A NASA instrument to measure the gas composition of the upper atmosphere of Mars and hardware to support a radio science experiment will fly on a Japanese spacecraft known as Planet-B. The Neutral Mass Spectrometer (NMS) instrument and Ultra Stable Oscillator are scheduled for launch aboard Planet-B on July 3, 1998, from the Kagoshima Space Center on Kyushu Island, Japan.

"The Neutral Mass Spectrometer will enable us to measure the chemical composition of the upper atmosphere of Mars on a global scale, which has never been done before," said Dr. Hasso B. Niemann, the NMS principal investigator at NASA's Goddard Space Flight Center's Laboratory for Atmospheres in Greenbelt, MD. Previous upper atmospheric composition measurements were done in only two locations as NASA's Viking landers entered the Martian atmosphere on July 20 and Sept. 3, 1976, respectively.

The radio science hardware was built by the Johns Hopkins University Applied Physics Laboratory in Laurel, MD, under contract to NASA. The ultra-precise signals generated by the oscillator serve as a very accurate clock to enable analysis of the Martian atmosphere and to help guide the spacecraft as it orbits the red planet.

Planet-B is designed to perform long-term studies of the upper Martian atmosphere and ionosphere, and its interaction with the solar wind. Launch of Planet-B is scheduled for 2:12 p.m. EDT on July 3. After launch, the Planet-B spacecraft will be placed into Earth orbit and will use two swingbys past the Moon to establish conditions for a final trajectory to Mars.

Once the spacecraft reaches Mars, which is now scheduled for Oct. 11, 1999, it will be placed into a highly elliptical or "egg- shaped" orbit stretching from 93-186 miles (150-300 kilometers) to about 17,000 miles (27,300 kilometers) above the surface. The low-altitude portion of the orbit will be used for remote sensing of the lower atmosphere and surface, and for direct measurements of upper atmosphere and ionosphere. The more distant parts of the orbit will allow instruments to probe the ions and neutral gas escaping from Mars, which interact with the charged-particle "wind" blowing outward from the Sun. Ionization of the upper atmospheric gas by solar radiation produces the charged-particle atmosphere (ionosphere) that acts as an obstacle to the solar wind.

This radiation produces species of gas not seen in Mars' lower atmosphere, such as nitric oxide, or dissociates the atmosphere into single atomic species, such as atomic oxygen. If these neutral or ionized species possess enough energy, they can escape the gravitational pull of Mars, resulting in a net atmospheric loss. Measurements of lighter species such as atomic hydrogen and deuterium also can provide clues about the evolution of the Martian atmosphere.

Mars has little or no intrinsic magnetic field to interact with this process, making it more like Venus in this respect than Earth. The upper atmosphere of Venus and its solar wind environment were studied for almost 14 years by the U. S. Pioneer Venus Orbiter spacecraft from a similar, highly elliptical orbit. The Planet-B NMS instrument is a state-of-the-art enhancement of the Pioneer Venus mass spectrometer, weighing only six pounds (2.8 kilograms). To conserve space and weight, electronic items such as transistors and integrated circuits were removed from their outer casings and placed in larger packages called hybrid circuits.

Data from previous Mars exploration spacecraft such as Mariner 9 indicate that dust storms near the surface can heat the lower atmosphere and increase the gas density in the upper atmosphere where Planet-B will make its measurements. The U.S. Mars Surveyor 1998 mission known as the Mars Climate Orbiter, due for launch this December, carries an instrument called the Pressure Modulated Infrared Radiometer, which will provide complementary information on the lower atmosphere and its response to dust storms.

The Planet-B project is managed by the Institute of Space and Astronautical Science (ISAS) within the Japanese Ministry of Education. Planet-B carries 14 instruments from Japan, Canada, Sweden, Germany and the United States. ISAS personnel will operate the spacecraft and its instruments. The spacecraft was built by the Nippon Electric Corporation and will be launched by the new M-5 rocket. This rocket is designed to expand Japan's launch capability for the inner planets and beyond.

Further information on the NASA portion of the Planet-B mission and related graphics can be obtained via the Internet at the following URL:

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