Quote from: lykos on 02/26/2022 05:04 pmIt had to have a big hole in the middle for the engine/thrust.And it was as light as possible.There was no hole in the descent stage, the ascent engine barely touched it; maybe what we see is the "marking" of ascent stage exhausts?
It had to have a big hole in the middle for the engine/thrust.And it was as light as possible.
Aug 07, 2022DFRS payload onboard Chandrayaan-2 orbiter detects high density plasma in the Lunar wake region.The Moon is believed to have a very tenuous atmosphere. Since the ionosphere derives its origin from the atmosphere the plasma density at the Moon is considered to be only a few hundred ions per cubic centimeter. Measurements made using the Dual Frequency Radio Science (DFRS) experiment onboard Chandrayaan-2 orbiter, however, have shown that Moon’s ionosphere has a plasma density of the order of 104 cm-3, in the wake region which is at least one order of magnitude more than that is present in the day side.Chandrayaan-2, the second Indian Lunar exploration mission launched by the Indian Space Research Organization (ISRO) on 22 July 2019, carried several payloads, among which the DFRS was designed to study the lunar ionosphere. It uses two coherent signals at the S-band (2240 MHz) and X-band (8496 MHz) of radio frequencies, transmitted from the Chandrayaan-2 orbiter and received at the ground station at Byalalu, Bangalore to explore the lunar plasma ambiance using the radio occultation (RO) technique. Simultaneous measurements by two coherent radio signals help to mitigate the effect of the Earth’s atmosphere and any uncertainties due to various sources during the experiments. The DFRS payload was conceptualized and jointly developed by Space Physics Laboratory (SPL) of Vikram Sarabhai Space Centre, Trivandrum; UR Rao Space Centre (URSC), Bangalore; and ISRO Telemetry, Tracking, and Command Network (ISTRAC), Bangalore. For the RO observations, an algorithm to estimate the integrated electron density profile was developed at SPL and used to study the Moon’s ionosphere in the lunar wake region, a region of the Lunar ionosphere that does not directly interact with the solar wind.A total of 12 radio occultation experiments have been conducted in campaign mode on four different occasions based on carefully selected geometry suitable for the RO measurements. Detailed analysis shows that the total electron content along the ray path in the Lunar ionosphere can be as large as 1.5 TECU (1 TECU = 1016 m-2) with the uncertainty of 0.15 TECU, in the Lunar wake region. Large electron content is also seen near lunar polar regions during solar twilight conditions. These findings are unique and first of its kind as they show substantial post-sunset enhancement in plasma density compared to the dayside values reported so far by earlier missions.The observed large enhancements in electron density in the Lunar wake region open new dimensions in understanding the lunar dark side plasma environment. In the wake region, neither the solar radiation nor the solar wind interacts directly with the available neutral particles, but still, the plasma is getting generated. Numerical simulations of the dark side of plasma environment using a 3-dimensional Lunar Ionospheric Model (3D-LIM) developed at SPL suggest that the production of ions by charge exchange reactions may play a pivotal role in producing a significantly large plasma density in the Lunar wake region, which can sustain for a longer period. The model suggests that the dominant ions in the wake region are Ar+, and Ne+ which have a comparatively longer lifetime than the molecular ions (CO2+, and H2O+ ) that are dominant at other regions. On the other hand, fraction of the ions produced by solar radiation during the daytime is swept by solar wind, leading to reduced daytime plasma density. These path-breaking scientific results on the Moon's ionosphere using DFRS payload onboard the Chandrayaan-2 orbiter and modeling of the physical mechanism are published in the Monthly Notices of the Royal Astronomical Society- Letter; "A study on the characteristic features of the Lunar ionosphere using dual-frequency radio science (DFRS) experiment onboard Chandrayaan-2 orbiter", Keshav R. Tripathi, R. K. Choudhary, K. M. Ambili, K. R. Bindu, R. Manikantan and Umang Parikh, DOI: 10.1093/mnrasl/slac058 (https://doi.org/10.1093/mnrasl/slac058)
Figure 1: Top left panel: Ray-tracing of radio signals in the lunar ionosphere. A1 is the point of impact factor on the given ray path, Top right panel: Comparison of observed and simulated ray-path integrated electron density for the Chandrayaan-2 orbit no. 1960 for the occultation experiment at a location inside the Lunar wake, Bottom left panel: Electron density profiles simulated using the 3D-LIM model for three different conditions (Lunar wake, Solar terminator, and local noon). The electron density is of an order of magnitude higher inside the lunar wake region, Bottom right panel: Altitude/latitude/longitude variations of electron density along the ray path as simulated by 3D-LIM. The color bar represents the electron density at different points along the radio path.
So here's my full explanation for why the Vikram Lander in Chandrayaan-2 crashed back in 2019. We still don't have much data to go on, but a few news pieces of information give us a working timeline.It crashed because of software problems and a valve not operating properly.
The next phase was to begin fine navigation for landing, cutting the thrust and performing a 38 second 'camera coast' where the spacecraft held a specific attitude while the navigation sensors checked the terrain to figure out where the landing site was.
Unfortunately, it appears that when they actuated the throttle control valves at the fastest rate they did not close as much as expected. The result was the spacecraft had more thrust than it was supposed to. Expected acceleration of 2.13m/s vs actual ~2.8m/s
We know the expected acceleration because we have start and end conditions for the 38s camera coast phase. The actual condition is approximated from the snippets of telemetry we saw.
During camera coast the spacecraft held steady and didn't try to fix any navigation errors, so the spacecraft built up a huge error from where it was supposed to be ~500m in space, and velocity ~25m/s. It was higher than it should be, moving slower, and further from target.
So it began to reorient to correct for this, and while I'm not 100% sure, I think it pitched forward and continued all the way though a 410 degree rotation. However, it may have rotated the other direction for all we know.
This took some time because the software limited the rotation to 10 degrees per second, in effect the rotation limits meant it was like dog chasing its tail to try and reach a desired velocity state. It quickly ran out of time and altitude.
The final telemetry came 7 seconds before impact, and it looks like the impact speed was about 140miles per hour.
You can see this in the doppler data from Cees Bassa @cgbassa - note the kink 38 seconds before the anomaly where the deceleration changes, then the acceleration as the spacecraft tries to make up lost ground, then tries to slow before hitting the surface.
So it's a combination of a valve which misperformed, software which inhibited from reacting until it was too late, and software limits which made recovery impossible. I'm not 100% sure, but all these parts now fit together into a sensible story.All of these are fixed on CY3.