As the Earth travels on its yearly circuit of the Sun it catches up with slower moving Mars every 780 days. When it does, Mars is at opposition; ie on the opposite side of the Earth to the Sun. Oppositions provide opportunities to view the polar caps and other features of Mars.
The distance between Mars and the Earth changes with each opposition due to the outer planet’s oval-shaped path around the Sun. As shown on the diagram, the following opposition in 2018 (see page) is more favourable than this year’s when Mars is 75.3 million km away. This year’s is still worth watching.
The dates of opposition between those of the two favourable ones in 2003 and 2018 are shown with the middle of each month indicated. Diagram Nick Lomb
Spacecraft
Space agencies take advantage of the relative closeness of Mars at oppositions to send spacecraft there. Two launches are planned for 2016.
Using a Russian rocket, the European Space Agency plans to launch the first of two ExoMars missions in January, with the second to follow at the 2018 opposition. This year’s mission is made up of an orbiter to sample trace gases that could have biological significance, especially methane, and a lander module. The lander will test the landing techniques on the planet’s surface that will be used on the next mission. Arrival of both the orbiter and lander is expected in October 2016.
NASA will launch the InSight spacecraft towards Mars in March 2016, landing on the surface to study the internal structure of the planet in September 2016.
Viewing
Opposition is on the evening of Sunday 22 May. However, the best evenings to view Mars will be in late May and early June, after the full Moon has passed. Plus Mars will still be approaching the Earth after opposition, reaching its closest point to the Earth on the morning of 31 May (AEST).