Explorer 9

Explorer 9: Pioneering Atmospheric Research

Mission Overview

• NSSDCA/COSPAR ID: 1961-004A

• Launch Date: February 16, 1961

• Reentry Date: April 9, 1964

• Launch Site: Wallops Island, Virginia

Mission Purpose

Explorer 9 was the first successful inflatable sphere deployed into orbit to study atmospheric density. It was part of a series of 3.66-meter inflatable satellites, designed to measure how Earth's atmosphere affects satellite motion. The mission was identical in design and objectives to the earlier, unsuccessful Explorer S-56 launch.

Spacecraft Design

• Constructed from alternating layers of aluminum foil and Mylar polyester film.

• Featured 5.1 cm-diameter white paint dots on the aluminum surface for thermal control.

• Initially stored in a 21.6 cm diameter, 48.3 cm long container within the fourth stage of the launch vehicle.

• Upon separation of the third and fourth stages, the sphere was inflated using nitrogen gas and ejected into orbit via a separation spring.

Communications & Tracking

• The satellite’s structure doubled as an antenna, with its two aluminum hemispheres separated by a Mylar gap at the equator.

• A 136 MHz, 15 mW beacon was included for tracking, but it failed after the first orbit.

• As a result, tracking relied on the Smithsonian Astrophysical Observatory (SAO) Baker-Nunn camera network.

• Power was supplied by solar cells and rechargeable batteries.

Historic Achievements

• First spacecraft placed in orbit using an all-solid-fuel rocket.

• First successful satellite launch from Wallops Island.

• Explorer 9 remained in orbit until it reentered Earth's atmosphere on April 9, 1964, marking a significant milestone in early space research and technology development.

The Scout Rocket: A Reliable Launch Vehicle

Explorer 9 was launched using a Scout 4 rocket, a four-stage solid-fuel rocket developed by engineers at NASA’s Langley Research Center. The Scout rocket was known for its cost-effectiveness and reliability, successfully launching small satellites for scientific research and military applications.

The first three attempts to launch an inflatable satellite failed due to problems with earlier launch vehicles. However, on February 16, 1961, Explorer 9 became the first successful launch of this type of satellite, proving the reliability of the Scout rocket.

Wallops Island: The Launch Site

Wallops Island, Virginia, played a key role in Explorer 9’s mission. This site was originally established in 1945 as a test facility for high-speed aerodynamics and rocket research. It became one of NASA’s major launch sites for small satellites and sounding rockets.

The first-ever attempted launch of a Scout rocket occurred at Wallops in 1960, though the test ended in failure. However, improvements were made, and Wallops continued to support Scout rocket launches throughout its history.

Scientific Discoveries from Explorer 9

The data collected from Explorer 9 helped scientists learn more about how solar radiation affects Earth's atmosphere. One of the biggest discoveries was a phenomenon called the "helium bulge", which showed how helium gas in the upper atmosphere shifts due to solar activity. This knowledge helped improve models of atmospheric density, allowing for better predictions of how long satellites stay in orbit.

Legacy of Explorer 9

Explorer 9 was the first in a series of inflatable satellites that continued with Explorer 19 (1963), Explorer 24 (1964), and Explorer 39 (1968). These missions helped scientists study how the density of Earth's upper atmosphere changes over a full solar cycle.

The mission also demonstrated the success of the Scout rocket, which went on to become one of NASA’s longest-running launch programs until its retirement in 1994.

Conclusion

The Explorer 9 mission was a milestone in space science. It provided important data about Earth’s atmosphere, demonstrated the effectiveness of the Scout rocket, and showcased the value of Wallops Island as a launch site. This mission helped pave the way for modern satellite tracking and space weather research, proving that even simple designs can lead to groundbreaking discoveries.