Near-Ultraviolet Emission Spectroscopy During an Airborne Observation of the Stardust Reentry

摘要

Thermal radiation of the heatshield and the emission of the postshock layer around the Stardust capsule, during its reentry, were detected by a NASA-led observation campaign aboard NASA's DC-8 airborne observatory involving teams from several nations. The German SLIT experiment used a conventional spectrometer, in a Czerny-Turner configuration (300 mm focal length and a 600 lines/mm grating), fed by fiber optics, to cover a wavelength range from 324 to 456 nm with a pixel resolution of 0.08 nm. The reentering spacecraft was tracked manually using a camera with a view angle of 20 deg, and light from the capsule was collected using a small mirror telescope with a view angle of only 0.45 deg. Data were gathered with a measurement frequency of 5 Hz in a 30-s time interval around the point of maximum heating until the capsule left the field of view. The emission of carbon nitride (as a major ablation product), N_2~+ and different atoms were monitored successfully during that time. Because of the nature of the experimental setup, spatial resolution of the radiation field was not possible. Therefore, all measured values represent an integration of radiation from the visible part of the glowing heatshield, and from the plasma in the postshock region. Further, due to challenges in tracking, not every spectrum gathered contained data. The measured spectra can be split up into two parts: 1) continuum spectra, which represent a superposition of the heatshield radiation and the continuum radiation of particles due to microspallation in the plasma, and 2) line spectra from the plasma in the shock layer. Planck temperatures (interpreted as the surface temperatures of the Stardust heatshield) were determined assuming either a constant surface temperature, or a temperature distribution deduced from numerical simulation. The constant surface temperatures are in good agreement with numerical simulations, but the peak values at the stagnation point are significantly lower than those in the numerical simulation if a temperature distribution over the surface is assumed. Emission bands of carbon nitride and N_2~+ were tracked along the visible trajectory and compared with a spectral simulation with satisfying agreement Values for the integrated radiation of the transitions of interest for these species were extracted from this comparison.