Solar System Exploration:

Update on recent spacecraft explorers (April, 2001)

by John Rummel

While NASA's NEAR Shoemaker stole the show this past month, there are lots of other exploration missions underway. Let's take a look at a few of the prominent ones.


This past December, the Cassini-Huygens probe passed Jupiter on its way to Saturn. In the process, it returned some stunning photos of the largest planet as well as reams of data on Jupiter's magnetosphere. NASA scientists considered the Jupiter flyby as a chance to fine-tune Cassini's instruments and to make sure everything is functioning properly. Cassini passed the test with flying colors. Cassini used Jupiter's immense gravity to sling it on to Saturn, where it will arrive sometime in mid 2004 to begin a long term mission investigating the Saturn system. The Huygens probe will be dropped into Titan's atmosphere and may give us a peek at conditions similar to those on Earth billions of years ago.


The Galileo Jupiter mission has become one of NASA's most successful missions. Galileo has been orbiting Jupiter since late 1995 and its mission has been repeatedly extended and lengthened as the spacecraft just seems to keep going and going. Galileo got off to a rocky start. During its Venus and Earth flyby gravity assists in the early 1990's, the main antenna used to transmit data back to Earth failed to deploy properly. Since this antenna was the sole channel through which information could be "downloaded" at high speed to Earth, it threatened to doom the mission. Repeated attempts to unfurl the stuck antenna failed and NASA eventually switched to a small low-gain (e.g., slow) antenna. The impact of this switch was that data would be sent back to Earth at a snail's pace compared to the speedy pipeline that had been planned. In spite of its difficulties, Galileo has returned an unprecedented treasure-trove of data to scientists. Its flybys of four largest Jovian moons has revolutionized planetary science and will keep generations of scientists busy analyzing the information.

Mars Global Surveyor

Another stunningly successful mission, the MGS has been orbiting Mars since 1997. During those four years, it has returned more information than all earlier Mars missions combined. Instruments onboard MGS have mapped the surface of Mars to a degree of accuracy not yet even obtained over much of the Earth! MGS came on heels of one of NASA's most painful planet-explorer losses ever - the loss of the Mars Observer in August 1993 just days before it was to enter orbit around the red planet. Three other Mars missions were attempted in the 1990's, the wildly successful Pathfinder/Sojourner mission, and the failed Climate Orbiter and Polar Lander. NASA is preparing to return to Mars in 2001 with the launch of Mars Odyssey, scheduled to launch April 7th of this year. Odyssey will map minerals and elements on the surface of Mars, study landforms, and evaluate the potential health risks of the Martian radiation environment for any future human explorers.


Scientists have two ways of obtaining material from beyond Earth for study: Wait for it to fall (meteorites, etc.) or go get it ourselves (e.g., Apollo samples from the moon's surface). The study of meteorites makes up several branches of geology and space science, and the moon rocks confirmed many theories about the origin of the solar system. However, no samples have been returned to Earth since the 1970s. The Stardust mission sets out to remedy that situation. Launched in 1999, Stardust just did an Earth flyby gravity assist in January of this year, and will rendezvous with Comet Wild 2 in 2004. It will attempt to fly past the comet at a distance of about 30 miles and actually capture samples of the material from the comet's "coma." Since comets are thought to be the leftover material of the early Solar System, created before the time of the formation of the planets, it is hoped that we will find out more about the formation of our solar system. The planets have been altered by weathering, tectonics and other factors. In contrast, comets are believed to be the most pristine objects in the solar system..

Voyager I & II

Though these two spacecraft are from a generation ago, and their primary missions of exploring the outer planets are long since concluded, NASA remains in contact with them and continues to use information returned from them to study the region of the solar system beyond Pluto. Voyager I, the most distant human-made object, is now nearly 7 1/2 billion miles from the sun (twice the distance of Pluto), and is traveling at about 35,000 miles per hour. Even at that great speed, it would take Voyager I almost 40,000 years to reach the nearest star, Proxima Centauri (though Voyager I is not actually headed in that direction - it's headed roughly toward the constellation Hercules).

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