The Sky Guy

Mars, meteors welcome night sky viewers

Posted: Sunday, August 14, 2005

 

  Graphic courtesy of Andy Veh

Graphic courtesy of Andy Veh

The astronomy season starts out in grandeur as the first major planet visible will be Mars at its best. It'll be joined by Venus and Jupiter early this fall and Saturn later this year.

The red planet has been an odd-year planet throughout the 1990s and this decade, meaning it became prominent in 2003, again this year and will be prominent again in late 2007. Mars needs about one year and 11 months for one orbit. So as Earth orbits our sun and returns to the same spot after exactly one year, Mars is almost on the other side of our sun. As we're trying to catch up during our second orbit, Mars makes the feeble attempt to elude us, but finally Earth catches up and get closest to Mars once again after two years and two months. Hence the two-year recurrence of Mars from our vantage point.

Mars is moving across Aries until January and will start retrograding in October, with our closest approach on Nov. 6. Mars will move across Taurus in February and March and through Gemini in April and May.

Astronomers like to be close to Mars to observe it because we can only see it when Earth and Mars are on the same side of the sun, or else the sun would be in between, rendering Mars obliterated by the bright daytime skies, as will be the case for the remainder of 2006. But we also like to be closer to a planet because it appears brighter and bigger, which is especially the case for Mars. Once again Mars is in the news for allegedly being closer than ever, just as it was two years ago.

You can't miss reddish Mars in the east or south, depending on the time of night, because it's the brightest object in the night sky, apart from our moon. The diagram shows the positions of Mars during August and September around midnight. View Mars due south about 15 degrees above the horizon, either above the Kenai Mountains at Kach-emak Bay or above Cook Inlet.

There are some bright stars out that could be mistaken for Mars. These stars are reddish Arcturus and Antares in the west and southwest. These are huge, cool stars that radiate at around 5,000 degrees F. That temperature corresponds to red, dying stars called red giants, unlike Mars which gets its color from its oxidized, rusted surface rocks. Other bright stars make up the summer triangle overhead, Vega, Deneb and Altair.

Every year August brings the most famous and probably best known meteor show, visible this weekend. As Earth orbits the sun, we intercept the orbits of comets — just the orbits, not the comets themselves since they're in other positions of their orbits — and are "pelted" by the the debris that is strewn throughout their comets' entire orbits. Mostly this debris is fine dust particles or at best, tiny rock pebbles.

In August, Earth passes through the orbit of comet Swift-Tuttle. When we look down its orbit we see the constellation Perseus. Therefore it appears that the dust particles in Swift-Tuttle's wake are coming from Perseus —hence the meteor shower is given the name Perseids. These dust particles burn up in our atmosphere, creating what's called meteors. Comets are named after their discoverers. In this case the comet was discovered independently by Americans Lewis Swift and Horace Tuttle, who noted the comet in 1862.

The comet also goes by the designation 109P, the P meaning it's a periodic comet that returns in regular intervals, the number meaning it's one among 164 such comets. Designation one goes to Halley and nine is Tempel, the one NASA's Deep Impact probe collided with last month.

Swift-Tuttle returns every 130 years or so. When it was rediscovered in 1992, its orbital data didn't correspond with the previous data. Instead, a nongravitational computation provided a better fit. That sounds counter-intuitive at first since gravitation really is the only force affecting orbits. But comets lose mass as solar wind pushes away dust and gas — which we describe as a comet's tail — and thus the gravitational force diminishes, changing a comet's orbit.

A new planet in our solar system has been discovered within the Kuiper belt, a zone extending tens of billions of miles past Neptune's orbit consisting of icy remnants from the original material that formed the solar system. The object, 2003 UB313 (a catalog number akin to 109P), is a whopping 10 billion miles from the sun and Earth on an orbit with an average distance of 6.8 billion miles, about twice that of Pluto. Its orbit is highly inclined at 44 degrees and it needs 557 years to revolve around our sun. It's a little bigger than Pluto, yet slightly smaller than our moon.

This discovery has once again spawned a discussion about what a planet is. There are no clear-cut criteria for the definition of a planet and it's not a scientific question but rather an etymological one.

Wikipedia.org points out that most definitions agree "that (a planet) must orbit a star, be massive enough to become spherical using its own gravity, and yet not big enough to produce nuclear fusion in its core." However, even that definition is not set in stone, as many moons are spherical and a few (our moon, Jupiter's moons Io, Europa, Ganymede and Callisto and Saturn's moon Titan) are even bigger than Mercury, Pluto and 2003 UB313. There also are rogue planets drifting through our galaxy, not orbiting any star.

Personally, my response is nonchalant — any discovery and new development in astronomy is so exciting that a question of definition is only secondary.

Andy Veh is the physics and astronomy instructor at Kenai Peninsula College.

He can be reached at aveh@uaa.alaska.edu.



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