James Webb Space Telescope Feed Post

Near to Mid-Infrared Spectroscopy of (65803) Didymos as observed by JWST: Characterization Observations Supporting the Double Asteroid Redirection Test

Median averaged slice through the MIRI MRS IFU showing Didymos. Note that Dimorphos was being occulted by Didymos during the entire period of MIRI observations. The measured median PSF FWHM is 0.332” in sub-band 2B, consistent with the predicted FWHM of 0.294”. The measured FWHM across other sub-bands were found to be 0.9—1.2 times the predicted FWHM, broadly consistent with expectations for a point source. The yellow box marks the 7x7 pixel aperture used in our MIRI spectral extraction; the red box denotes the inner edge of the region used for background estimation. Abstract: The Didymos binary asteroid was the target of the Double Asteroid Redirection Test (DART) mission, which intentionally impacted Dimorphos, the smaller member of the binary system. We used the Near-Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI) instruments on JWST to measure the 0.6-5 µm and 5-20 µm spectra of Didymos approximately two months after the DART impact. These observations confirm that Didymos belongs to the S asteroid class and is most consistent with LL chondrite composition as was previously determined from its 0.6-2.5-µm reflectance spectrum. Measurements at wavelengths > 2.5 µm show Didymos to have thermal properties typical for an S-complex asteroid of its size and to be lacking absorptions deeper than ~2\% due to OH or H2O. Didymos' mid-infrared emissivity spectrum is within the range of what has been observed on S-complex asteroids observed with Spitzer Space Telescope and is most consistent with emission from small (< 25 µm) surface particles. We conclude that the observed reflectance and physical properties make the Didymos system a good proxy for the type of ordinary chondrite asteroids that cross near-Earth space, and a good representative of likely future impactors.