27th July 1997

An exciting way to more discoveries

By Ranjit T Edward

Mars rover riding the Martian surface.
An exciting and awe some demonstration of engineering and technical competence was witnessed on the surface of Mars on the 4th of July 1997, when the Pathfinder spacecraft pierced the upper atmosphere of Mars. Mars Pathfinder was a NASA Discovery program mission scheduled for launch in early December 1996. The spacecraft used an innovative, unconventional method of directly entering the Martian atmosphere. Tethered to a huge, billowing parachute, the spacecraft successfully descended to the surface. Seconds before impact, Pathfinder inflated a cocoon of airbags to cushion its landing.

Within hours of a safe landing, the lander opened up and deployed a mini-rover weighing in at 10 kilograms (22 pounds). Sojourner, the name given to this free-spirited rover, set out to explore the mouth of an ancient outflow channel called Ares Vallis, using a tool kit of miniature science instruments to study the composition of rocks and take close-up photographs of Martian surface features.

Mission Profile

Mars Pathfinder was launched atop a Delta II expendable launch vehicle during the early part of December 1996, and spent seven months cruising to Mars. The tetrahedral shaped spacecraft entered the upper Martian atmosphere at 7.6 kilometers per second (17,000 miles per hour) at a 14.2-degree angle. It reached its peak atmospheric shock of 25 times Earth’s gravity at 32 kilometers (20 miles) above the surface. A parachute was deployed at 6-10 kilometers (3.7-6.2 miles) above the ground and rockets inside the backshell were fired to further slow the lander’s descent. A tether attaching the lander to the backshell was severed and a collection of airbags inflated to soften the impact. Pathfinder landed as scheduled on Mars on July 4, 1997 (2310 Hours Sri Lanka Standard Time).

The rover mounted on one of three panels inside the lander was tied down with a connector cable. During cruise, the lander-cruise stage provided the rover with structural and thermal support, and limited data collection and transmission.

The entry and landing sequences were designed to minimize gravitational forces, called "g levels" on the payload and keep them to about 40 g’s on impact with the Martian surface. The spacecraft, enveloped in airbags, bounced many times - and as high as a 4 storey building - across the surface for nearly 10 minutes, before coming to a halt. Once it had stopped, the lander deployed its three solar panels for power and raised a camera to view the surroundings. The rover was then prepared for deployment to the surface.

To keep costs down, the surface mission was designed to achieve the most important objectives within the first few days of the mission. The lander’s first task was to transmit engineering and science data collected during descent through the thin atmosphere of Mars. Then its camera took a panoramic image of its surroundings and began transmitting the data directly to Earth at a few thousand bits per second. Much of Pathfinder’s mission after this will be focused on supporting the rover with imaging telecommunications and data storage.

The Rover and Science Instruments

The rover with its mounting and deployment equipment weighed about 17.5 kilograms (38.5 pounds) at launch. Once mobile it weighs a mere 10 kilograms (22 pounds).

The rover relies on a six-wheel rocker-bogie mobility system to navigate the surface Equipped with three cameras - a forward stereo system and rear colour imaging system - Sojourner has taken several black-and-white images of the lander to assess any damage that may have been incurred on impact.

Sojourner runs on a 0.2 square meter (1.9 square feet) solar array, sufficient to power the rover for several hours per day, even in the worst dust storms. As a backup and augmentation, lithium thionyl chloride D-cell batteries are enclosed in the rover’s thermally protected warm electronics box. Thermal protection is provided by a nearly weightless material called silica aerogel. The rover control system features operator designation of targets and autonomous control to reach targets and perform the tasks.

Science objectives

While Pathfinder is designed to demonstrate an innovative engineering design for delivering spacecraft to Mars, the spacecraft will also accomplish a focused set of exciting science investigations. In the first few days of the mission, the lander used its stereo, multi-colour imager to take several panoramic photographs of the Martian landscape. Pathfinder’s atmospheric instrument will study the upper atmosphere of Mars. A meteorological package will monitor Martian weather. Regular tracking of the lander can be used to more precisely determine the Martian pole of rotation, its precession or orbital changes since Viking era measurements and the moment of inertia, which should validate or disprove theories about Mars’ interior, whether the planet has a metallic core and internally generated magnetic field. The landing site will give geologists an opportunity to sample a wide variety of different Martian crystal materials and rocks. Ancient floods in Ares Vallis are thought to have carved out mini-island structures in the soil, Viking data suggest this region is about as rocky as the Viking sites, but perhaps a bit less dusty. The site appears to have been created by smooth layers of sediment and punctuated with a few large hills and small secondary craters.

Rover science revolves around the alpha proton x-ray spectrometer, which will be used to measure the elemental composition of rocks and surface soil and determine their mineralogy. This data will help scientists understand more about the crust of Mars, its differentiation and the impact of weathering on surface features. Pathfinder will be the first step in beginning to understand the early evolution and geological history of Mars. Traces of water vapor in the atmosphere and soil will help scientists understand more about Mars’ early atmosphere. Ultimately these clues will lead to a better understanding of how Mars’ evolution differed from Earth’s.

(Courtesy - NASA Discovery Program Mission
JPL for NASA’s office of Space Science, Washington, D.C.
Ames Research Center in Mountain View, Calif.,
Lewis Research Center in Cleveland, Ohio,
Langley Research Center in Hampton, Va.,
Jet Propulsion Laboratory in Pasadena, Calif).

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