possibly large numbers of other Trojan objects in Jupiter's orbit, are likely composed of water ice, with a layer of dust, and the hypothesized large amounts of water ice on the closer, main-belt asteroid 1 Ceres, suggest that importing volatiles from this region via the Interplanetary Transport Network may be practical in the not-so-distant future. These possibilities are dependent on complicated and expensive resource utilization from the mid to outer Solar System, which is not likely to become available for a significant period of time. The lack of a substantial atmosphere for insulation results in temperature extremes and makes the Moon's surface conditions somewhat like a deep space vacuum with surface pressure (night) of 3 × 10−15 bar. It also leaves the lunar surface exposed to half as much radiation as in interplanetary space (with the other half blocked by the Moon itself underneath the habitat), raising the issues of the health threat from cosmic rays and the risk of proton exposure from the solar wind. In 2020 scientists reported the first ever measurements, made via China's Chang'e 4 lander, of the radiation exposure dose on the lunar surface. Lunar rubble can protect living quarters from cosmic rays. Shielding against solar flares during expeditions outside is more problematic. When the Moon passes through the magnetotail of the Earth, the plasma sheet whips across its surface. Electrons crash into the Moon and are released again by UV photons on the day side but build up voltages on the dark side. This causes a negative charge build up from −200 V to −1000 V. See Magnetic field of the Moon. Moon dust is an extremely abrasive glassy substance formed by micrometeorites and unrounded due to the lack of weathering. It sticks to everything, can damage equipment, and it may be toxic. Since it is bombarded by charged particles in the solar wind, it is highly ionized, and is extremely harmful when breathed in. During the 1960s and 1970s Apollo missions, astronauts were subject to respiratory problems on return flights from the Moon, for this reason. Growing crops on the Moon faces many difficult challenges due to the long lunar night (354 hours), extreme variation in surface temperature, exposure to solar flares, soil with almost no nitrogen and little potassium, and lack of insects for pollination. Due to the lack of any atmosphere of substance on the Moon, plants would need to be