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Register a new userAn Unprecedented Constraint on Water Content in the Sunlit Lunar Exosphere Seen by Lunar-Based Ultraviolet Telescope of Change-3 Mission
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The content of $mathrm{OH/H_2O}$ molecules in the tenuous exosphere of the Moon is still an open issue at present. We here report an unprecedented upper limit of the content of the OH radicals, which is obtained from the in-situ measurements carried out rm by the Lunar-based Ultraviolet Telescope, a payload of Chinese Change-3 mission. By analyzing the diffuse background in the images taken by the telescope, the column density and surface concentration of the OH radicals are inferred to be $<10^{11} mathrm{cm^{-2}}$ and $<10^{4} mathrm{cm^{-3}}$ (by assuming a hydrostatic equilibrium with a scale height of 100km), respectively, by assuming that the recorded background is fully contributed by their resonance fluorescence emission. The resulted concentration is lower than the previously reported value by about two orders of magnitude, and is close to the prediction of the sputtering model. In addition, the same measurements and method allow us to derive a surface concentration of $<10^{2} mathrm{cm^{-3}}$ for the neutral magnesium, which is lower than the previously reported upper limit by about two orders of magnitude. These results are the best known of the OH (MgI) content in the lunar exosphere to date.
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We reported the photometric calibration of Lunar-based Ultraviolet telescope (LUT), the first robotic astronomical telescope working on the lunar surface, for its first six months of operation on the lunar surface. Two spectral datasets (set A and B) from near-ultraviolet (NUV) to optical band were constructed for 44 International Ultraviolet Explorer (IUE) standards, because of the LUTs relatively wide wavelength coverage. Set A were obtained by extrapolating the IUE NUV spectra ($lambda<3200AA$) to optical band basing upon the theoretical spectra of stellar atmosphere models. Set B were exactly the theoretical spectra from 2000AA to 8000AA extracted from the same model grid. In total, seven standards have been observed in 15 observational runs until May 2014. The calibration results show that the photometric performance of LUT is highly stable in its first six months of operation. The magnitude zero points obtained from the two spectral datasets are also consistent with each other, i.e., $mathrm{zp=17.54pm0.09}$mag (set A) and $mathrm{zp=17.52pm0.07}$mag (set B).
The Doppler tracking data of the Change 3 lunar mission is used to constrain the stochastic background of gravitational wave in cosmology within the 1 mHz to 0.05 Hz frequency band. Our result improves on the upper bound on the energy density of the stochastic background of gravitational wave in the 0.02 Hz to 0.05 Hz band obtained by the Apollo missions, with the improvement reaching almost one order of magnitude at around 0.05 Hz. Detailed noise analysis of the Doppler tracking data is also presented, with the prospect that these noise sources will be mitigated in future Chinese deep space missions. A feasibility study is also undertaken to understand the scientific capability of the Change 4 mission, due to be launched in 2018, in relation to the stochastic gravitational wave background around 0.01 Hz. The study indicates that the upper bound on the energy density may be further improved by another order of magnitude from the Change 3 mission, which will fill the gap in the frequency band from 0.02 Hz to 0.1 Hz in the foreseeable future.
The Asteroid Redirect Mission (ARM) proposes to retrieve a near-Earth asteroid and position it in a lunar distant retrograde orbit (DRO) for later study, crewed exploration, and ultimately resource exploitation. During the Caltech Space Challenge, a recent workshop to design a crewed mission to a captured asteroid in a DRO, it became apparent that the asteroids low escape velocity (<1 cm s$^{-1}$) would permit the escape of asteroid particles during any meaningful interaction with astronauts or robotic probes. This Note finds that up to 5% of escaped asteroid fragments will cross Earth-geosynchronous orbits and estimates the risk to satellites from particle escapes or complete disruption of a loosely bound rubble pile.
We here report the photometric performance of Lunar-based Ultraviolet telescope (LUT), the first robotic telescope working on the Moon, for its 18-months operation. In total, 17 IUE standards have been observed in 51 runs until June 2015, which returns a highly stable photometric performance during the past 18 months (i.e., no evolution of photometric performance with time). The magnitude zero point is determined to be $17.53pm0.05$ mag, which is not only highly consistent with the results based on its first 6-months operation, but also independent on the spectral type of the standard from which the magnitude zero point is determined. The implications of this stable performance is discussed, and is useful for next generation lunar-based astronomical observations.
The Lunar University Network for Astrophysics Research (LUNAR) undertakes investigations across the full spectrum of science within the mission of the NASA Lunar Science Institute (NLSI), namely science of, on, and from the Moon. The LUNAR teams work on science of and on the Moon, which is the subject of this white paper, is conducted in the broader context of ascertaining the content, origin, and evolution of the solar system.
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