Hitomi problems explained !

Astro-H (Hitomi) is still tumbling, but at least it is known why.

Case of Astro-H (named Hitomi) seems to be far away from the lucky end. JAXA still is trying to contact with spacecraft which probably came apart during flight on 26 March 2016 at around 01:20 GMT while satellite remained on altitude of 575 km. Since then, JAXA was able to receive only short radio signals from Hitomi without any data on current state of satellite.  According to JAXA, anomaly started on 26 March 2016 - temperature was increasing and problems with attitude and powering were spotted. On March 26, 27 and 28 Hitomi managed to send only very short radio signals. According to JAXA report announced on April 11, 2016, largest piece which remained from Hitomi is tumbling with rotation period of 5.2 s - it was confirmed by University of Tokyo which performed optical observation of anomaly. Previous actions performed by JAXA have been limited to observing of Hitomi with NORAD ID: 41337, 41442, 41440, and 41441 by following space observation centers: Kamisaibara Space Guard Center, Bisei Space Guard Center with support of University of Tokyo Astronomical Observatory and National Astronomical Observatory of Japan.

On April 15, 2016 JAXA announced updated report on Hitomi. It put new light on understanding reasons of failure. It seems that problem probably started on February 28, 2016, when JAXA uplinked to Hitomi new RCS (Reaction Control System) control parameter to correct changes in mass after extending Extendable Optical Bench (EOB). Investigation unveiled that RCS control parameter was incorrect. After uplink and resetting RCS thrusters were not started and system was not checked properly. On March 26, 2016, JAXA planned maneuver to orient Hitomi telescope toward the nucleus of galactic with deployed solar arrays toward to Sun. At that moment Hitomi switched to use Star Tracker (STT, one of its two attitude instruments along with Inertial Reference Unit IRU). After maneuver IRU bias rate estimation became larger than actual, with parameter at around 21,7°. In such case data from IRU should be replaced with correct data from STT. Problem appeared during uploading data from STT to Hitomi along with procedure, which determines to continue flight only according to IRU in case of lack data from STT or in case when difference between data from two instruments is greater than 1°. While difference was 21° (on Z axis), ACS decided to perform action to prevent anticipated rotation. Reaction wheels in ACS stopped to rotate what caused that spacecraft started to rotate what happens is satellite is in stable flight. Construction of ACS was based not on sun sensor, but on calculations performed by software algorithm. Due this fact, magnetic torque failed to decrease the momentum kept in RW within the correct range due the anomaly caused by IRU data and lead to accumulating angular momentum. Telemetry data received on Earth showed that angular speed was at 112 Nms while upper limit is 120 Nms. ACS switched spacecraft into Safe Hold mode which determines using thrusters for attitude control instead of reaction wheels. After firing thrusters to recover attitude, spacecraft started using RCS which was basing on incorrect parameter uploaded on February 28. It caused further increasing rotation velocity until EOB and solar panels broke off. In this case Hitomi lost power source which is now limited to onboard battery and its main science payload is heavily damaged along with change of carrier frequency for about 200 kHz comparing to normal condition. Cooling system of Soft X ray Imager (SXS) is also broken and Helium level, which is cooling factor, is decreasing. At the moment main fragment of the spacecraft is ID 41337 - it is at the moment of slightly different orbit that was planned for Hitomi, with apogee at 589.1 km and perigee 571.3 km at with inclination at 31°.

JAXA unveiled their further actions to bring Hitomi back to even partial operational condition. Due the fact that spacecraft's onboard battery is low, first of all JAXA will try to uplink to Hitomi command to start charging battery using solar arrays attached to fuselage of the satellite, of course if they are still working.  Resuming communication and recharging are crucial for further actions. JAXA will still follow Hitomi from Earth with astronomical observatories. After recovering communication it will be possible to check condition of the Hitomi and perform additional actions to recover control on flight.