Topic of the month: Observing Mars Oppositions

The planet Mars is my second favorite planet. As a child I dreamed that I would someday work on the red planet as a colonist. As near as our solar system neighbor may be at times, the technological challenges of spanning that gulf still make colonization a distant dream.

Observing the planet Mars presents it own challenges. Although it is a neighbor in our solar system, Earth and Mars are vastly different. Early telescopic observers noted that the planet had polar caps and a changing surface that seemed to indicate a seasonal nature. Mars was touted as the planet that was most likely populated with intelligent beings. Early Mars missions of Mariner and Viking soon wiped out hope of 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, planet Earth passes Mars on its orbital track, lapping the red planet like horses at a race track. (Well, not quite like horses, both Earth and Mars have stable orbits around the Sun.) Opposition occurs when Earth overtakes Mars. Mars is then directly opposite from the Sun in the sky. Mars will be seen all night after the Sun sets and present a full disk. 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.

Each Mars opposition is a little bit different, but because the planets are in stable orbits, after 15.8 years they retrace the same basic pattern. There are 7 oppositions in the cycle as shown in the Mars Oppositions image. This image was created by NASA, ESA, and Z. Levay (STScI) and was modified by D. Jenkins to reflect later oppositions of Mars. It is licensed under Creative Commons 4.0 International. The image is a composite intended to show how the appearance of the red planet changes as the 15.8-year cycle of Mars oppositions progresses. The orbits for solar system planets are true and the disk images were taken by the Hubble space telescope during oppositions from 1997 - 2007. For the 2018 opposition, the disk of Mars will grow to 24.3" at closest approach.

Note the 2018 opposition is represented by the Mars planetary disk located in the lower right. The disk shows features that may be seen in a telescope at a size relative to other Mars oppositions apparent disk size. Each opposition's Mars disk is shown next to Mars orbital path in red. The Mars orbit (red ring) is closest to the Sun at the time of Mars perihelion. This year perihelion occurs on September 16. Although the Earth's orbit is much more circular than Mars, it also has apsides, the date of Earth's aphelion is July 6. Thus it can be seen that anytime Earth-Mars reach opposition near those positions in their orbits, a favorable opposition will occur.

When Earth and Mars are making a close pass, it will be summer in the northern hemisphere on Earth and 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 smallest Mars disk diameter 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.) One last note, the next opposition occurs in October 2020. The Oppositions image 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 is obeying Kepler's laws of planetary motion and sweeping through this part of its orbit faster than it does near aphelion.

The color of Mars is often described as ochre, a name that covers a wide range of pigmentations. I usually see the surface as a pale salmon color when viewing through a telescope. With the unaided eye, the color of Mars is often described as red. This can be seen more readily when the planet is compared to a nearby bright field star. Unfortunately, this opposition, Mars is located in an area of the sky that contains few bright stars. It may be easier to compare Mars to Saturn, that typically appears white or a bit yellowish to observers. Although Saturn is bright (around 0 mag) it can't compete with Mars near opposition but still provides an excellent contrast. It may also be amusing to compare the red planet to Antares, a first-magnitude red giant star in nearby Scorpius. The Antares name is intended to describe its color - - anti-ares. Mars, Saturn, and Antares may be seen together above the horizon at the same time this month after midnight for locations with unobscured horizons. I find the visual color of Mars can be affected by atmospheric conditions, its position on the ecliptic, and (as always) personal color variations due to the optical detector--the human eye. One may see orange, red or just plain white when looking up at the sky

If you are not familiar with looking for planetary features through a telescope, be patient. It takes the eye awhile to get used to looking for those delicate features. Avoid observing where stray light can reach your eye as you are looking through the telescope. Averted vision can sometimes help to bring out faint details. Planets are very sensitive to atmospheric conditions. Nights when the stars are crisp and sharp may not be the best for planets. It is a rare night when deep sky objects look fantastic and so do the nearby planets. A general rule for observing Mars and other planets is to use about 30x per inch of aperture. This means a 6" telescope will need about 180x. This is easily attained with my 6" f/9.6 instrument, but a small telescope with a shorter focal length will be challenged. One technique used to overcome telescopes with short focal lengths is placing an aperture stop over the end of the tube. (Often done with reflectors.) By decreasing the size of the objective, the focal length becomes longer. Sometimes these are off-axis, meaning that the entire secondary mirror and support structure may be avoided. If that is not possible, place your aperture stop where there is minimal obstruction. What I find most helpful is using a barlow with my eyepieces. Barlows help to bring up the magnification. They may also allow you to use an eyepiece with better eye relief instead of a high-power magnifying eyepiece. Lastly, make sure your instrument is well collimated!

So what features are we looking for? The smallest telescope (about 2" or 5cm) should show the polar caps. This opposition, the southern hemisphere is pointing at Earth, or really the Sun. Mars southern hemisphere will be experiencing spring through opposition and summer will begin in October. The southern polar cap is now visible and will be shrinking. As Mars' rotation is only a bit slower than Earth's, features appear about 40 min later each night. The black eye of Mars, aka Solis Lacus, should prove to be an interesting surface feature this opposition. It bears close watching. The "dark continent" of Syrtis Major is reminiscent of the subcontinent of India. The Tharsis region where large volcanoes like Olympus Mons are located may be in view at times during this apparition. Don't expect features to jump out at you like they do on Hubble telescope images, but do search out some on the internet so that you can see what a large orbiting telescope is capable of and clarify just what is out there. Dust storms, clouds, and other albedo features should be visible. During Summer in the southern hemisphere there is the possibility of planet-wide dust storms, but the season for these is well after opposition and hopefully we will avoid that this time.

For dedicated observers, this is a good time to try to see the 2 moons of Mars: Phobos and Deimos. They are very faint - Phobos is 11.4 mag and Deimos is even fainter at 12.5 mag. In contrast, Mars' opposition disk can be dazzling (mag. -1.2 to mag -2.8 from now until September.) Observers need to use an occulting bar to cover that dazzling primary. It should be easy to obtain an ephemeris for the satellites during the time you want to search for them. It is highly unlikely they will be glimpsed without one. A good 10" telescope might work, but 12"+ would be better.

Many observers like to draw the disk of Mars near opposition. To do this you should have paper, pencils - - drawing and colored, smudging tool, pastels, art eraser, and a comfortable chair. It is nice to have a telescope with a clock drive, although it is not essential. You probably have a red flashlight, but be careful if you are drawing in color because a red light will distort your drawing. I always checked mine when I was finished in white light. You should start with blank disks, about 42mm in diameter is suggested by Jeffrey D. Beish at ALPO. Begin by sketching in the terminator and blacking out that portion of your disk--this is the preceding edge and new features will come into view from the opposite side of the disk. (The terminator will be on the following edge after opposition.) Try not to look at maps or determine what should be on the disk before you begin. Just record what you see. You will want to note name, location, date, time, observing conditions, latitude of Mars central meridian, etc., especially if you want to present them to others.

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