Thermal Vacuum Testing of the Orbiting Carbon Observatory Instrument 2008-01-2036

The Orbiting Carbon Observatory (OCO) instrument is scheduled for launch onboard an Orbital Sciences Corporation LEOStar-2 architecture spacecraft in December 2008. The instrument will collect data to identify CO₂ sources and sinks and quantify their seasonal variability. OCO observations will permit the collection of spatially resolved, high resolution spectroscopic observations of CO₂ and O₂ absorption in reflected sunlight over both continents and oceans. OCO has three bore-sighted, high resolution, grating spectrometers which share a common telescope with similar optics and electronics. A 0.765 μm channel will be used for O₂ observations, while the weak and strong CO₂ bands will be observed with 1.61 μm and 2.06 μm channels, respectively. The OCO spacecraft circular polar orbit will be sun-synchronous with an inclination of 98.2 degrees, mean altitude of 705 km and 98.9 minute orbit period. The OCO mission forms part of NASA's A-Train and leads the afternoon constellation ahead of the Aqua spacecraft.

The instrument thermal design provides two cryogenic temperature zones for the detectors at 120 K and 180 K, one near room temperature zone for the spectrometers at 268 K and one for the electronics and cryocooler at 290 K. It contains three focal plane arrays (FPAs) in three separate housings, two of which are cooled to 120 K, and one to 180 K by a single Northrop Grumman Space Technology (NGST) pulse tube cryocooler. The cryocooler with a coldtip at 110 K, provides refrigeration to all three detectors via a high conductance flexible thermal strap. A variable conductance heat pipe (VCHP) based heat rejection system (HRS), with space viewing radiators, provides cooling for the electronics, cryocooler and maintains the spectrometer at -5°C. The VCHPs, with closed-loop temperature control, transport waste heat from the instrument located inside the spacecraft to the outboard radiators and provide tight temperature control. In survival mode, the instrument is off and the VCHPs are shutdown to minimize survival heater power.

The instrument was fully integrated in December 2007 and has undergone EMI/EMC, vibration, and two thermal vacuum (TV) tests. This paper reports on the results from the TV and thermal balance (TB) tests with an emphasis on the VCHP-based HRS and the cryogenic subsystem (CSS). A brief overview of the thermal control design is presented as well as key test results from the instrument-level thermal vacuum/balance test.