Douglas Isbell Headquarters, Washington, DC July 21, 1995 (Phone: 202/358-1753) RELEASE: 95-117 ATMOSPHERIC INSTRUMENT SELECTED FOR 1998 MARS ORBITER NASA officials have selected an atmospheric science instrument called the Pressure Modulator Infrared Radiometer (PMIRR) to fly aboard the Mars Surveyor '98 orbiter, a Mars- bound spacecraft scheduled for launch in late 1998. This selection helps define the mass and electrical power requirements of the scientific payload for the mission. The payload also will include an optical camera, which is currently the subject of an open selection competition due to be concluded in October. The selection of PMIRR follows the recommendation of an ad hoc science definition committee chaired by Dr. Geoffrey Briggs, director of the Mars Mission Research Center at NASA's Ames Research Center, Mountain View, CA. PMIRR was chosen over a Gamma Ray Spectrometer (GRS) instrument that would study the chemical composition of the Martian surface. A GRS is now tentatively planned for flight on a NASA Mars orbiter that will be launched in 2001. "The PMIRR instrument will make a fundamental contribution to our Mars program science goals," said Jurgen Rahe, director of the Solar System Exploration Division at NASA Headquarters. "The GRS instrument can more easily be accommodated on the 2001 Mars Surveyor orbiter." PMIRR will produce a vertical picture of the tenuous Martian atmosphere by measuring a profile of the infrared radiation reflected from the horizon of the planet. These data can be used to derive other atmospheric components such as temperature, water vapor levels and dust content, from the Martian surface level through altitudes as high as 50 miles. Such information helps produce more accurate global models of the Martian environment, a necessary precursor for future robotic and human exploration missions. The principal investigator for PMIRR is Dr. Daniel McCleese of NASA's Jet Propulsion Laboratory, Pasadena, CA. The Mars Surveyor '98 orbiter and a separate Mars lander to be launched in 1998 are being built for NASA by Lockheed Martin Astronautics, Denver, CO. PMIRR and GRS are the two remaining instrument spares from the Mars Observer program waiting for an opportunity to fly again. Mars Observer fell silent on August 21, 1993, three days before it was to have entered orbit around Mars. An independent investigation panel determined that the most likely cause of this loss of communications was a high-speed spacecraft spin that resulted from a propellant system leak. The other five of the seven Mars Observer science instruments will be carried by NASA's Mars Global Surveyor orbiter spacecraft, due for launch in November 1996. -end- EDITOR'S NOTE: An artist's concept of the Mars Surveyor '98 orbiter is available for media representatives by calling the News Branch at 202/358-1900. The photo number is: Color -- 95-HC-133.