P78-1 (Solwind)

Launch data: Designation 11278 / 79017A Launch date 24 Feb 1979 Launch site Va, SLC-3W Launch vehicle Atlas-F/OIS Mission Scientific: high atmosphere study Earth orbit: Perigee/Apogee 310/317 km Inclination 97.8° Period 90.8 min

P78-1 satellite

Spacecraft data:

End of life

Description Instrumentation:

• The Orbiting Solar Observatory (OSO) platform included a solar-oriented sail and a rotating wheel section. Ball Aerospace was the primary contractor for design and construction, and provided the attitude control and determination computer programs. The P78-1 carried a gamma-ray spectrometer, a white light spectrograph, an extreme ultraviolet spectrometer, a high latitude particle spectrometer, an aerosol monitor, and an X-ray monitor. The X-ray monitor, designated NRL-608 or XMON, was a collaboration between the Naval Research Laboratory and Los Alamos National Laboratory.
• The instrument payload consisted of a gamma-ray spectrometer, a white light spectrograph, an extreme ultraviolet spectrometer, a high latitude particle spectrometer, an aerosol monitor, and an X-ray monitor. The X-ray monitor, designated NRL-608 or XMON, was a collaboration between the Naval Research Laboratory and Los Alamos National Laboratory.
• The X-ray monitor consisted of 2 collimated argon proportional counters. The instrument bandwidth of 3-10 keV was defined by the detector window absorption (the window was 0.01 inch beryllium) and the upper level discriminator. The active gas volume (P-10 mixture) was 2.54 cm deep, providing good efficiency up to 10 keV. Counts were recorded in 2 energy channels. Slat collimators defined a FOV of 3 degrees x 30 degrees (FWHM) for each detector; the long axes of the FOVs were perpendicular to each other. The long axes were inclined 45 degrees to the scan direction, allowing localization of transient events to about 1 degree. The centers of the FOVs coincided, and were pointed 40 degrees below the scan equator of the wheel in order to avoid scanning across the Sun. The spacecraft wheel rotated once every 6 seconds. This scan rate corresponds to 1 degree every 16 milliseconds; counts were telemetered in 64 or 32 millisecond bins to minimize smearing the collimator response.
• The platform spin vector was perpendicular to the plane of the Sun- synchronous orbit. Because the detector FOV was not symmetric about the spin vector, the scan pattern repeated with a 1 year cycle, rather than the 6 month cycle of the HEAO platforms. Some sources, as Vela X-1, spend as much as 2 months within the half response bands. Such a long exposure is particularly useful for studying sources with intrinsic time scales of days to weeks, such as eclipsing binaries.
• P78-1 was in a noon-midnight, Sun-synchronous orbit at 600 km altitude. The orbital inclination of 96 degrees implied that a substantial fraction of the orbit was spent at high latitude, where the particle background prevented detector operation. In-flight experience showed that good data were obtained between 35 degrees N and 35 degrees S geomagnetic latitude outside the South Atlantic Anomaly. This yields an instrument duty cycle of 25-30%. Telemetry data were obtained for about 40-50% of the orbits, yielding a net data return of 10-15%. Though this data rate appears low, it means that about 10e8 seconds of good data reside in the XMON data base.
• The instrument parameters and data yield imply a 3 sigma point source sensitivity of 30 UFU in one day's operation (1 UFU = 2.66e-12 erg/cm2-s-keV). Each detector was about 0.1 of the area of the Uhuru instrument. The instrument background at low geomagnetic latitudes was ~16 cts/s. Of this background, ~6 cts/s comes from the diffuse cosmic X-ray background, with the rest being instrumental. Assuming a conservative 10% data return, the net source duty cycle in scanning mode is 1.4 X 10-3, implying a source exposure of 120 seconds per day. For a background of 16 cts/s, the 3 sigma error in determining the flux from a given sky bin is then 4.5 cts/s, or about 45 UFU, after 1 day. A limiting sensitivity of 30 UFU can be obtained by combining both detectors. A comparable error will exist in the flux determination for moderately bright galactic sources. Source confusion due to the 5 degrees FOV projected along the scan direction will complicate the observation of sources in the galactic bulge region (approximately 30 deg > l > -30 deg, |b| < 10 deg).

Mission details:

• P78-1 or Solwind was a United States satellite launched aboard an Atlas F rocket from Vandenberg Air Force Base in California on February 24, 1979. The satellite operated until September 13, 1985, when it was shot down in orbit by an F-15 Eagle fighter aircraft during an US Air Force ASM-135 ASAT test. The test outraged some scientists because although five of P78-1's instruments had failed at the time of the test, two instruments remained in operation, and the satellite was what one solar physicist called "the backbone of coronal research through the last seven years."
• However, for several months before the ASAT test, the satellite batteries were degrading. This caused more and more frequent 'under-voltage cutoffs', a condition where the satellite detected low main bus voltage and automatically shut down all non-vital systems. As a result, an ever-increasing amount of time and network resources were spent reconfiguring the satellite for normal operation. Data collection from the few remaining payloads was severely limited. Because of the additional burden on the Air Force Satellite Control Network (e.g., extra support and antenna time at the tracking stations), serious discussions were already underway to terminate the mission prior to the ASAT test. It was extended for several weeks solely to support the test. During this final phase, it was often allowed to remain in the under-voltage condition for several days at a time.

Science:

• Data from the P78-1 X-Ray Monitor experiment offer source monitoring with a sensitivity comparable to that of instruments flown on SAS-3, OSO-8, or Hakucho, and the advantages of longer observing times and unique temporal coverage. Five fields of inquiry are particularly well suited for investigation with P78-1 data. These are:
  • study of pulsational, eclipse, precession, and intrinsic source variability on time scales of tens of seconds to months in galactic X-ray sources.
  • pulse timing studies of neutron stars.
  • identification and study of new transient sources.
  • observations of X-ray and gamma-ray bursts, and other fast transients.
  • simultaneous X-ray coverage of objects observed by other satellites, such as HEAO-2 and 3, as well as bridging the gap in coverage of objects in the observational timeline.