Mars and Earth both orbit the sun, but at different distances, and thus, different speeds. Every two years or so, Mars, Earth and the sun form a straight line during the course of their orbits, with Earth in the middle — an event known as opposition.
This summer, opposition occurs on July 27, and Mars will reach its closest approach to Earth at 3:50 a.m. EDT (0750 GMT) on July 31. The Red Planet will also be at its brightest since 2003, when Mars made its closest approach to Earth in almost 60,000 years. [Mars Close to Earth! Where to See the Red Planet Online and In Person]
Mars arrives at opposition on the same day as July's full moon. For some lucky skywatchers, that means they also have the chance to see a total lunar eclipse. That eclipse, which will not be visible from the United States, will be the longest lunar eclipse of the 21st century. You can see our full guide for that event here: Blood Moon 2018: Longest Total Lunar Eclipse of Century Occurs July 27
Closest since 2003
During the Mars opposition in 2003, the Red Planet was only 34.6 million miles (55.8 million kilometers) from Earth. This was the closest the two planets had come to each other in almost 60,000 years, and this record won't be broken until Aug. 28, 2287, according to NASA.
In comparison, when Mars is on the other side of the sun and thus at its greatest distance from Earth, it is about 250 million miles (401 million km) away. However, the average distance between the two planets is roughly 140 million miles (225 million km). [Look Up This Weekend for Closest Mars Views Since 2003]
Leading up to this year's opposition in July, Mars will continue to brighten in our sky. By June 26, Mars will be just 44 million miles (70.8 million km) away, and from the perspective of skywatchers on Earth, it will appear five times brighter than usual. By the time the Red Planet reaches its closest point to Earth, it will be a mere 35.8 million miles (57.6 million km) away and will have nearly doubled in brightness since June 26. [8 Cool Destinations That Future Mars Tourists Could Explore]
What, exactly, is an opposition?
Mars orbits the sun at a greater distance than Earth. As the distances increase, the orbital period also increases, so Mars takes about two Earth years to complete one orbit around the sun. Due to these different orbital speeds, every two years or so, Earth passes between Mars and the sun. This means that Mars and the sun are on directly opposite sides of Earth.
Also, because Mars is directly opposite the sun during opposition, Mars rises as the sun sets, and it sets as the sun rises. As a result, the Red Planet shines prominently in our night sky. [Who Discovered Mars, Anyway? A Look Back at the History of the Red Planet]
However, since both planets have elliptical orbits, some Earth-Mars encounters are closer than others. In July, Mars will be closer to Earth than it has been since 2003, though the Red Planet's next-closet approach won't occur until 2035.
In addition, this year, Mars will reach what is called "perihelic opposition". Perihelion refers to the point in Mars' orbit when it's closest to the sun. Therefore, when Mars is closest to the sun, it is even closer to Earth during opposition.
How to see Mars
For about two months, between July 7 and Sept. 7 this year, Mars will brighten dramatically, outshining Jupiter and moving up in rank as the fourth-brightest object in Earth's sky after Venus, the moon and the sun. [Previewing 2018's Year of Spectacular Mars Using Mobile Apps]
Throughout May, both Mars and Saturn were visible, and the best views will be in the predawn hours of the Northern Hemisphere. Mars was easily distinguished by its reddish color, while Saturn appeared golden. Mars moved east of Saturn by mid-May, into the constellation Capricornus, according to EarthSky.org.
Starting in mid-June, Mars officially entered the evening sky and noticeably brightened and grew in size, leading up to opposition on July 27. The planet will appear brightest between July 21 and Aug. 3. As sunsets occur earlier in late summer and early autumn, viewers will be able to see the planet higher in the evening sky. [How to Use Mobile Apps to Watch Mars' Closest Approach Since 2003]
Beware of the Mars Hoax
In 2003, during that year's epic Mars conjunction, a hoax that went viral claimed that Mars would be as big as the moon. Since then, the meme comes up every now and then, usually around September, and it may rear its ugly head again this year. [Viral Hoax Claims Mars Will Look As Large As the Moon This Weekend]
But don't be fooled — while Mars will shine big and brightly leading up to and during opposition, the Red Planet is only about half the size of Earth. This means that even at its closest approach this summer, it will only be 24.3 arc seconds across when viewed from Earth. In comparison, the angular diameter of the moon is 1,800 arc seconds, which is about 75 times larger.
Editor's note: If you snap an amazing photo of Mars and would like to share it with Space.com for a story or photo gallery, send comments and images to email@example.com.
- Samantha Mathewson, Space.com Contributor
Samantha Mathewson joined Space.com as an intern in the summer of 2016. She received a B.A. in Journalism and Environmental Science at the University of New Haven, in Connecticut. Previously, her work has been published in Nature World News. When not writing or reading about science, Samantha enjoys traveling to new places and taking photos! You can follow her on Twitter @Sam_Ashley13.
- Samantha Mathewson, Space.com Contributor on