On a mission to Mercury

DURING the next few weeks, the tiny planet Mercury will be visible low in our western evening sky.

For the next several days, it will appear quite close to the sun’s largest planet, Jupiter, at twilight. The two will appear at their closest on Monday night, with Mercury a little above Jupiter and to its left.

As I gaze at the smallest of the planets in the solar system, I am delighted that the latest mission to Mercury is on its way, aiming to find out more about Mercury than ever before.

It is the BepiColombo mission, which was successfully launched at lunchtime last Saturday, Tasmanian time. It was named after Italian scientist Giuseppe Colombo, who contributed greatly to the success of the very first mission to Mercury, Mariner 10, in the 1970s.

BepiColombo is a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency. Both have had great successes in interplanetary science missions. Probably the best known one by the ESA was the Rosetta mission to the rubber duck-shaped Comet

MARTIN GEORGE Space

Churyumov-Gerasimenko, and a few weeks ago I wrote about JAXA’s spectacular recent achievements in landing some tiny craft on the surface of the asteroid Ryugu.

Back to Mercury. Not only is it the smallest of the sun’s planets after the famous reclassification of Pluto as a dwarf planet in 2006, but it is also the closest planet to the sun.

Orbiting at an average distance of only 58 million kilometres from the sun — just 39 per cent of the Earth’s distance — it looks somewhat like a larger version of our moon. However, there are important differences, and not only because it is a planet in its own right, orbiting the sun independently.

Mercury was first imaged by a spacecraft when the US’s Mariner 10 flew past three times in 1974 and 1975, and a detailed study was made between 2011 and 2015 by the US Messenger craft, which was the first, and so far the only, spacecraft to be placed into orbit around the planet. These missions have shown Mercury to be a cratered world with a surface composition that is rather unlike that of our moon. Also, there are more smooth plains, and we have seen evidence of relatively recent volcanism. Mercury certainly seems to have been a more “active” body than the moon.

There are many unanswered questions about Mercury, including the interesting measurement that there is much less iron in the crust of the planet than one would expect, especially given that there is a huge iron core, whose presence itself needs a lot more investigation.

It may seem that getting to Mercury would be easy, as it is much closer to us than the giant planets Jupiter and Saturn, each of which has been visited by several spacecraft.

But the journey to Mercury will be a long one, lasting seven years. Firstly, BepiColombo will return to the vicinity of the Earth, flying past us in April 2020. Then it will visit Venus for two fly-bys, following which it will pass Mercury six times before being able to finally settle into orbit in December 2025.

The mission is made up of two separate craft. One is called the Mercury Planet Orbiter and the other is the Mercury Magnetospheric Orbiter. The former was built by ESA, and the latter by JAXA.

Results from the MMO, especially, will be of great interest in understanding more about Mercury. The planet’s magnetic field was discovered using data from Mariner 10 in the 1970s, and it is about 1 per cent of the strength of the Earth’s magnetic field. Accurately measuring the field, and how it interacts with the stream of solar particles called the solar wind, is a major goal of the mission, and this will also give us more information on the iron core in its interior.

We have come a long way since the days when astronomers struggled to make out surface features on Mercury using their telescopes. One of the most keen of many such observers was Eugene Antoniadi, who in 1933 published a map of the planet showing the locations of dark shadings, and even named many of them. They are very hard to pick out, even when using very large telescopes, and it is difficult to match them with the features imaged by our spacecraft.

It is unfortunate that Antoniadi and other early Mercury observers are not around today to see the great pictures we have. Martin George is manager of the Launceston Planetarium (QVMAG).