Solrad 9

Solrad 9 (Explorer 37): A Key Player in Solar Monitoring

Mission Overview

• Alternate Names: Explorer 37, Explorer SE-B

• Program: SOLRAD (Solar Radiation) Series

• Launch Date: March 5, 1968

• Launch Site: Wallops Flight Facility, Virginia, USA

• Launch Vehicle: Scout

Solrad 9 was part of the SOLRAD program, initiated in 1960 to provide continuous solar radiation monitoring using standardized photometers. Its mission played a crucial role in studying solar activity, particularly during the early Apollo crewed missions.

Satellite Design & Operations

• Stabilization: Spin-stabilized, with its spin axis perpendicular to the Sun-satellite line, ensuring that its 14 solar X-ray and ultraviolet (UV) photometers were consistently aligned to observe the Sun.

• Data Collection:

  • Transmitted real-time data via FM/AM telemetry.

  • Stored data in a core memory, retrievable on command.

  • Scientists and institutions were invited to access transmitted data through the 136 MHz telemetry band, using standard IRIG channels (3-8).

• Continuity of Solar Data:

  • Solrad 8 ceased operations in August 1967.

  • During the gap before Solrad 9’s launch, solar data was collected by OSO-4 and OGO-4 satellites.

Significance for the Apollo Program

Solrad 9 was a particularly critical mission in the SOLRAD series because its solar activity data helped ensure astronaut safety during the Apollo program.

• Apollo 7 (1968): The first crewed Apollo mission benefited from its solar behavior predictions.

• Apollo 9 & Apollo 10 (1969):

  • Solrad 9 recorded one of the strongest solar storms occurring between these missions.

  • Had astronauts been outside Earth's protective magnetosphere, this storm could have posed serious radiation risks or even fatal exposure.

End of Operations & Reentry

• In July 1971, NASA shifted to using Solrad 10’s memory storage, which continued until June 1973, when it experienced malfunctions.

• NASA then resumed reading stored data from Solrad 9 until February 25, 1974, when the satellite exhausted its gas reserves needed for attitude control.

• Once stabilization was lost, the satellite became inoperative and was decommissioned.

Unlike Solrad 8, which remained in orbit, Solrad 9 reentered Earth’s atmosphere and disintegrated on November 16, 1990. This marked the end of its contributions to solar research, but its data remained valuable for years to come.