THE 2020-2021 PERIHELIC APPARITION OF MARS
By: Jeffrey D. Beish (04-17-2012)
 

INTRODUCTION

Mars appears more Earth-like to us than most of the other planets because we can observe its surface, atmospheric clouds and hazes, and its brilliant white polar caps.  The latter are composed of frozen CO₂ and underlying water ice, and wax and wane during the Martian year. These aspects, along with the changing seasons and the possibility of life, have made Mars one of the most studied planets in our solar system.

The Red Planet Mars offers both casual and serious observers many challenges and delights, as well as providing astronomers a laboratory to study another planet’s atmosphere and surface. Some Martian features even appear to shift position around the surface over extended periods of time.

There are several cooperating international Mars observing programs under way to assist both professional and amateur astronomers. These include the International Mars Patrol (I.M.P.) coordinated by the Mars Section of the Association of Lunar and Planetary Observers (A.L.P.O), the International MarsWatch and the Terrestrial Planets Section of the British Astronomical Association (B.A.A.).  

Information for observing Mars during a typical apparition is presented in a separate report titled, “General Information for Apparitions of Mars.”  Also, you can find gobs of information at this site.

With the advent of modern CCD camera technology the amateur can produce useful images of Mars when it is as small as 3.5 arcsec . Early in an apparition, Mars rises in the east or morning sky and sets with the rotation of the Earth in the western or evening sky.  During the past few apparitions (2001-2018), observers began to take CCD images when Mars was only 30 degrees away from the Sun.  Since Mars was only a visual magnitude of ~1.8 then the planet would have been difficult to locate bright twilight hours.

In the pre-apparition reports the observer will find the motion of Mars in our sky, the characteristics for that particular apparition, information pertaining to the polar cap(s) and any special events that may be seen during that particular apparition.  As usual a calendar of events will be included with each report that contains cardinal dates for seasonal activity and orbital information of Mars.

MOTION OF MARS IN OUR SKY

As a general rule, an "apparition" begins when a planet emerges from the glare of the Sun shortly after conjunction. Mars will be in conjunction with the Sun on September 02, 2019 (74.3° Ls); however, it will not be safe to observe Mars until after October 07, 2019 when it is at least 12 degrees away from the glare of the Sun.  

The apparent declination of Mars begins at -22.3° in early March 2020 the constellation Sagittarius and will ascend north into the constellations Caprcornus, Aquarius and Pisces .  Mars will be south of the celestial equator throughout the apparition until July 12, 2020. This is not good news for those observing in the Northern Hemispheres because Mars will be seen fairly low in their sky during the entire apparition. Good news for southern hemisphere observers however. 

By March 20, 2020, a ‘0.9’ visual magnitude Mars will be seen rising early in the morning sky in the constellation Sagittarius and it will be at western quadrature with the phase defect or terminator of 38.1°.   NOTE: The Solar Elongation for Mars is the angle between the lines of sight from Earth to the Sun and from Earth to Mars.  When these lines of sight form a right triangle then Mars is at quadrature (eastern or western). For detailed definitions and graphics for the motion of Mars in our sky see these excellent web sites:  Planetary Aspects and Elongations and Configurations.

Figure 1. A heliographic chart of the orbits of Mars and the Earth showing the relative positions of both planets.  Quadrature is when Mars is directly east or west of Earth as shown.

The 2020-2021 apparition of Mars begins retrogression, or retrograde motion against the background stars eleven months after on September 09, 2020 (274.2° Ls) and continues through November 14, 2020 (314.1° Ls).  Each night for this brief period of time; before, during and after opposition the Red Planet will appear to move backwards toward the western sky in the Pisces .   Since the Martian year is about 687 Earth days long -- nearly twice as long as ours, the Martian seasons are similarly extended. While the Earth’s seasons are nearly equal in duration, the Martian seasons can vary by as much as 52 days from each other due to that planet’s greater orbital eccentricity (see Figure 2).

Figure 2. A heliographic chart of the orbits of Mars and the Earth showing the relative seasons of both planets in the planetocentric longitude system Ls. Graphic Ephemeris for the 2020 Perihelic Apparition of Mars. Original graph prepared by C.F. Capen and modified by J.D. Beish.

2020 APPARITION CHARACTERISTICS

Another general rule for predicting oppositions of Mars is from the following: the planet has an approximate 15.8-year periodic opposition cycle, which consists of three or four Aphelic oppositions and three consecutive Perihelic oppositions. Perihelic oppositions are also called "favorable" because the Earth and Mars come closest to each other on those occasions. We sometimes refer to this as the seven Martian synodic periods. This cycle is repeated every 79 years (± 4 to 5 days) and, if one were to live long enough, one would see this cycle nearly replicated in approximately 284 years. The 2020 Mars apparition is considered Perihelic because the orbital longitude at opposition will be 45° from the longitude of perihelion (250° Ls).

