European Space Astronomy Centre (ESAC) in Spain, while observing the event near the summit of Paranal.
We can use the Mercury transit as an exoplanet analog, to see how the transit looks from Earth and then comparing that with transits of distant worlds orbitng other stars.
Missions such as NASA's Kepler space telescope watch for the slight dimming of the light of stars, revelaing that an exoplanet has paassed in front, blocking a tiny amount of starlight. This is exactly what Mercury did on Monday (and Venus last did in 2012), so these transits close to home can be used as a tool to understand transits far away.
Also, the Mercury transit can be used to study the planet's exosphere — an extremely thin atmosphere that fizzes into space. Sunlight that passes through the exosphere is encoded with spectroscopic data that can then be used to work out the nature of the environment surrounding Mercury. As we've had missions that have explored the planet, such as NASA's MESSENGER mission (and future ESA BepiColumbo) we have the “ground truth" as to the characteristics of that exosphere, so we can compare direct measurements with transit observations to better calibrate astronomical techniques.
“For (the Venus transit) it was better in that Venus has a dense atmosphere, much denser than Earth's," Pérez Ayucar told Discovery News. “The effect of that atmosphere is very clear in the signal of the sun so you could study the atmosphere by looking at the planet's light-curve. However, the last Venus transit was in 2012 and the next one isn't until 2117 … so I probably won't see it."
ESAC's Manuel Castillo chats to #MeetESO participants at Paranal Observatory next to the ESA's solar telescopes. Ian O'Neill
Despite the cool modern science that can be done, the Mercury transit acts as a reminder as to how cosmic distances were calculated. Hundreds of years ago, the first measurements of the distance between the Earth and the sun were carried out by observing the transits of Mercury and Venus from distant locations on the planet. By precisely recording the time of the start and finish of the transit, this calculation could be made.
“There is something special about being hands on, doing the historic calculations yourself," said Manuel Castillo, also of ESAC. “We can set up a telescope here and with a team in another place, you can calculate the distance from the sun to the Earth yourself."
Although the measurement isn't particularly accurate, when considering more precise modern methods that can be applied, “you're replicating the way man built the scales of the universe through history," he said.
And then things get really interesting. “When you (know the distance between the sun and the Earth) you can calculate the parallax to the stars," added Castillo, and from there, astronomers through history have built on these classical measurements to calculate the distance to the stars, then to the galaxies, ultimately realizing we're living in an expanding universe.
In short, astronomy doesn't only unite the world in a common fascination for the cosmos, it unites the discoveries made through history. We may have cutting-edge equipment and a better grasp on astronomical techniques, but making these basic astronomical observations of Mercury transit the sun, we connect in a very special way with the pioneers who painstakingly made those first, historic measurements.
To keep up with our adventures in the Atacama Desert, be sure to follow the #MeetESO hashtag across social media platforms — 7 other participants are providing their unique view on the most powerful observatories on the planet, so don't miss out!