Explorer program: Explorer 9 (S-56a) is launched.

The NASA Explorer Program: Pioneering Space Science and Exploration Since 1958

The Explorer program stands as a cornerstone of NASA's enduring legacy in space exploration, offering unparalleled flight opportunities for a diverse range of scientific investigations from orbit. This initiative specifically targets critical research in physics, geophysics, heliophysics, and astrophysics, providing a platform for breakthroughs in our understanding of the universe, our solar system, and our home planet.

Its inception marked a pivotal moment in the Space Race. Launched on January 31, 1958 (having been designed and built by the Jet Propulsion Laboratory and instrumented by the University of Iowa), Explorer 1 became the first spacecraft successfully placed into Earth orbit by the United States. This historic achievement followed the Soviet Union's Sputnik 1 and Sputnik 2, galvanizing American efforts in space science. Explorer 1 carried a Geiger-Müller tube, which famously led to the discovery of the Van Allen radiation belts, a monumental finding that reshaped our understanding of Earth's magnetosphere.

Since that groundbreaking start, the Explorer program has overseen the launch of over 90 space missions, a testament to its longevity and scientific productivity. While early missions were managed by various entities, the program has been consistently operated by NASA since Explorer 6, launched in 1959. A hallmark of the Explorer program is its emphasis on collaboration, regularly partnering with a wide array of other institutions, including universities, research centers, and numerous international partners from countries around the globe. This collaborative model fosters global scientific cooperation and leverages diverse expertise and resources.

Launch Vehicles Powering Explorer Missions

The Explorer program has utilized a remarkable variety of launch vehicles throughout its history, reflecting the evolution of rocketry and space technology. These include the early Juno I and Juno II rockets, various configurations of the Thor family (such as Thor-Delta), the cost-effective solid-fueled Scout rockets, the versatile Delta series, the air-launched Pegasus vehicle, and more recently, the powerful Falcon 9. This diversity in launch capability has allowed the program to adapt to changing mission requirements and technological advancements, ensuring access to space for its scientific payloads.

Current Structure of the Explorer Program

To accommodate a broad spectrum of scientific objectives and budget constraints, the Explorer program is currently structured into three distinct classes:

In addition to these core classes, the program also facilitates select Missions of Opportunity. These are typically smaller contributions to missions led by other agencies (domestic or international), or scientific instruments flown aboard non-Explorer spacecraft, maximizing scientific return through synergistic partnerships.

Explorer 9 (S-56A): A Balloon Satellite for Atmospheric Studies

Among the program's many notable missions is Explorer 9, which carried the pre-launch designation S-56A. Launched in February 1961, this NASA satellite was specifically designed to investigate the density and composition of Earth's upper thermosphere and lower exosphere. Understanding these outermost layers of our atmosphere is crucial for predicting satellite orbital decay, managing space debris, and understanding atmospheric drag effects on spacecraft.

Explorer 9's mission was particularly significant as it represented a successful reflight of the earlier Explorer S-56 mission, which had unfortunately failed due to a launch vehicle malfunction. The satellite itself was an innovative design: a lightweight, inflatable sphere weighing just 7 kg (15 lb) and expanding to a diameter of 3.66 meters (12.0 ft) once in orbit. This large surface area-to-mass ratio made the balloon highly sensitive to subtle changes in atmospheric density, allowing scientists to precisely measure atmospheric drag and infer density variations at its orbital altitudes.

Deployed into a medium Earth orbit (MEO), Explorer 9's orbital characteristics allowed it to sample a wide range of atmospheric conditions over time. The mission was expertly conducted by NASA's Langley Research Center, a facility renowned for its atmospheric and aeronautical research, further underscoring the scientific rigor applied to this critical study of Earth's upper atmosphere.

Frequently Asked Questions about the NASA Explorer Program

What is the NASA Explorer Program?
The NASA Explorer Program is a long-running space exploration initiative established in 1958. It provides flight opportunities for scientific investigations across various fields, including physics, geophysics, heliophysics, and astrophysics, using cost-effective spacecraft and instruments.
When was the Explorer Program established, and what was its first mission?
The Explorer Program was established in 1958. Its first mission, Explorer 1, launched on January 31, 1958, becoming the first successful American satellite to orbit Earth and famously discovering the Van Allen radiation belts.
What types of scientific investigations does the Explorer Program support?
The program supports a broad range of investigations in physics (e.g., cosmic rays, dark matter), geophysics (e.g., Earth's magnetosphere, ionosphere), heliophysics (e.g., solar flares, solar wind), and astrophysics (e.g., exoplanets, black holes, galactic evolution).
How many missions has the Explorer Program launched?
The Explorer Program has launched over 90 space missions since its inception in 1958, making it one of NASA's most prolific and successful programs.
What are the current classes of Explorer missions?
Currently, the program operates three primary classes: Medium-Class Explorers (MIDEX), Small Explorers (SMEX), and University-Class Explorers (UNEX). It also includes select Missions of Opportunity, which involve contributions to non-Explorer missions.
What was Explorer 9's primary mission?
Explorer 9, launched in February 1961, had the primary mission to study the density and composition of Earth's upper thermosphere and lower exosphere, using atmospheric drag measurements to infer these properties.
What made Explorer 9 unique in its design?
Explorer 9 was unique for its inflatable, spherical balloon design. It was a very lightweight, 7 kg (15 lb) satellite that expanded to 3.66 meters (12.0 ft) in diameter, giving it a large surface area-to-mass ratio ideal for precise atmospheric drag measurements.
Which NASA center managed the Explorer 9 mission?
The Explorer 9 mission, including its design and operations, was managed by NASA's Langley Research Center.