NOTE: Ls is the planetocentric longitude of the Sun along the ecliptic of Mars’ sky. 0° Ls is defined as that point where the Sun crosses the Martian celestial equator from south to north, that is the planet’s northern hemisphere vernal equinox. The other Ls values that define the beginnings of Martian northern hemisphere seasons are: summer, 90° Ls; autumn, 180° Ls; and winter, 270° Ls. For Mars’ southern hemisphere these values represent the opposite seasons. Distance (A.U.) - Distance from Earth to Mars in astronomical units, where one (1) A.U. equals 92,955,807.267 miles or 149,597,870.691 km.

Closest approach occurs at 1419 UT on October 06, 2020 (291.0° Ls) with an apparent planetary disk diameter of 22.6’’ at a distance of 0.414909454147 astronomical units (AU) or 38,568,243 mi (62,069,571-km).   During closest approach in 2020 the apparent diameter of Mars will be 1.7 arcsec smaller than it was at the same period in 2018; however, it will be 31 degrees higher in the sky – good for observing the Red Planet for observers in the northern and southern hemispheres of Earth.  It should also be noted that closest approach between Earth and Mars is not necessarily coincident with the time of opposition but varies by as much as two weeks.

Opposition occurs 12 months after conjunction when Mars is on the opposite side of the Earth from the Sun. At that time, the two planets will lie nearly in a straight line with respect to the Sun, or five weeks after retrogression begins. Opposition will occur at 2320 UT on October 13, 2020 (295.2° Ls) with an apparent planetary disk diameter of 22.4 arcsec. Mars will remain visible for more than 12 months after opposition and then become lost in the glare of the Sun around September 01, 2021 as it approaches the next conjunction (October 08, 2021). The cycle is complete in 780 Earth days.

Figure 3. A simulated view of the appearance of Mars during opposition at 2320 UT on October 13, 2020 (295.2° Ls, CM 162°)

The observable disk diameter of Mars will be greater than 6 arcsec from March 18, 2020 [-22.5° d ] (168.0° Ls) and will not fall below this value until March 12, 2021 (15.9° Ls), lasting nearly 12 months or 208 degrees Ls.  Imaging by CCD devices may begin with a disk diameter of 4.3 arcsec or more, commencing on or about January 15, 2020.

The Sub-Earth (De) and Sub-Solar (Ds) points are graphically represented in Figures 4 and 5. The 2019-2021 Ephemeris of Mars is tabulated on Internet in this web site. A glossary of Terms appears at the end of this table.

Figure 4. As it approaches Earth, it will swell from a small apparent disk of 6" in March 18, 2020 to a maximum diameter of 22.6” at closest approach on October 06, 2020 , and then shrink as it moves away.   Images shown at 0h UT.

Figure 5. Graphic Ephemeris of Mars during the 2020-2021 apparition from March 20, 2020 through February March 11, 2021.    Opposition and 6 arcsec apparent diameter range are indicated.  Plot illustrates the Declination (black line),   the latitude of the Sub-Earth point (De) or the apparent tilt ( green line ) in areocentric degrees, and the latitude of the Sub-Solar point ( brown line ) in areocentric degrees.  The areocentric longitude (Ls) of the Sun, shown along the bottom edge of the graph defines the Martian seasonal date.  The value of Ls is 0° at the vernal equinox of the northern hemisphere, 70° when Mars is at aphelion, and 90° at the summer solstice of the northern hemisphere 250° when Mars is at perihelion, and 180° is northern autumn.

Figure 6. Graphic Ephemeris of Mars from March 20, 2020 through February March 11, 2021.  Plot illustrates the apparent diameter of Mars in seconds of arc.  The areocentric longitude (Ls) of the Sun, shown along the bottom edge of the graph defines the Martian seasonal date.

THE SOUTH POLAR REGION

Astronomers will have a view of the Martian South Polar Region (SPR) will be tilted toward the Earth and will remain so throughout the apparition when it reaches 0° De on April 07, 2020 and should be free of its hood. For more detailed information on the south polar cap click to this web site.

DUST STORMS

Observations of Mars indicate that major dust storms tend to be more frequent when Mars is closest to the Sun – during southern hemisphere spring and summer. While predicting these events is nearly impossible to make our studies show that the Martian dusty season should begin about the third week in July (241° Ls) throughout the first week in September 2018 (270° Ls). The highest probability of dust storms occurring will be on or about August 10, 2020 (255° Ls) and a sensitive area for the development of dust storms is in northwest Hellas.    Massive, planet-encircling storms usually occur in southern hemisphere summer and that will come by the middle of November (315° Ls).  Observers should be alert for dust clouds in the northeast Hellas Basin, the Serpentis-Noachis region, and the Solis Lacus region.   For more detailed information on Martian dust storms on this web site. 

 

Table 2. CALENDAR OF EVENTS -- MARS, 2020-2021