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The problem with a computer mounted externally to the station, known as Multiplexer / Demultiplexer (MDM): omskärelse its repair is required to maintain redundancy in the systems of the ISS, and will require an EVA currently scheduled for April 22; NASA and SpaceX would still be willing to confirm the launch of the third Dragon, currently scheduled for Friday 18th (weather permitting). The MDM is essentially an electronic unit that sends and receives data through multiple streams, and therefore these elements are essential to the flow of information ISS. The MDM are part of the Command and Data Management, which oversees all the main functions of the U.S. segment of the station; some of them are installed on board, while others, such as fault, are outside. More precisely, the two external MDM group Tier 2 run the equipment of the truss segment, including Structures and Mechanisms, System Exterior Thermal Control System and Electricity. The MDM labeled with the number 2 has not made a successful boot the last time it was activated: currently absorbs 33 watts of power (compared with a nominal value of 50) and does not respond to commands. It is an element of about 25 kg of mass, with dimensions of about 40x40x25 omskärelse centimeters. It has "slots" to accommodate up to 16 electronic cards, but at the moment it fits only 5. E 'was launched in 2002 with STS-110, together with the S0 truss. Its removal should omskärelse be relatively simple, given that the network connection is by means of an inlet that is located on the back of the unit and that connects directly during assembly. It provides for a time of two and a half hours for the repair, even though the activity is included among the so-called "big 12", the twelve most challenging EVA. For foruna the replacement is already on board, and the defective part is easily accessible from the hatch of the form Quest. The EVA will be undertaken by two of the three astronauts currently aboard NASA's share (Rick Mastracchio, Steve Swanson and Koichi Wakata of JAXA, all experts); 3011 will be used in the suit (which has been recently replaced the element Fan Pump Separator) and the number omskärelse 3005 (which still has to undergo replacement surgery FPS). As a further guarantee, aboard Dragon is also a brand new suit, the number 3003. Once replaced, the MDM failure could be repaired on board (through replacement of defective single card), or be sent ashore to further examination. It should be emphasized that the MDM external number 2 that is spoiled habitually backup functions omskärelse compared to the number 1, and therefore there was no loss of operability. The maintenance, as I said, you only need to restore the usual security levels. A fault in the main MDM now make it impossible for the crew (and ground) the control equipment of the S0 truss; as the SSRMS arm is connected to Node 2 during the moorings of the Dragon, it would not be involved in the failure. For this reason, the current situation has no bearing on the planning of the launch capsule filling: the take-off of Dragon will take place as soon as they were settled the inconvenineti of these hours. Rather, it will be necessary to move the item MT (Mobile omskärelse Transporter), which currently prevents access MDM failure. omskärelse His movement is also required to provide additional crew shots during the capture of the Dragon. The displacement of the MT is governed dall'MDM survivor, number 1: if this should fail, it would become impossible to control the system. It is thought, therefore, to anticipate the possible transfer of the MT to the worksite 2, but in this way the umbilical cord TUS (Trailing omskärelse Umbilical System) would remain exposed too long to micro-meteorites. A fault in MDM 1, in the absence of his backup MDM 2, would result in the inability to govern the main two Thermal Radiator Rotary Joint (TRRJs), which control the radiators to save them from the illumination of the sun. For this they have already been locked in the safety position. The two Solar Alpha Rotary Joint (SARJs), which allow the panels to follow the sun on the main Alpha, would be useless in the event of failure MDM still working. Finally, several gyroscopic systems are under the control of