How Close Mars?
by John Rummel
When it comes to observing the planets, Jupiter and Saturn probably amount to 90% or more of amateur astronomers’ observing time. Both planets reach opposition about once per year. Both are large, spectacular targets, rich in detail, and endlessly rewarding of either cursory or close study. And between Jupiter’s moons and Saturn’s rings, both are real crowd pleasers at star parties as well.
Mars is most often an inconspicuous orange, starlike object that wanders eastward among the stars. But about every 2 years, it begins to brighten, reverses its course and is prominent as a fiery orange-red star in the evening sky. At times it even exceeds mighty Jupiter in brightness. Then, after a month or two, it begins to fade and resumes its anonymity among the starry host.
The oppositions of Mars, much less frequent than those of Jupiter and Saturn, are generally disappointing to observers. Its disk is tiny. Even at good oppositions like the one this summer, its angular size is just a bit more than half of Jupiter’s average apparent size. In short, for all but the most dedicated observers, Mars often gets a quick glance, and the observer moves on to more interesting targets.
Having said all that, this summer will be of great interest to both casual and dedicated Mars observers. This year Mars reaches a rare “perihelic” opposition, in which it will be at its closest point in a cycle that causes its close-approach distance to vary by nearly half over a period of years.
How close this year?
Much has been made of the fact that this opposition of Mars in 2003 will bring the red planet closer to Earth than it has been at any time in thousands of years. Depending on who you read, the figures can vary widely. I’ve read credible reports that say Mars will be closer than at any time within the last 100,000 years. Or 70,000 years says another publication.
What exactly is going on here?
Part of the problem is in how you define your terms. Part of it lies in how mathematically rigorous you want to be. Here, in a nutshell, are the elements contributing to this issue.
Though Mars reaches opposition roughly about every 26 months, all these oppositions are not equal. Mars’ orbit is significantly more elliptical than Earth’s, so only those oppositions that occur close to Mars’ perihelion will be extremely favorable for observation from Earth. These so-called perihelic oppositions occur roughly every 15 to 18 years, and always occur around late August or early September.
All perihelic oppositions tend to be pretty good. But this summer, the opposition is very close even by typical perihelic standards. For instance, in 2003, Mars will be 19,000 kilometers closer to the Earth than it was during the last exceptionally close opposition, the one of August 1924. This difference in distance is all but insignificant as far as observational variables are concerned (less than one-hundredth of an arcsecond growth in apparent size, for example).
Just how close these perihelic oppositions are depend on a number of variables, including the moment of Mars’ own perihelion, or closest approach to the sun (this year it occurs on August 30th, about two days after the Earth-Mars opposition). The closer an opposition is to Mars’ own perihelion, the closer the two planets are.
If this was the whole story, any yahoo with a computer and a copy of Starry Night software could settle the issue. Unfortunately, the situation is quite a bit more complex. First of all, like all planets, the orbits of Earth and Mars slowly precess around the sun. Their inclinations and points of perihelion slowly shift position. These critical changes in position of perihelion and aphelion must be taken into account, but most desktop astronomy software uses static orbital elements and can’t account for such long term periodic changes. Right now, the position of Mars’ perihelion and Earth’s aphelion are slowly converging, and in a few thousand years they will line up neatly. This situation is responsible for the fact that the close oppositions have been getting gradually closer over the past few thousand years. Dr. Myles Standish, orbital wizard at the Jet Propulsion Laboratory, has provided the table reproduced above. He notes that “...there is a slow evolution of the Earth and Mars orbits, so that the close approach distance is increasing into the past; decreasing into the future. Of course, the evolution of the orbits is periodic, so that the distance will eventually [begin to increase again]”
Ast. Units Kilometers Date
0.37200418 55651033. 2729 SEP 08
0.37200785 55651582. 2650 SEP 03
0.37217270 55676243. 2934 SEP 05
0.37225400 55688405. 2287 AUG 28
0.37230224 55695623. 2808 SEP 11
0.37238224 55707590. 2571 AUG 30
0.37238878 55708568. 2366 SEP 02
0.37271925 55758006. 2003 AUG 27
0.37279352 55769117. 2208 AUG 24
0.37284581 55776939. 1924 AUG 22
0.37292055 55788120. 2887 SEP 16
0.37296343 55794535. 2445 SEP 05
0.37302110 55803163. 1845 AUG 18
0.37305741 55808594. 1482 AUG 03
0.37310445 55815632. 2855 AUG 31
0.37321735 55832521. 2492 AUG 24
0.37325251 55837780. 1561 AUG 07
0.37326031 55838948. 1766 AUG 13
0.37327582 55841268. 2129 AUG 19
0.37339761 55859488. 1119 JUL 31
Table: The 20 closest Mars oppositions 3000 BC to 3000 AD, computed by Dr E Myles Standish; JPL, and posted to DOME-L, a newsgroup for planetarium professionals, in response to a question about Mars’ close approach this year. Dr. Standish’s data is based primarily on the convergence of Earth’s and Mars’ aphelion/perihelion points. Note there is no historical opposition closer than the 2003 event, but seven even closer ones coming up over the next several hundred years
That’s not the end of the story though. To split hairs even further, the eccentricities and inclinations of the planets’ orbits also slowly change over time. Though this process is a bit less well understood, and therefore slightly more theoretical, the changes are predictable up to a point. Mars’ orbit is gradually becoming even more eccentric than it is now, and this will continue for several thousands of years, before it will gradually begin to decrease toward its current value again. Mathematical astronomer Jean Meeus has investigated this question in some depth. In an article published in the March 2003 issue of The Planetarian (published by the International Planetarium Society), he elaborates on his earlier work in his excellent books Astronomical Tables of the Sun, Moon, and Planets, Mathematical Astronomy Morsels and More Mathematical Astronomy Morsels. His discussion includes several tables showing the evolutions of the eccentricities of the orbits of Earth and Mars, covering a period of some 2 million years. His work, and his collaborations with others, leads him to conclude that the last time Mars was closer to Earth than it will be in August of 2003 was September 12, 57617 BC. Accordingly, it has been about 60,000 years since Mars has been as close as it will be this year.
All of these cumulative sources of change reinforce the idea that no two perihelic oppositions of Mars are ever exactly the same. This fall, Mars will be extremely well placed for amateur observation. Professional astronomers, by the way, care little about such issues. Most professional examination of Mars is based on research by orbiting probes such as Global Surveyor and Mars Odyssey. The Hubble Space Telescope typically examines Mars around opposition, but the variations in distance discussed in this article are of no consequence.
If somebody knows you are an amateur astronomer, they may ask you about Mars being closer than it has in thousands of years. Disabuse them of any notions that Mars will loom larger than the full moon in the sky, and then tell them the truth.
Or maybe just let them take a look at Mars in your telescope.
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