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tokens: for the water production rate of both asteroids. In Section 5 , we calculate the fraction of ice coverage required to achieve this upper limit, discuss our results, and present our conclusions. 2. Observations and Data Analysis 2.1. Herschel HIFI Observations Themis was observed with HIFI on Herschel on 2012 January 30 (see Table 1 ). The asteroid was traveling inbound (2.92 au) and close to perihelion (2.75 au) when the observation was carried out. Cybele was observed with HIFI on 2012 December 21 (see Table 1 ) when the asteroid was 3.52 au from the Sun and inbound, having already passed aphelion (3.8 au). It is important to highlight that with the low escape velocity from these asteroids, any escaping water vapor would have been visible to Herschel independent of its source location, as the footprint of the telescope was much larger than the asteroids themselves. Table 1 Observations and Observing Conditions for the Previously Unpublished Measurements Used in This Paper Note. Columns: target (the two asteroids), observatory and instruments used, ObsID (observation ID reference number), date and time in UT, rh (heliocentric distance), Δ (Earth to asteroid distance), α (phase angle). The line emission from the fundamental ortho-H 2 O 1 10 –1 01 line at 556.936 GHz was searched for in the lower sideband of the HIFI band 1b mixer. The observations were performed in the frequency-switching observing mode using both the wide-band spectrometer (WBS) and the high-resolution spectrometer (HRS). The spectral resolution of the WBS is 1 MHz (0.54 km s −1 at the frequency of the observed line), while the HRS was used in its high-resolution mode with a resolution of 120 kHz (0.065 km s −1 ). Initial processing of the HIFI data set was carried out using the standard HIFI processing pipeline of the Herschel Interactive Processing Environment v15.0.1 (HIPE; Ott 2010 ), followed by processing with the GILDAS CLASS software. In CLASS, vertical and horizontal polarizations were folded and averaged. The frequency-switching observing mode introduces quite a strong baseline ripple, which was removed through a polynomial baseline correction performed on the final spectra. The averaged and baseline-corrected HRS spectra of the two asteroids are shown in Figures 1 (a) and (b). The pointing offset of both the vertical and the horizontal polarization spectra is 35 in band 1b with respect to the target position. The half-power beam width is 381 at 557 GHz. The main beam brightness temperature scale was computed using a forward efficiency of 0.96 and a beam efficiency of 0.62. The output line area rms values are provided in Table 2 . Figure 1. Averaged HIFI spectra and thermophysical modeling output. Shown are the HRS-averaged spectra for (a) asteroid Themis and (b) asteroid Cybele with TPM outputs (c) for Themis (left) and Cybele (right). These outputs represent the Herschel view with orientation “ecliptic sky” at the specific HIFI observing start epochs. The plots in