Very-low Earth orbit (VLEO) space below 200 km is essential for high-quality communications and near-Earth space environment detection. Due to the significant atmospheric drag, orbital maintenance is required for spacecraft staying here. Based on air-breathing electric propulsion (ABEP) technology, this paper analyzed the orbital boundary conditions of the spacecraft under the constraints of parameters including slenderness ratio, thrust-to-power ratio, drag coefficient, and effective specific impulse. The energy balance is the key constraint for low VLEO orbits, which is determined by the drag coefficient, slenderness ratio, and thrust-to-power ratio. Under the existing technical conditions, the lowest circular orbit (along the terminator) is about 170 km. An elliptical orbital flight scheme is also analyzed to reach a 150 km perigee. A half-period control method was proposed based on the on–off control method for the elliptical orbit, which could enable the spacecraft to maintain a stable 150–250 km elliptical orbit.

With the development of deep space exploration technology, manned missions to Mars are expected to be realized in the near future. However, the journey to Mars requires more supplies and fuel than near-Earth missions, including moon landings. Using the traditional Apollo-style mission method will result in the spacecraft reaching the LEO mass of 2000 tons, which is not easy to achieve. The use of the modular design method and multiple launches will significantly reduce the mass of a single launch. In addition, the use of nuclear power engines (Nuclear Thermal Propulsion, NTP, and Nuclear Electric Propulsion, NEP) can greatly improve propulsion efficiency, reducing the mass of the propellant. This paper uses the Mission Architecture Matrix (MAM) method to concisely and precisely analyze a series of mission architectures in the case of impulse maneuver transfer and low-thrust transfer. The results show that when only chemical propulsion (specific impulse is 440 s) is used, the maximum LEO launch mass in the optimal mission architecture is 325 tons, and the total LEO mass of the system is 1142 tons. With the usage of NTP (specific impulse is 900 s) and NEP (the specific impulse is 6000 s) technology, the maximum LEO launch mass in the optimal mission architecture is only 85 tons, and the total LEO mass of the system is only about 400 tons. Considering the current rocket technology, the total cost is about USD 1149 million US.

Aims . The rotational states of comet 103P/Hartley 2 (103P) during its perihelion passage are strongly modulated by nongravitational forces. By fitting its attitude-variation measurements, this study aims to infer the spatial distributions of H ₂ O and CO ₂ ices on the nucleus and to assess how these inhomogeneities drive the nucleus’s rotational behavior. Methods . We adopted a dust-mantle thermal model of the nucleus that incorporates H ₂ O and CO ₂ ices as the principal volatiles. A coupled attitude-thermal simulation framework was developed, in which a neural-network surrogate greatly accelerates the iterative computations. The nucleus was partitioned into three morphological regions – large end, waist, and small end – and a localized activity model described the uneven sublimation across these zones. Results . Our model successfully reproduces both the observed rotational variations and the measured volatile production rates of 103P during its 2010 encounter. At the observation instant, in addition to the pronounced CO ₂ sublimation at the sunlit small end, the dust mantle covering the large end insulated the underlying CO ₂ , allowing it to continue sublimating on the shadowed side. We find that the H ₂ O ice-front depth is on the order of millimeters, whereas the CO ₂ ice front lies between centimeters and decimeters, with the large end exhibiting stronger activity. The fitted CO ₂ production rate closely matches the observations, while direct H ₂ O sublimation contributes only a minor fraction. The total H ₂ O production rate of 103P may primarily originate from H ₂ O ice ejected by CO ₂ sublimation at the nucleus’s ends. Moreover, CO ₂ sublimation at the large end produces the majority of the recoil torque, driving the nucleus’s enhanced long-axis excitation following the encounter.