Since last December, Chandra mission planning guidelines prohibit observations which point greater than 170 degrees from the sun. This does not exclude the possibility of pointing at any celestial position, but does potentially restrict constrained observations. For example, a point on the ecliptic plane will be inaccessible for 24 days while it is situated opposite the sun, and simultaneous observations with optical observatories will not be possible for targets near zenith at local midnight.
This exclusion has been implemented to prevent any possibility of freezing two hydrazine propellant lines, monitored by thermistors PLINE03 and PLINE04. Ironically, the possibility has arisen because of a warming trend in a third propellant line, with thermistor PLINE02. The explanation is that the heater circuit which warms all three lines is controlled by a single thermostat near PLINE02. By design, it was expected that PLINE02 would be the coolest of the three, and this was indeed the case for the first several years of the mission. This is no longer the case, as Chandra is generally becoming warmer through the years due to the degradation of its multi-layer insulation.
Hydrazine freezes at 35 deg F, and the thermal control was set so that the lines would never fall below 40 deg F. This is a hardware setting which cannot be altered. About a year ago, NGST Flight Operations Team engineers noticed PLINE03 and PLINE04 dipping below 45 deg F for the first time. By last November, 18 such cases had been observed, with temperatures reaching as low as 42 deg F. It was established that these cool excursions were correlated with ChandraÕs pitch angle, occurring as low as 158 deg pitch from the sun. At that time we therefore established an immediate, temporary restriction against pitch angles greater than 150 deg from the sun.
Hydrazine contracts as it freezes. This allows more liquid to flow into the resulting void and also freeze. When the lines subsequently warm up, they are not expected to thaw uniformly, so that the liquid expands against the remaining frozen "plug" and stresses the walls of the pipe. In ground testing, repeated such cycles have been observed to eventually split the line. It is impossible to analyze the exact consequences, but obviously release of such a contaminant into the Chandra HRMA or equipment volumes could have disastrous consequences, and must absolutely be avoided.
Further analysis involving the original design engineers has established that the thermistors do read the coldest portion of the lines. With all tolerances on local gradients, thermistor errors and quantization, and a temperature difference in the event that the primary heater circuit failed over to the redundant heater/thermostat, the hydrazine will not freeze as long as the temperature reading remains above 42.5 deg F. Analysis of current thermal performance shows that this can be achieved allowing pitch angles as large as 170 deg F, hence our current operational limit.
Although this restriction excludes only a very small percentage of the sky, about 1.1%, it has the additional effect of constricting the regions where we can point to cool our radiation monitor, EPHIN, prior to dwelling at attitudes where EPHIN is subject to overheating. We will monitor performance as the mission proceeds. Continued change in thermal performance could lead to greater restriction on the maximum pitch angle. However, the team is also investigating special operational scenarios which could allow some targets at attitudes beyond 170 deg. (Chandra Electronic Bulletins, such as Bulletin #35, will continue to update users on restrictions. -Editor)
Dan Schwartz, Sabina Bucher, Dan Shropshire