On Sept. 11, a little after 9 p.m., the trajectory of the spacecraft Mars Global Surveyor will cross Mars' orbit 186 miles ahead of the planet. A 466-million-mile journey that began on Nov. 7, 1996, will end then as the main engine fires to slow the vehicle by 2192 miles per hour. Surveyor will be captured into an orbit around Mars by the red planet's gravity. NASA scientists say the accuracy of this is equivalent to hitting the torch on the Statue of Liberty in New York harbor with a baseball thrown from San Francisco.

        Surveyor's main science instrument is a camera with a wide angle lens to produce photographs similar to weather-satellite photos of Earth and a narrow angle lens that can image objects less than 5 feet across. Surveyor also carries a laser altimeter that will measure variations in the distance from it to the surface. These distances will be combined with the photographic observations to provide a detailed topographic atlas of Martian maps. A thermal emission spectrometer will determine the general mineral composition of the ground in areas as small as 3.5 square miles and will provide data about atmospheric weather and clouds. A magnetometer that will study Mars' weak magnetic field should provide clues about the planet's interior structure. The very stable frequency of Surveyor's radio transmitter will provide information about variations in the Martian gravitational field, and attenuation of the radio waves by varying thicknesses of atmosphere each time Surveyor passes behind the planet will provide details of Mars' atmospheric structure.

        The 48-hour orbit into which Surveyor is captured, with a low point of 186 miles and a high point 34,800 miles above Mars' surface, will be much too eccentric for a mapping mission. Mission design engineers at the Jet Propulsion Laboratory sacrificed fuel, which could have slowed the spacecraft further and inserted it into a shorter, more circular orbit, for more and heavier instruments to use in its surveying mission. They will circularize and reorient its orbit about Mars by a technique called "aerobraking," first tested on the spacecraft Magellan which recently completed a radar mapping mission around Venus. Aerobraking will slow Surveyor slightly by friction with the thin upper atmosphere of Mars each time it reaches the low point of its orbit. This will gradually lower the high point and reduce Surveyor's orbital period. Not until March of next year will Surveyor's orbit be suitable for its mapping mission.

         Surveyor's final orbit will have a high point of 250 miles, an average altitude of 235 miles, and an orbital period of 117 minutes and 39 seconds. In this orbit Surveyor will pass over the north pole, travel southward over the day side of Mars, pass over Mars' south pole, and then pass northward over the night side of Mars. As Mars rotates beneath the orbiting Surveyor, its camera will be able to image the entire surface every 7.2 Earth days. The orbit is designed so that as Surveyor crosses over the equator on the day side, the Martian time on the surface below it will be 2:00 p.m. This will help to eliminate variations in surface properties that would result from the shadows being different and the heating of the ground and air being different at different times of day. Any differences will be primarily due to the progression of the Martian seasons. The scheduled mapping mission of one Martian year -- 687 Earth days -- will result in a better understanding of the seasonal variations on Mars that were already known from terrestrial observations last century.

        After Surveyor's mapping mission ends in January of 2000, the orbiting spacecraft will be used to relay radio signals to Earth from vehicles on Mars' surface as part of NASA's extended Martian exploration plan.