Optimal design of near-Earth asteroid sample-return trajectories in the Sun–Earth–Moon system

In the 6th edition of the Chinese Space Trajectory Design Competition held in 2014, a near-Earth asteroid sample-return trajectory design problem was released, in which the motion of the spacecraft is modeled in multi-body dynamics, considering the gravitational forces of the Sun, Earth, and Moon. It is proposed that an electric-propulsion spacecraft initially parking in a circular 200-kin-altitude low Earth orbit is expected to rendezvous with an asteroid and carry as much sample as possible back to the Earth in a 10-year time frame. The team from the Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences has reported a solution with an asteroid sample mass of 328 tons, which is ranked first in the competition. In this article, we will present our design and optimization methods, primarily including overall analysis, target selection, escape from and capture by the Earth-Moon system, and optimization of impulsive and low-thrust trajectories that are modeled in multi-body dynamics. The orbital resonance concept and lunar gravity assists are considered key techniques employed for trajectory design. The reported solution, preliminarily revealing the feasibility of returning a hundreds-of-tons asteroid or asteroid sample, envisions future space missions relating to near-Earth asteroid exploration.

Interactions between invasive Eurasian watermilfoil and native water stargrass in Cayuga Lake, NY, USA

Aims Eurasian watermilfoil(Myriophyllum spicatum L.)is a common invasive plant in American lakes and has many negative impacts on invaded ecosystems.Drastic decline of this plant at the north-ern end of Cayuga Lake in the New York State has occurred since the 1980s,with a much smaller magnitude of decline of Eurasian watermilfoil at the southern end(Johnson et al.2000)During the same period,increases in the abundance of native plants,partic-ularly water stargrass(Heteranthera dubia(Jacq.)MacMill.),have been observed(Johnson et al.(1998))We aimed to investigate the mechanisms responsible for the decline of Eurasian watermilfoil and evaluate the responses of co-occurring plants at the two ends of Cayuga Lake over time.We hypothesized that plant interactions might have contributed to the drastic decline of Eurasian watermil-foil,particularly allelopathy by native water stargrass.Methods A lake survey was conducted to assess distribution and abundance of plant communities at the northern end and the southern end of Cayuga Lake.Additionally,two sets of greenhouse experiments were conducted to investigate the interactions between invasive Eurasian watermilfoil and native water stargrass.A competition experiment evaluated intra-versus inter-specific competition among plants grown together;an allelopathy experiment examined the responses of plants to each other’s extracts.Important Findings The lake survey showed that water stargrass was extremely abun-dant at the northern end,whereas Eurasian watermilfoil was sparse at the northern end but one of the most common species at the southern end.The survey also revealed that water stargrass was more abundant than Eurasian watermilfoil at sites where the two species coexisted in the lake.Results from greenhouse experi-ments revealed no effects of Eurasian watermilfoil on water star-grass growth.However,Eurasian watermilfoil biomass was reduced by 46%when treated with high concentration of water stargrass extracts.This is likely due to osmotic effects rather than allelopathic effects of water stargrass.We proposed several possible reasons for the drastic decline of Eurasian watermilfoil and the increase in water stargrass abundance at the northern end of Cayuga Lake,including space competition,nutrients,substrates,wind exposure and water clarity in addition to insect herbivory and mechanic harvesting.

A nonlinear optical loop mirror-based linear cavity switchable multi-wavelength erbium-doped fiber laser

A nonlinear optical loop mirror(NOLM)-based linear cavity switchable multi-wavelength erbium-doped fiber(EDF) laser is proposed and experimentally demonstrated.Due to the characteristics of the intensity-dependent transmissivity,the NOLM can effectively mitigate the mode competition of the homogenous broadening gain medium,so that the multi-wavelength lasing can be achieved at room temperature.By adjusting the states of the polarization controllers(PCs),the number of the lasing wavelengths in the proposed laser can be adjusted flexibly from 11 to 13 with a wavelength spacing of 0.4 nm around the wavelength of 1 530 nm.