Carbonaceous (C-type) asteroids
are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of ...carbonaceous chondrites
and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth's atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)
onboard the spacecraft Hayabusa2
, indicating that the asteroid's boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m
s
K
(300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites
and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect
. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites
. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity
of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies
.
In order to improve the thrust force of the μ 10 microwave discharge ion thruster, its ion source which is biased at a screen voltage is separated into three parts; 1.waveguide, magnet, and a grid ...holder, 2.magnet spacers, and 3.a screen grid by ceramic rings. Connection of a high voltage power supply is selectable among each component independently, and maximum beam currents are measured in all six cases in addition to the nominal setup. Except for one case, the maximum thrust force is increased from the HAYABUSA's 8mN and HAYABUSA2's 10mN. The highest thrust force 11.2mN is recorded at Isp 3150sec when the magnet spacers are not connected to the power supply. At this time, the potential of the isolated magnet spacers is increased from 1500V to 1525V. The increase of the potential of the spacers regulates collisions of ions, resulting in increase of the ion currents. This study firstly demonstrates the effect of the segmentation of the discharge chamber of microwave ion thrusters and achieves the improvement of 4% of the total efficiency and of 40% of the thrust force since the HAYABUSA's flight model.
In microwave discharge ion thrusters, the plasma mode transition and hysteresis are two of the most important phenomena. The plasma mode-transition restricts the maximum thrust, whereas the ...hysteresis splits two mode of thrust in each flow rate. The previous thruster development and optical measurement indicates that there are two important parameters; the volume of magnetic confinement region where electrons are continuously heated by ECR and confined by magnetic mirrors, and waveguide plasma conditions. In this study, several kinds of thrusters that have different volume of magnetic confinement were tested. In addition, the enhancement of cut-off density inside the waveguide using larger diameter of waveguide was proposed. Experimental results show that the volume of magnetic confinement region is a key parameter for determining the maximum thrust. It was also found that the enhancement of cut-off is effective to eliminate the plasma hysteresis, whereas it does not affect the plasma mode transition. Hence, to further improve the thrust, one possible solution is to enhance the cut-off density at downstream of waveguide outside of magnetic field confinement region.
A cluster of gridded microwave discharge ion thrusters μ10 contributed to the success of the deep space exploration missions of the Japanese asteroid explorers Hayabusa and Hayabusa2. To meet the ...demands of the next mission DESTINY+, the maximum ion beam current was increased to 200 mA from the 170 mA used in Hayabusa2 by redesigning the magnetic field and grid configuration. However, this thruster exhibited the plasma hysteresis caused by a high-voltage breakdown. Specifically, the ion beam current (i.e. thrust) has two modes at a given propellant flow rate, namely high- and low-current modes. In nominal operation, the thruster operates in high-current mode. However, once a high-voltage breakdown occurs, the ion beam current transitions to and remains in low-current mode, which significantly decreases thrust efficiency. In this paper, based on an investigation of the physical mechanism that causes the hysteresis, we proposed a recovery method to return the thruster operation to high-current mode. In the investigation, we focused on the plasma mode transition, where plasma is generated inside the waveguide. We experimentally investigated the hysteresis by varying the grid configuration and the waveguide geometries while maintaining the magnetic field. The results show that the neutral density and plasma conditions inside the waveguide are strongly related to the hysteresis, which indicates that the main cause of the hysteresis is plasma generation inside the waveguide.
•Enhanced thrusters have plasma hysteresis caused by a high-voltage breakdown.•The hysteresis is caused by plasma generation and extinction inside the waveguide.•Temporarily gas stop is effective for the recycling after high-voltage breaks down.•The hysteresis depends on the thruster geometry even with the same magnetic field.•Ion optics design is one of the most important parameters for the hysteresis.
Presented here are the observations and interpretations from a comprehensive analysis of 16 representative particles returned from the C-type asteroid Ryugu by the Hayabusa2 mission. On average Ryugu ...particles consist of 50% phyllosilicate matrix, 41% porosity and 9% minor phases, including organic matter. The abundances of 70 elements from the particles are in close agreement with those of CI chondrites. Bulk Ryugu particles show higher δ18O, Δ17O, and ε54Cr values than CI chondrites. As such, Ryugu sampled the most primitive and least-thermally processed protosolar nebula reservoirs. Such a finding is consistent with multi-scale H-C-N isotopic compositions that are compatible with an origin for Ryugu organic matter within both the protosolar nebula and the interstellar medium. The analytical data obtained here, suggests that complex soluble organic matter formed during aqueous alteration on the Ryugu progenitor planetesimal (several 10’s of km), <2.6 Myr after CAI formation. Subsequently, the Ryugu progenitor planetesimal was fragmented and evolved into the current asteroid Ryugu through sublimation.
