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Buzzoni, A., Fan, S., Frueh, C., Altavilla, G., Foppiani, I., Micheli, M., Nomen, J., and Sanchez-Ortiz, N.:
"The puzzling case of the deep-space debris WT1190F: a test bed for advanced SSA techniques",
2017, Proc. of the Stardust Final Conference: Advances in Asteroids and Space Debris Engineering and Science. Eds. M. Vasile, E. Minisci, L. Summerer, P. McGinty. ESA ESTEC (Springer: Berlin) (in press)


Summary:
We report on somewhat unique photometric and spectroscopic observations of the deep-space debris WT1190F, which entered Earth atmosphere off the Sri Lanka coast, last 2015 November 13. This striking case has been imposing to the worldwide SSA community as an outstanding opportunity to effectively assess origin and physical nature of such extemporary impactors and appraise their potential threat for Earth. Our observations indicate for WT1190F an absolute magnitude R = 32.45 ±0.31, with a flat dependence on the phase angle, and slope 0.007 ±0.002 mag deg−1. The detected short-timescale variability suggests a "four-facet" geometry, with the body likely spinning with a period P = 2.9114 ±0.0009 s. In the BVRI color domain, WT1190F closely resembled the Planck deep-space probe, a feature that points to an anthropic origin of the object. This match, together with a depressed reflectance around 4000 and 8500 Å may be suggestive of a "grey" (aluminized) surface texture. An analysis is in progress to assess the two prevailing candidates to WT1190F's identity, namely the Athena II upper stage of the Lunar Prospector mission, and the ascent stage of the Apollo 10 lunar module (LEM LM-4) "Snoopy", by comparing observations with the synthetic photometry from accurate mock-up modeling and reflectance rendering.



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   Browse figures in hires (click on the thumbnails)
Figure 1 -
An illustrative frame of WT1190F from the Loiano observatory in the night of 2015 November 7-8. The image is taken in the Johnson-Cousins R band with a 420s exposure time. The object was 518,000 km away from us. Thanks to the "on-target" telescope tracking, WT1190F is clearly detected as a point source near the field center, with an apparent magnitude R = 20.45.
Figure 2 -
The derived photometry (in absolute magnitude scale) from a 30s R trailing image of WT1190F taken with the "Cassini" telescope of Loiano observatory along the 2015 November 13 observations. The object appears to "flash" with an apparent period shorter than ~1.5s, likely as a consequence of a quick spinning motion. The photometric behavior is sampled in time with a step of 0.189s, as reported in the plot. The dashed line marks the mean absolute luminosity along this set of observations.
Figure 3 -
The (B−V) versus (V−R) average colors of WT1190F along the three observing runs of 2015 November, from Loiano (red/orange dots). The target colors are compared with other reference objects, namely the Planck spacecraft, the Sun, and the locus of Main Sequence stars (Pecaut & Mamajek 2013, solid curve, labeled with the stellar spectral type). Note that WT1190F appears to be slightly "redder" than the Sun, but very close in color to Planck. It would also quite well match the colors of a star of spectral type K3.
Figure 4 -
The deep-space astronomical observatory Planck. The probe, in operation between 2009 and 2013, was placed in an Earth-corotating orbit around the Sun-Earth Lagrangian point L2, some 1.5 million km away. Note the prevailing grey color of the surface texture.
Figure 5 -
The WT1190F reflectance curve, together with its 1σ statistical uncertainty band, as obtained from the Loiano spectroscopy. The curve is normalized at the 7000 Å value. Note two clear "dips" in the curve about 4000 and 8500 Å, a possible signature of some "aluminized" material onboard.
Figure 6 -
Our preliminary mock-up modeling of the Lunar Prospector TLIS (upper panel) and the Apollo 10 LEM "Snoopy" (lower panel). The derived synthetic photometry may likely provide an effective tool to assess the ultimate nature of WT1190F.


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AB/May 2017