Spacecraft to hunt cosmic and earthly wisps

The SPIDR probe will map filaments of the intergalactic cosmic web.

(CNN) -- From dozens of low-cost proposals, NASA chose two missions to send into space in the coming years, including a $89 million satellite that will search for an intergalactic cosmic web.

The Explorer for Spectroscopy and Photometry of the Intergalactic Medium's Diffuse Radiation orbiter, or SPIDR, which should launch in 2005, will map the distribution of hot gas filaments in the nearby universe.

The observations could shed light on how galaxies form, evolve and cluster with other galaxies, according to NASA, which announced the Explorer-class mission selections this week.

"From the time Explorer I was launched more than 40 years ago and discovered the Van Allen radiation belts, Explorer satellites have made impressive discoveries at the lowest cost," NASA deputy chief Ed Weiler said in a statement.

"The two missions we've selected will continue in the Explorer tradition by investigating some of the most fundamental questions raised in space science," he said.

While SPIDR looks up to the heavens, another Explorer mission will look down in the direction of Earth, searching for the planet's highest clouds.

The AIM mission will study how greenhouse gases affect the upper atmosphere.

The Aeronomy of Ice in the Mesosphere mission, or AIM, slated to launch in 2006, will help scientists investigate the causes of noctilucent clouds, which form about 50 miles (80 kilometers) above the surface.

The enigmatic wisps, which seem to glow at night, form much higher than other clouds, most of which reside within the first six miles (10 kilometers) of the atmosphere.

The number of noctilucent clouds over the polar regions has risen steadily in recent decades, possibly the result of increasing concentrations of greenhouse gases at high altitudes, scientists theorize.

"One theory is the atmosphere 50 miles above the surface is getting colder because CO2 [carbon dioxide] is building up and causing the atmosphere to cool," said AIM scientist Russell James of Hampton University in Hampton, Virginia.

"CO2 warms the atmosphere near the Earth surface, but would cool the high atmosphere."

Other possible causes include an increase in atmospheric water vapor, which could result from growing methane emissions, he said.

The $95 million AIM mission will monitor the clouds and measure water vapor and temperatures in the upper atmosphere, which could help establish a link between the environment and the clouds.

NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Explorer Program, designed to produce inexpensive space access for small and medium-sized scientific probes.