Robotic Missions in Sun-Earth Space - Nasa's Explorer Program

The Explorer program includes robotic missions that conduct relatively low-cost scientific investigations with specific objectives in the fields of astronomy or space physics. The program dates back to the late 1950s. The first satellite launched into space by the United States was called Explorer 1. It was the first of a series of scientific satellites named Explorer.

In 1958 the Explorer 1 satellite used temperature gauges and a Geiger counter to collect data in space. A Geiger counter is an instrument that detects the presence and intensity of radiation. Explorer 1 detected radiation levels as expected during most of its orbit. However, at the highest altitudes the Geiger counter recorded no radiation. This was most puzzling. Scientists expected radiation levels to increase, not decrease, farther from Earth. Data from later Explorers and the Soviet Sputnik satellites revealed that Explorer 1 had encountered radiation levels so high, its detector was overwhelmed.

These missions led to the discovery of the Van Allen radiation belts. These are two doughnut-shaped regions of high radioactivity that encircle Earth. The inner belt is centered approximately 2,000 miles above Earth and is roughly 4,000 miles thick. The outer belt lies between approximately 11,500 and 25,000 miles from Earth. All future spacecraft venturing very far from Earth had to be designed to withstand the intense radiation of the Van Allen belts.

The Explorer program continued over the next three decades with mission series given different names. These series included AE (Atmosphere Explorer), EPE (Electrostatic Particle Explorer), IMP (Interplanetary Monitoring Platform), and SOLRAD (Solar Radiation Satellites). There were also Explorer spacecraft with colorful names such as Hawkeye, Injun, and Uhuru. Uhuru means "freedom" in the African language Swahili.

During the 1990s NASA specified that Explorer missions be dedicated to three specific themes in space science:

• Astronomical search for origins and planetary systems
• Structure and evolution of the universe
• The Sun-Earth connection

NASA also established different classes of Explorer missions based on their total cost to the agency.

• University-Class Explorer (UNEX)/Student Explorer Demonstration Initiative Program (STEDI)—Total cost less than $15 million
• Small Explorer (SMEX)—Total cost less than $120 million
• Medium-Class Explorer (MIDEX)—Total cost less than $180 million

Another type of Explorer mission is called a Mission of Opportunity (MO). This is a mission operated by another office within NASA or by the space agency of another country in which an OSS investigation "hitches a ride" with the mission of the other agency or country. The total cost to NASA of an MO mission cannot exceed $35 million.

As of 2006 there have been nearly ninety Explorer program missions. Only four spacecraft failed. The other missions are considered successes. Operational Explorer missions as of February 2006 are listed in Table 6.1. Most of the spacecraft are small observatories in low-Earth orbit (LEO, 125-1,200 miles above Earth's surface). The Advanced Composition Explorer (ACE) and Wilkinson Microwave Anisotropy Probe (WMAP) are positioned at Lagrange points around Earth (designated L1 and L2 in Table 6.1). These are points at which the gravitational pulls of the Sun and Earth are relatively even, meaning that a spacecraft can stay "parked" between the two heavenly bodies. Point L1 is approximately one million miles from the Earth toward the Sun; Point L2 is approximately one million miles from Earth in the other direction (away from the Sun). IMAGE is in orbit just outside the outer Van Allen radiation belt, which extends to about seven Earth radii. (The radius of the Earth is approximately 3,960 miles.)