The SEPICA sensor on ACE has apparently suffered a valve failure in the
isobutane system that provides proportional counter gas to one of its three
sensors.  Nominal valve operation suddenly ceased on February 13 and the
pressure in this sensor decayed to zero. Subsequent attempts to open the valve
have been unsuccessful; the most likely explanation is that the valve has
failed in a closed position. The valves on the other two counters are now
being operated in an open position, with the pressure regulated by the outflow
through the calibrated leaks of the counters.  These sensors are both
performing nominally. In the meantime, laboratory testing of a fourth valve on
the ground is continuing.

Although all three SEPICA sensors measure charge states over the same energy
interval, the sensor that failed has a smaller aperture and higher resolution
than the other two.  Data from this sensor obtained during the large solar
events of last November will be used to help calibrate the response of the two
lower-resolution sensors, thereby allowing the extraction of more detailed
information from these sensors during future events. Overall, SEPICA will
still provide average charge state measurements with much greater sensitivity
and statistical accuracy than in the past, allowing ACE to meet its scientific
objectives.

The remaining eight instruments on ACE all continue to operate nominally.

The ACE Science Center (ASC) has processed and delivered data to the instrument
teams through March 31st, 1998. 

The Flight Operations Team reports that the third mission station-keeping
maneuver was successfully executed last week. A test was also performed that
verified the operation of Sun Sensor B. All spacecraft subsystems are
performing as expected.