Spacecraft that will observe the Earth’s highest clouds
and detect hidden matter in the universe have been chosen as
the next two missions in NASA’s Small Explorer (SMEX)

“From the time Explorer 1 was launched more than 40 years ago
and discovered the Van Allen radiation belts, Explorer
satellites have made impressive discoveries by obtaining
significant science at the lowest cost,” said Edward Weiler,
Associate Administrator for Space Science, NASA Headquarters,
Washington. “The two missions we’ve selected will continue in
the Explorer tradition by investigating some of the most
fundamental questions raised in space science.”

The first mission, to be launched in 2005, is the Explorer
for Spectroscopy and Photometry of the Intergalactic Medium’s
Diffuse Radiation (SPIDR), a mission to map the “cosmic web”
of hot gas that spans the universe. Half of the normal matter
in the nearby universe is in filaments of hot gas, and SPIDR
will observe its emission and distribution for the first

SPIDR’s data will answer fundamental questions concerning the
formation and evolution of galaxies, clusters of galaxies,
and other large structures in the universe as well as address
a number of questions related to hot gas in our own galaxy.
An innovative and sensitive spectrograph design will be
placed into a high earth orbit to observe emission lines from
important atomic tracers of mass in the intergalactic medium.
Supriya Chakrabarti of Boston University will lead SPIDR at a
total cost to NASA of $89 million.

The second mission, to be launched in 2006, is the Aeronomy
of Ice in the Mesosphere (AIM) Explorer, which will determine
the causes of the highest altitude clouds in the Earth’s
atmosphere. There are indications that the number of clouds
in the upper atmosphere (mesosphere) over the Earth’s poles
has been increasing over the past couple decades, and it is
hypothesized that this results from increasing concentrations
of greenhouse gases at high altitudes.

AIM will measure atmospheric temperatures and water vapor
concentrations in the cloud forming region, as well as the
properties of the clouds themselves. This will help determine
the connection between the clouds and their environment and
serve as a baseline for the study of long-term changes in the
upper atmosphere. James Russell III of Hampton University in
Hampton, Va., will lead AIM at a total mission cost to NASA
of $92 million.

The Explorer Program is designed to provide frequent, low-
cost access to space for physics and astronomy missions with
small to mid-sized spacecraft. Six SMEX missions have been
launched since 1992, and five of them are still operating and
returning science data. The Galaxy Evolution Explorer (GALEX)
will be the seventh SMEX mission and is scheduled for launch
in September 2002. GALEX will study the history of star
formation in the universe by measuring the ultraviolet light
from newly formed stars in galaxies out to a distance of 10
billion light years.

The selected proposals were among 33 SMEX and 13 mission-of-
opportunity proposals originally submitted to NASA in
February 2000 in response to an Explorer Program Announcement
of Opportunity issued in November 1999. NASA selected seven
proposals in September 2000 for detailed feasibility studies.
Funded by NASA at $450,000 each, these studies focused on
cost, management and technical plans, including small
business involvement and educational outreach. NASA’s Goddard
Space Flight Center, Greenbelt, Md., manages the Explorer
Program for the Office of Space Science, Washington.

Information and artist’s concepts of these missions are
available at:

For more information of the Explorer program, visit: