Topic of the month: Mars at Opposition

As a child I dreamed that I would someday work on the red planet as a colonist. Even today, the technological challenges of spanning that vast distance makes colonization a distant dream. Just observing the planet Mars presents it own challenges. Early telescopic observers noted that the planet had polar caps and a changing surface that seemed to indicate a seasonal nature. Earthlings believed that Mars was the planet most likely to be populated with intelligent beings. Early space probes soon wiped out hopes for oceans and intelligent beings completely. The average amateur astronomer was left to follow the red planet through its periodic cycles with a small instrument.

Every 780 days, or roughly 2 years and 50 days, Earth passes Mars on its orbital track, lapping the red planet and quickly leaving it behind. Opposition occurs when Earth overtakes Mars on July 27, 2018. That day Mars is directly opposite from the Sun in the sky, it will be seen all night after the Sun sets and present a full disk. The next time Earth catches up with Mars another opposition occurs, but at different positions on the planets' orbital paths. Their orbits are stable and after 15.8 years they retrace the same basic pattern. Again Earth and Mars will pass in the night at a very close position in their orbits.

To represent the Earth/Mars opposition cycle, I have created this diagram. It shows the solar system as though we are looking down on the Sun's north pole. Although the orbits are shown here with some accuracy, the images of the Sun, Earth, and Mars are not drawn to scale. The 2018 opposition is represented by the disks of Earth and Mars. Earth s is shown with the north pole tipped toward the Sun and Mars has its south pole toward the Sun. Notice their axes are aligned in different directions! For each upcoming opposition the position of Mars is shown on the orbital path in red while Earth’s position is marked in blue. Because of the speed of their orbits Earth travels for 2 years, plus an extra 50 days or so when it catches up again with Mars. The Mars orbit is closest to the Sun at the time of Mars perihelion, marked P on the diagram. This year perihelion occurs on September 16. The Earth's orbit is much more circular than Mars, its aphelion date is July 6 marked A on the diagram. When Earth and Mars reach opposition near Mars perihelion, a favorable opposition occurs called a “perihelic” opposition.

When Mars emerges from conjunction and becomes visible again, (as it did in September last year) a new march toward opposition begins. Astronomers refer to that observing period as an apparition. This apparition will continue until Mars once again disappears behind the Sun. Most amateurs start seriously peering at Mars when its disk grows over 10"(that's arc seconds.) This year Mars reached that size in April beginning a seven-month period when it will be most easily observed.

Each Mars opposition is a little bit different, as seen by the 7 oppositions in the cycle diagram. Notice how smoothly and regularly the various oppositions occur. Despite what some might say, planetary lineups and conjunctions are typical for objects moving around the Sun in regular orbits. This cycle repeats 3 times, then Earth and Mars take up very similar positions as they did the last time. For the 2018 opposition, the disk of Mars will grow to 24.3" at closest approach. If you never saw Mars before or if you are a regular observer, now is the time for you to look for the red planet. As July opens, Mars is rising about 11pm, but by opposition, it will rise when the Sun is setting almost 3 hours earlier. (For EDT time zone denizens.)

Because of its axial tilt Mars experiences seasons like Earth, although because of its more eccentric orbit, Mars seasons are not equal in number of days. Southern summers are shorter, but hotter. When Mars is making a close pass, it will be summer in the northern hemisphere on Earth. The planet Mars will be low on the ecliptic, because Mars will be in Earth's sky near the winter solstice Sun position. For that reason perihelic apparitions are never seen best from the northern hemisphere of Earth. Find the Mars opposition on the opposite side of the diagram. This aphelic opposition occurs when Earth is closer to the Sun and Mars is farthest away from it. Mars will be found near the Sun's summer solstice position (high in the north.) The next opposition occurs in October 2020. The diagram shows how far Earth must travel to lap Mars from their previous positions and that Mars is on a slower track. There is a greater distance between 2018 Mars and 2020 Mars because Mars obeys Kepler's laws of planetary motion and sweeps through this part of its orbit faster than it does near aphelion.

With the unaided eye, the color of Mars is often described as red. This may be more notable when Mars is compared to a nearby bright star. This opposition, Mars is located in an area of the sky that contains few bright stars. Compare Mars to Saturn that typically appears white or a bit yellowish. Then try the first-magnitude red giant star in nearby Scorpius, Antares. The name describes its color - - anti-Ares. The three objects can be seen together above the horizon at the same time. I find the visual color of Mars can be affected by atmospheric conditions, its position on the ecliptic, and personal color vision. One may see orange, red or just plain white when looking up at the sky. The red or orange color is caused by rust on the surface.

In July 2018, Mars is hanging out in the constellation of Capricornus. It arrived from Sagittarius in mid-May and it will remain in Capricornus until November. During the period of opposition the motion of Mars against the star background is quite intense. The finder shows how the red planet is moving through Capricornus. It reaches its first stationary point on Jun 28, when it begins to move in the opposite direction from its usual pace. This tracing back is called retrograde motion. (So now we know why the ancients called planets "the wanderers.") The disk will grow from 21" in diameter on July to 24.3" when it reaches closest approach on July 31, 2018. Yes, closest approach is 4 days later this time around because Mars will not yet be at its perihelion apside on Sep 16. Only when Mars is at opposition on July 27, 2018 will it show us its entire disk, then we will be able to see 90% of its surface. It actually shows a little more than that on the day of opposition because of its rotation.

One other thing you may like to note is that the Moon will be full at the same time Mars is at opposition. This means that we will be blasted by the Moon while we patiently try to observe Mars' delicate colors. Even more exciting than a full moon is a moon that is being totally eclipsed. Yes, a very deep eclipse occurs when the Moon is at apogee! Unfortunately for observers based in N. America we do not see even a slice of the Moon in shadow. The eclipse center is in the Indian Ocean E. of Madagascar and should be seen all over the world, except for some reason North America will be the worst place to be for this one. More accurately, opposition occurs at 5 hours UT while the eclipse begins at 18:24 UT so even though it's on the same day, Mars will already be past the 100% disk visible stage.

Last month, I speculated that we might avoid a global wide dust storm. Mars proved me wrong almost as quickly as I could enter the words on my computer. Most of the global dust storms during favorable oppositions come after perihelion. They kick up out of the Hellas Basin at a high latitude and slowly cover the planet. This storm came as the northern hemisphere was transitioning into fall. On May 30, a dust storm kicked up from a high northern latitude that turned into a planet-wide dust storm. The storm has been declared a PEDE or Planet-Encircling Dust Event. NASA’s Opportunity rover has powered down until the sky clears but no one can be sure when that will be. The storm will broaden our knowledge of these massive events, but will hamper observers from seeing the features clearly. Solar system planets are expert at behaving differently then we believe they should and Mars is no exception. Hopefully Opportunity will revive and continue its exploration mission, then we will also be able to see those faint surface details.

When Mars is out these July nights, it will be next to impossible to miss. It is so bright and looks so red or orange, especially as it is rising. I see the Mars surface as a pale salmon color when viewing through a telescope. When looking for planetary features through a telescope, be patient. It takes the eye awhile to get used to looking for those delicate features. The first few minutes of staring at Mars disk may reveal nothing - - so keep trying!

Find out which side of Mars is visible while you are out viewing the red planet through a telescope. Sky & Telescope magazine features an interactive application, Mars profiler. Use this tool to find out what side of the martian surface is currently in your viewfinder or check up on the time of your observation.

--See You Under the Stars!
Astra for Astra's Almanac