Editing Mars Observer (section)

== Communications loss ==
{{multiple image
| footer    = Investigators believe oxidizer leaked through check valves and mixed with fuel when pyro-valves 5 and 6 were opened.
| image1    = Mars Observer - loss of telemetry.png
| width1     = 114
| alt1      =
| caption1  = <small>Loss of telemetry</small>
| image2    = Mars Observer - bipropellant system failure diagram.png
| width2     = 86
| alt2      =
| caption2  = <small>Suspected failure</small>
}}
On August 21, 1993, at 01:00 UTC, three days prior to the scheduled [[Mars]] [[orbital insertion]], there was an "inexplicable" loss of contact with ''Mars Observer''.<ref name=commloss/> New commands were sent every 20 minutes in the hopes that the spacecraft had drifted off course and could regain contact. However, the attempt was unsuccessful.<ref name="commloss" /> It is unknown whether the spacecraft was able to follow its automatic programming and go into Mars orbit or if it flew by Mars and is now in a [[heliocentric orbit]].

On January 4, 1994, an independent investigation board from the [[Naval Research Laboratory]], announced their findings: the most probable cause in the loss of communication was a rupture of the fuel pressurization tank in the spacecraft's propulsion system.<ref name=iiboard/> It is believed that [[hypergolic fuel]] may have leaked past valves in the system during the cruise to Mars, allowing the fuel and oxidizer to combine prematurely before reaching the combustion chamber.  The leaking fuel and gas probably resulted in a high spin rate, causing the spacecraft to enter into the "contingency mode"; this interrupted the stored command sequence and did not turn the transmitter on.<ref name=iiboard/> The engine was derived from one belonging to an Earth orbital satellite and was not designed to lie dormant for months before being fired.{{cn|date=September 2024}}

{| class="wikitable" style="margin:1em auto;"
! Quoted from the report<ref name=iiboard/>
|-
| scope="col" width="750" | Because the telemetry transmitted from the Observer had been commanded off and subsequent efforts to locate or communicate with the spacecraft failed, the board was unable to find conclusive evidence pointing to a particular event that caused the loss of the Observer.

However, after conducting extensive analyses, the board reported that the most probable cause of the loss of communications with the spacecraft on August 21, 1993, was a rupture of the fuel (monomethyl hydrazine (MMH)) pressurization side of the spacecraft's propulsion system, resulting in a pressurized leak of both helium gas and liquid MMH under the spacecraft's thermal blanket.  The gas and liquid would most likely have leaked out from under the blanket in an unsymmetrical manner, resulting in a net spin rate.  This high spin rate would cause the spacecraft to enter into the "contingency mode," which interrupted the stored command sequence and thus, did not turn the transmitter on.

Additionally, this high spin rate precluded proper orientation of the solar arrays, resulting in discharge of the batteries.  However, the spin effect may be academic, because the released MMH would likely attack and damage critical electrical circuits within the spacecraft.

The board's study concluded that the propulsion system failure most probably was caused by the inadvertent mixing and the reaction of nitrogen tetroxide (NTO) and MMH within titanium pressurization tubing, during the helium pressurization of the fuel tanks.  This reaction caused the tubing to rupture, resulting in helium and MMH being released from the tubing, thus forcing the spacecraft into a catastrophic spin and also damaging critical electrical circuits.
|}