Infrared photometry and spectrometry of satellites and debris at UKIRT

The characterization of deep space debris poses a significant challenge in Space Domain Awareness (SDA). Multi-color photometry and the resultant color indices offer the potential to rapidly discriminate between debris and intact space objects such as rocket bodies and satellites. These multi-color techniques can also identify anomalous members of objects in certain groups and cue higher fidelity data collections and studies. However, multi-color photometry can be difficult to interpret, as the effects of phase and rotation become conflated with the more fundamental material properties of the satellite. Additionally, the broad astronomical photometric bands may not identify key spectral features that can be diagnostic for SDA applications. With our recent observational campaign we have been able to collect 5-color photometry in the near-IR with WFCAM as well as overlapping near-IR spectra with the UKIRT 1–5 μm Imager-Spectrometer (UIST). On a small set of objects, we also have mid-IR spectrophotometry with the Mid-IR esCHELLE (Michelle) imaging spectrograph.

Our previous measurements with the United Kingdom Infrared Telescope (UKIRT) Wide Field Camera (WFCAM) characterized a wide range of space objects with the goal of developing techniques to rapidly discriminate between different classes of objects and to identify anomalous members of these groups. The survey has produced a comprehensive database of 5-color photometry in the Z, Y, J, H, and K bands, analogous to the bands that are anticipated to be exploited by future U.S. ground-based SDA systems. Our current data set includes:

• (a)
  United States Centaur rocket bodies (RBs),
• (b)
  Molniya communication satellites including the −1 K, 1T, −2, and −3 variants,
• (c)
  Russian FREGAT and SL-6 upper stage RBs in Molniya orbits,
• (d)
  Russian Breeze-M rocket bodies disposed of in GEO-crossing graveyard like orbits, and the Angara-5/Breeze-M mass simulator, also disposed of in a near-GEO orbit,
• (e)
  Intact payloads selected from satellites using the Boeing HS-376 buses—including four different generations of solar panel technology.

Many of the objects we have studied have significant orbital inclination or drift in the GEO belt. The interpretation of photometry of these objects is especially difficult as phase angle can no longer be considered simply as a single-dimensional quantity. During 2023, we have endeavored to expand our phase angle coverage of a handful of objects and comprehensively sample brightness and color in both components of phase angle. In this paper we present exemplary “phase-phase” diagrams demonstrating this technique and highlight some of the practical and observational difficulties in achieving comprehensive phase angle coverage and interpreting these results.