Recent work has shown that a significant contributor to the afterbody aeroheating during Mars entry is radiation1-3. However, relevant ground test data is not available to help assess the uncertainty associated with prediction of the radiation when designing the thermal protection system for the aeroshell afterbody. The present work is aimed at studying the afterbody radiation experienced during Mars entry through experiments. The X2 expansion tube at the University of Queensland is used to generate the relevant experimental freestream flow conditions. Analysis is carried out to characterize the generated experimental freestream conditions. A two dimensional wedge model is used to produce the expanding flow which simulates aspects of the afterbody flow around Mars entry vehicles. This generated expanding flow has a test time of about 50-110 μs, thus, allowing flow measurements to be conducted. Horizontal emission spectroscopy measurements at 3.25 mm above the test model was taken for the carbon dioxide 4.3 μm band at freestream velocities of 2.8, 3.4, and 4.0 km/s. The measurements were compared to numerical calculations of the radiation emission generated from three-dimensional flowfield calculations. The comparison showed that the numerical result was significantly overpredicted for the 2.8 km/s condition and underpredicted for the 4.0 km/s condition.