Sep 13, 1999

Mars meets anti-Mars





This week Mars and Antares pass less than 3 degrees apart.
IN PREPARATION: Mars &  Antares above an old barn
September 13, 1999: After approaching one another for months, Mars will pass just north of the bright red star Antares on Wednesday. Separated by less than 3 degrees, the two will resemble a pair of red embers smoldering above the southern horizon.

Right: Duane Hilton's rendering of Mars, Antares, and the crescent Moon above an old barn just after sunset on Sept. 15, 1999, two days before closest approach. While you're enjoying the view of Mars and Antares in the southwest, don't forget to look slightly eastward to view the Galactic Center in Sagittarius. How to find Mars, Antares, and the galactic center.
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The name Antares means "rival of Mars" or "anti-Mars," probably because the star's vivid reddish hue reminded ancient astronomers of the Red Planet. This week the pair can be seen from both hemispheres of Earth as soon as the sky begins to get dark. Simply go outside and look to the south-southwest. From mid-northern latitudes Mars appears about 25 degrees above the horizon with Antares just below and to the left. Because the pair will be low in the sky for most of North America, an unobstructed view of the horizon and dark skies are recommended.

This week Mars will appear to be about twice as bright as Antares. Like all planets, Mars shines by reflected sunlight and is intrinsically dimmer than a star. Mars seems to be brighter because it is much closer than Antares. The distance from Mars to Earth is a mere 191 million km -- just a hop, skip, and a jump by cosmic standards -- while Antares lies 30 million times further away (6x1015 km).

Antares is 700 times larger, 9000 times more luminous and about 15 times more massive than the Sun. Although it's big and bright, most of Antares is really fluffy and insubstantial. The star's density is a million times less than water. In fact, the outer layers of Antares are so thin that you could fly a spaceship through them and barely notice the difference.
image credit D. Malin (AAO), AATB, ROE, UKS Telescope
Antares is in a region of space that includes star-forming molecular clouds and glowing nebulae. Seen through a powerful telescope, the area around Antares is filled with colorful clouds and dark dust lanes. In this image (left) from the Anglo-Australian Observatory, Antares lights up the yellow-red clouds on the upper left. Another star, Rho Ophiuchi lies at the center of the blue nebula on the right.

Above: This picture shows what the sky just around Antares looks like in a long photographic exposure obtained with a large telescope. Why is the sky near Antares and Rho Ophiuchi so colorful? [Click here to find out.]

The close approach of Mars and Antares is a great opportunity to observe a phenomenon that is well-known to avid stargazers: Stars twinkle but planets do not (at least, not much).
Credit and Copyright: Tony Phillips
Right: This black and white video clip shows the planet Mars (above) and the star Spica (below) during a 1.7 degree close encounter earlier this year. It shows how Spica twinkles but Mars does not. The images were obtained by Tony Phillips on June 6 using an Astrovid 2000 CCD video camera. The animation consists of 10 video frames each separated in time by 67 milliseconds.

Neither stars nor planets twinkle as seen from outer space, but the view from Earth's surface is a different matter. Tiny irregularities in the density and temperature of the air above us drift in front of stars and planets as we view them through the atmosphere. When a beam of starlight travels through the atmosphere, these irregularities disturb the beam making it dance back and forth slightly. That's what causes stars to "twinkle." (In scientific terms: tiny inhomogeneities in the atmospheric index of refraction cause point sources like stars to "scintillate.") Stars are so far away that they look like unresolved points of light even in large telescopes. Planets, on the other hand, are relatively nearby. They look like disks even in small telescopes. Planets don't twinkle because the scintillation of light from one side of the disk mixes with scintillation from other parts of the disk, and the disturbances cancel out. The light from planets arrives as a steady stream, not a babbling brook. It's all because planets appear much, much bigger than stars as seen from Earth.
Left: Facing south just after sunset this week, stargazers in the northern hemisphere will see the constellations Scorpius and Sagittarius low on the horizon. Antares is the brightest star in Scorpius, just below reddish-colored Mars. Observers in dark sky locations will be able to see the pearly white glow of the Milky Way slicing through Sagittarius and the tail of Scorpius. The Milky Way seems to brighten in the vicinity of Sagittarius and, indeed, the Galactic Center is located just above the spout of the familiar teapot asterism.


Web Links


This week's sky at a glance -- from Sky & Telescope

The Planet Mars - from the SEDS Nine Planets web site

Mars Global Surveyor - home page

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A close encounter with the Red Planet -- Mars makes its closest approach to Earth in 1999. Apr 23, 1999 NASA Science News

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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: Frank M.