The Frontier Solid State Power Amplifier (SSPA), developed at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) to support NASA's Interstellar Mapping and Acceleration Probe (IMAP) mission, is scheduled to launch in 2025. In order to provide reliable communications within a cost, schedule, and mass constrained mission, an SSPA was selected. The SSPA can operate over the International Telecommunication Union (ITU) Near Earth and Deep Space downlink frequency band of 8400–8500 MHz and provides a minimum of 10 W of RF output power, with a nominal efficiency of 31.5%, and a mass of less than 1.1kg. The SSPA consists of two standalone boxes, an RF Amplifier Box and a Power Converter Unit (PCU), connected via an interbox harness. The RF Amplifier box design utilizes three single-string hybrid amplifiers, an isolator, DC bias circuitry, and thermistor-based gain control. The hybrid amplifiers were designed using commercially available GaN transistors that have successfully been screened for the harsh space environment. The transistor is impedance matched at the input and output using microstrip transmission lines and thin-film resistors on alumina substrates, and sealed in a hermetic package. The PCU is a buck-boost converter, which provides 30 V and -5 V secondary voltages from a primary voltage ranging from 22 V to 35 V, while maintaining proper turn on and turn off voltage sequencing. The Engineering Model (EM) SSPA assembly was subjected to a 3-axis sine and random vibration followed by thermal vacuum testing from -25°C to +55°C. The design methodology, analyses, such as thermal, structural, and multipactor, along with a comparison of pre and post environmental testing are presented.