This paper presents the design methodology and performance testing of an additively manufactured resistojet operating on hydrogen as propellant. Additive manufacturing allows to produce complex ...monolithic resistors, resulting in reliable high efficiency thrusters. The concept, to be used in combination with advanced cryogenic storage technologies, is proposed for short time and high specific impulse orbit transfers. The simplified two-dimensional thermal design approach adopted is discussed, and its application to the engineering of the resistor is shown for both Inconel 718 and tungsten. The paper reports the performance testing of the proof-of-concept version of the thruster, manufactured in Inconel 718. Experiments on hydrogen show near ideal performance, demonstrating a peak thermal efficiency of 96%. The thruster proves the validity of the design methodology proposed, and the feasibility of the approach to develop monolithic additively manufactured hydrogen resistojets as main propulsion units.
•Developed a simple modeling approach for concentric resistojets.•Successfully 3D printed a thin wall cylindrical multi-layer heater.•Demonstrated the first 3D printed hydrogen resistojet.•Achieved near ideal thermal efficiency in the thruster.
Japan's second asteroid explorer Hayabusa2 was successfully launched on Dec 3, 2014, to return a sample from asteroid 162173 Ryugu by 2020. Four xenon ion thrusters based on electron cyclotron ...resonance discharge propelled the spacecraft for 547 h during its first year in space. Hayabusa2 completed an Earth gravity assist on Dec 3, 2015, followed by 798 and 2593 h of ion thruster operation, called the first and second transfer phases of delta-v, respectively. The third transfer phase of delta-v was conducted from Jan 10, 2018, to Jun 6, 2018, in which the final 2475-h ion thruster operation was executed before the rendezvous with Ryugu. The cumulative operating times for the four ion thrusters are 6,450, 11, 5,193, and 6418 h. This paper summarizes the 6515-h powered flight by the ion engine system, which produced 1015 m/s delta-v, in terms of thruster performance change, roll torques generated by various combinations of ion thrusters, and spacecraft surface erosion history measured by two quartz crystal microbalances located near the thrusters. In parallel with the space flight operation, an engineering model of the microwave discharge neutralizer has been under long-duration testing on the ground since 2012. It has accumulated 55,170 h of diode-mode operation as of Mar 15, 2019.
•Hayabusa2, JAXA's asteroid explorer arrived at its destination, asteroid Ryugu.•A cluster of four microwave discharge ion engines worked very well.•The cumulative operation time was 6515 h, producing 1015 m/s delta-v.•An engineering model of the neutralizer has passed 55,170 h of ground operation.
To improve the performance of the 10-cm-class microwave discharge ion thruster μ10 for use in future deep space exploration missions planned by the Japan Aerospace Exploration Agency (JAXA), a new ...discharge chamber was designed, and its performance was tested. The maximum beam current in the new discharge chamber geometry was 16% higher than that in the original geometry, which was used in the Hayabusa 2 space explorer, under the same discharge power. To investigate the reason for this performance improvement, the multi-charged ion ratio in the plume, the beam current density profiles, and the ion current in the discharge chamber were measured by probes. It was found that the multi-charged ion efficiency and the beam divergence efficiency in the redesigned configuration were not significantly different from those in the Hayabusa 2 configuration. This shows that the increase in the ion beam current enhances the thrust. In addition, it was confirmed that the total ion current inside the new discharge chamber is higher than that in the Hayabusa 2 configuration. The ion extraction efficiency, however, was lower than that in the Hayabusa 2 configuration. This suggests that the increase in the total ion current per unit of incident microwave power is the cause of the performance improvement. In the redesigned configuration, the thrust is 12.0 mN, the specific impulse is 3122 s, the discharge loss is 162 W/A, and the propulsion efficiency is 39.6% at the peak performance point.
•An ion beam current increase of 16% was obtained by redesigning the discharge chamber of the μ10 thruster.•It was verified by E×B probe measurement that the beam current improvement was not due to multi-charged ion current.•It was confirmed by electrostatic probe measurement that total plasma current was increased.•A thrust of 12 mN and a total efficiency of 39.6% have been achieved at the peak performance point.
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