Mars will continue to be visible through the winter and spring 2008. Our Monday twilight and Friday evening telescope viewing sessions will continue to feature Mars and other celestial objects through the end of the spring 2008 semester.
The Red Planet puts on its best showing in December, when it reaches its closest approach to Earth on the 18th and opposition with the Sun on the 24th. Mars is now quite prominent in the northeastern sky during the evening hours, resembling a brilliant, orange-red star in the constellation Gemini. It rises around 6:30 pm EST in early December, but around 4:30 pm, or at sunset, at opposition. Mars's brightness now rivals and even surpasses that of blue-white Sirius, the brightest star in the sky, which rises about two and a half hours after Mars.
No other planet has inspired the human imagination the way that Mars has, primarily due to its many similarities to the Earth. For instance, Mars has a rotation period of just a little longer than 24 hours, and the Martian rotation axis is tilted at an angle of 25°, very close to Earth’s value of 23.5°. Mars experiences seasonal changes just as Earth does, and a telescope reveals that the Martian polar caps actually grow and shrink. During this opposition, it will be summer in the Martian southern hemisphere and winter in the northern hemisphere.
A telescope also shows that Mars has changing weather patterns that include haze, water ice clouds, and ground fog. Moreover, spacecraft have revealed that the surface of Mars contains mountains, canyons, volcanoes, and even dry riverbeds called arroyos. On the other hand, Mars has only half the diameter of Earth, and, because Mars’s orbit lies over 50 percent farther from the Sun than does Earth, the average surface temperature of Mars is much colder than that of Earth.
The Martian atmosphere is also much thinner than Earth’s, and is composed primarily of carbon dioxide, instead of nitrogen and oxygen which are the primary constituents of the air we breathe on Earth. Much of the Martian surface is covered by iron oxide, which gives the planet its distinct ruddy red color.
On December 18, Mars will be closest to Earth, and less than a week later, on December 24, Mars will reach what astronomers refer to as its opposition point. Opposition occurs when an outer planet like Mars lies on the exact opposite side of the Earth from the Sun. Hence, on that date Mars will rise at sunset and set at sunrise.
An outer planet is closest to Earth when at or near opposition. (The 6-day discrepancy between closest approach and opposition of Mars is due to its elliptical orbit.) Earth is the faster moving inner planet, with an orbital period of 365 days while Mars, which lies further out, takes 687 (Earth) days to orbit the Sun. This means that every 26 months Earth “laps” Mars as these two planets orbit the Sun. When this occurs, Mars lies in the opposite side of Earth’s sky from the Sun, and hence is visible all night long. Not all oppositions are equally favorable, however.
This year when Mars reaches opposition, it will be located in the constellation Gemini and lie at a distance of about 55 million miles, while during the previous opposition in November 2005 Mars was at a distance of about 44 million miles. And at the opposition before that in late August 2003, Mars was at its closest viewing position in recorded history, about 35 million kilometers away, and located in the constellation of Aquarius.
While Mars will be over half again as far from Earth in 2007 as it was in 2003, a somewhat compensating factor is that this year Mars is positioned at a much more northerly declination than it was then, which means it will be much higher in the sky and therefore more easily accessible for viewers in the northern hemisphere and less subject to atmospheric distortion.
A telescope will usually show polar caps and dark markings on the Martian surface. Mars sometimes experiences planet-wide dust storms which obscure its surface features, and if one occurs you won’t be able to see any surface markings at all. Still, with or without a telescope, Mars will be a spectacular sight in the night sky.
In 2001, a dust storm which started as a local disturbance in the Martian southern hemisphere in July spread rapidly across the entire planet by August, effectively obscuring the planet’s surface. Such global dust storms are relatively rare, but when they occur they obscure the surface details as seen from Earth. These storms are powered by the heat of the Sun’s rays, which are especially strong when Mars is near its perihelion point.
Mars was first visited by a robotic spacecraft, the Mariner 4, in July of 1965. The images radioed back to Earth showed for the first time that Mars has a cratered surface, but little else. Subsequent Mariners missions visited the Red Planet in the late 1960s and early 1970s revealed many more surface details, as described in the previous paragraph. The Viking I spacecraft made the first successful soft landing on Mars in July of 1976, and the Viking II followed in September of the same year. The Viking cameras revealed a desolate desert landscape, and several tests concluded that there was no life on Mars.
The Mars Pathfinder mission in 1997 also beamed back pictures of the surface, and took samples of soil. On October 23 of 2001, NASA’s Odyssey Spacecraft reached the Red Planet. Mars Odyssey carries an infrared imaging system, called THEMIS, that uses infrared spectroscopy to determine the composition of mineral deposits on the surface. THEMIS is designed to work in concert with the spacecraft's gamma-ray spectrometer, which has been used to map the abundance of hydrogen (a proxy for water) just below ground level.
On June 10, 2003, the first Mars Exploration Rover was launched from Cape Canaveral aboard a Delta II rocket. The second rover was launched later in the month. NASA announced the names for the two Mars buggies: Spirit and Opportunity. The landing sites lie on opposite sides of Mars. Spirit landed in a large, sediment-filled crater called Gusev. Opportunity landed in the flatlands of Meridiani Planum.
The Phoenix spacecraft was successfully launched on August 4, 2007 by a Delta II rocket from Kennedy Space Center, Cape Canaveral, Florida, and is currently on its interplanetary cruise. It is scheduled to arrive at Mars on May 28, 2008. It is a lander with the task to provide an in-situ volatile and organic molecule survey, proposed and directed by LPL/University of Arizona.
Source: Lunar and Planetary Laboratory, University of Arizona