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Treebeard's Stumper Answer
12 September 2003

Close Encounters

It's hard to miss the planet Mars in the eastern sky after sundown. It's so bright because it's so close. Mars was closest to us on August 27, but it's still plenty bright and even easier to see now because it's rising a bit earlier every day. We've all heard that this was the closest approach to Earth in 60,000 years. Should we worry that Mars is getting closer? Earth orbits the sun in 365 days, and Mars orbits in 687 days, so why aren't these close encounters more common? What kind of natural cycle in the solar system takes 60,000 years to repeat?

At it's closest approach last month, Mars was a "mere" 34,647,397 miles from Earth. It's still plenty bright. If you haven't noticed it yet, get outside tonight!

I did my duty as a science teacher on the night of August 27, 2003 and went out at 1:30 A.M. PDT to get a photo of Mars at closest approach. (See this NASA page.) My method was brute force: set my Olympus 2040Z digital camera up on a tripod at full telephoto and take the longest manual exposure possible, F2.6 @ 16 seconds. It`s still just a (over-exposed) red dot, so I enlarged the 30x30 pixel planet to fill the frame. Kind of pretty. I also pasted the original size image in the upper right corner for comparison.

Mars at closest approach, August 27, 2003 at 1:30 A.M. "Black frame" photo with lens covered for 16 second exposure. .

A problem with such long exposures is that digital photo sensors fire off +/- random pixels over time. Astronomers use elaborate schemes to keep their cameras VERY COLD. Another way is to put your hand over the lens and take a "black frame" shot and then subtract it from your image using a program like the free MediaChance Black Frame Noise Reduction utility. This works because the hot pixels aren`t quite random, they will most likely be in the same place during a short period at the same temperature. (I have three hot pixels that show up in every photo.) The photo on the right is a 360x360 pixel section of my 16 second "black frame" exposure with the lens covered. Looks pretty busy for nothing!

I got another photo last week on September 10, 2003 showing the almost full Moon and the bright planet Mars rising above the trees at my home. They were even closer together two nights before, but lost in the fog. The exposure is F2 @ 1 second, short enough that noise is not a problem.


The Moon and Mars on September 10, 2003

This photo raises another stumper. Mars and the Moon are barely above the trees. Last winter, Saturn and Jupiter were much higher and easier to see. Why the difference?


Mars is close to Earth, but it's not getting closer in any dangerous way. It's like we occasionally have an extra-low tide when many conditions come together just right. Earth and Mars have a close encounter about every two years, but both orbits are ellipses not circles. This month, Mars is near perihelion, closest to the Sun, at about the same time that Earth is near aphelion, farthest from the sun. It is this exact combination of events that has not happened for 60,000 years. Enjoy the show while you can! Mars will dim noticeably early next month.

Notes:

My very first stumper of the year and I'm already behind, yikes! Maybe it's just as well since our close encounter with Mars has been so acclaimed in the media.

DMS students Travis, Trevor, Bryan, and Vince all answered this stumper right away. But there is a deeper question. Earth and Mars have a close encounter about every 26 months, so why is this 2003 encounter especially close? There are other factors including the slowly changing tilt and eccentricity of our orbits. I think of watching the "12:00" un-set time blink on several VCRs and wondering when they will all blink together. The basic problem is like finding the Least Common Multiple (LCM) of several numbers.

It's even more complicated because the solar system is a physical system with gravitational interaction between all the planets, so the position of one slightly changes all the others. The long-term stability of the solar system is still an open question because of these interactions!

I don't know if I'll have time to get back to this stumper, but there's lots on info on the Web if you look for it. I believe the main source for the 60,000 year claim is Aldo Vitagliano's Solex program. It's a freeware program that can record planetary encounters based on a physical model over millions of years. I haven't had time to play with it yet. The author talks about Mars close encounters here and presents this graphic showing close encounters 100,000 years back and forwards:

This chart shows that the difference between a close encounter and a REALLY close encounter isn't that much, but it's real. This reminds me of a tide chart made by combining harmonic sine waves, see my Time and Tide (6 December 2002) stumper. I'll get back to this when I can!

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Copyright © 2003 by Marc Kummel / mkummel@rain.org