Philip T. Chen

The COR1 inner coronagraph for STEREO-SECCHI

The Solar Terrestrial Relations Observatory (STEREO) is a pair of identical satellites that will orbit the Sun so as to drift ahead of and behind Earth respectively, to give a stereo view of the Sun. STEREO is currently scheduled for launch in November 2005. One of the instrument packages that will be flown on each of the STEREO spacecrafts is the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), which consists of an extreme ultraviolet imager, two coronagraphs, and two side-viewing heliospheric imagers to observe solar coronal mass ejections all the way from the Sun to Earth. We report here on the inner coronagraph, labeled COR1. COR1 is a classic Lyot internally occulting refractive coronagraph, adapted for the first time to be used in space. The field of view is from 1.3 to 4 solar radii. A linear polarizer is used to suppress scattered light, and to extract the polarized brightness signal from the solar corona. The optical scattering performance of the coronagraph was first modeled using both the ASAP and APART numerical modeling codes, and then tested at the Vacuum Tunnel Facility at the National Center for Atmospheric Research in Boulder, Colorado. In this report, we will focus on the COR1 optical design, the predicted optical performance, and the observed performance in the lab. We will also discuss the mechanical and thermal design, and the cleanliness requirements needed to achieve the optical performance.

Incorporation of molecular adsorbers into future Hubble Space Telescope instruments

The Hubble Space Telescope (HST) has been designed to accommodate changeout and/or repair of many of the primary instruments and subsystem components, in an effort to prolong the useful life of this orbiting observatory. In order to achieve the science goals established for this observatory, many HST instruments must operate in regimes that are greatly influenced by the presence of on-orbit propagated contaminants. To insure that the required performance of each instrument is not compromised due to these contaminant effects, great efforts have been made to minimize the level of on-orbit contamination. These efforts include careful material selection, performing extensive pre-flight vacuum bakeouts of parts and assemblies, assuring instrument assembly is carried out in strict cleanroom environments, performing precision cleaning of various parts, and most recently, the incorporation of a relatively new technology -- molecular adsorbers -- into the basic design of future replacement instruments. Molecular adsorbers were included as part of the wide field/planetary camera 2 (WFPC-2) instrument, which was integrated into the HST during the servicing mission 1 (SM1) in 1993. It is generally recognized that these adsorbers aided in the reductio of on-orbit contamination levels for the WFPC-2 instrument. This technology is now being implemented as part of the basic design for several new instruments being readied for the servicing mission 2 (SM2), scheduled for early 1997. An overview of the concept, design, applications, and to-date testing and predicted benefits associated with the molecular adsorbers within these new HST instruments are presented and discussed in this paper.

The use of molecular adsorbers for spacecraft contamination control

In recent years, the technologies associated with contamination control in space environments have grown increasingly more sophisticated, due to the ever expanding need for improving and enhancing optical and thermal control systems for spacecraft. The presence of contaminants in optical and thermal control systems can cause serious degradation of performance and/or impact the lifetime of a spacecraft. It has been a goal of the global contamination community to develop new and more effective means for controlling contamination for spacecraft. This paper describes an innovative method for controlling molecular contaminants in space environments, via the utilization of Molecular Adsorbers. It has been found that the incorporation of appropriate molecular adsorbing materials within spacecraft volumes will decrease the overall contamination level within the cavity, thereby decreasing the potential for contaminants to migrate to more critical areas. In addition, it has been found that the placement of a Molecu...

TRMM project contamination control using molecular adsorbers

The Tropical Rainfall Measuring Mission (TRMM) is a spacecraft under development by the National Aeronautics and Space Administration (NASA) and the National Space Development Agency of Japan (NASDA) and is scheduled for launch in August 1997. The spacecraft design includes the use of numerous optical instruments and the thermal control surfaces. In addition to the inherent contamination sensitivities of the optical and thermal systems, TRMM has had the added challenge of designing systems to function at a relatively low altitude (350 km), with solar exposure. Under these conditions, high atomic oxygen densities and potentially high levels of backscattered contamination (self‐contamination), as well as UV photopolymerization effects, all pose major threats to sensitive TRMM elements. In considering the various contamination control paths to follow, the TRMM project management has opted for pursuing a relatively new, but very promising technology for the TRMM spacecraft in order to lower the on‐orbit conta...

Applying Contamination Modeling to Spacecraft Conventional Propulsion System Designs and Operations

Molecular and particulate contaminants generated from the operations of a propulsion system can impinge on spacecraft critical surfaces. Plume depositions or clouds can hinder the spacecraft and instruments from per- forming normal operations. The interconnection between the functions of spacecraft contamination modeling and propulsion system implementation is presented. An innovative contamination engineering approach is addressed during a spacecraft mission, which includes concept design, manufacturing, integration and test, launch, and on-orbit operations. A summary of the implementation on several successful missions is also presented. NE potential source of concern facing the instruments of or- biting spacecraft is the effect of molecular contaminant inter- action with sensitive thermal control and optics surfaces. Typically, the sources of these on-orbit contaminants can be categorized into e ve general areas: 1 ) material outgassing (water, hydrocarbons, sil- icones) from materials of construction; 2 ) spacecraft and multiple- layer insulation venting; 3 ) e uid leakage from pressurized vessels (e.g., cryogen tanks ), dumps, and lubricant loss; 4 )exhaust material generated through thruster e rings; and 5 ) extravehicular activity. 1 Once released, contaminants can propagate to the receiving sur- faces through direct line-of-sight transport (direct e ux), ree ections with spacecraft surfaces, and scattering through self-scattering or with the local ambient atmosphere (return e ux). The efe ciency of these transport mechanisms is a complicated function of spacecraft geometry, mission /e ight operations, and environmental effects. In the past the purpose of computer modeling was concentrated in the assessment of contamination damage during the late design phase, integration and test, and on-orbit operation. The impact of modeling on the mission was limited to minor design changes (such as vent locations ), verie cation (for meeting contamination require- ments), and on-orbit operation (such as operational constraints im- posed to avoid contamination ). Becauseofincreasedsensitivityofspacecraftcomponentstocon- tamination effects, contamination engineering has begun to play a more notable role in overall spacecraft development. Early involve- mentrepresents themosteffectivedirectionoffuture contamination modeling efforts. By ine uencing the early design, cost savings can be very signie cant because many inefe cient contamination avoid- ance remediesestablishedlate in thedesign cycle canbeeliminated. In recent years improved contamination modeling techniques have been used extensively by contamination sensitive projects to improve spacecraft and instrument performance during the early design stage. One good example is the detailed modeling effort for the Tropical Rainfall Measuring Mission (TRMM). Contami- nation modeling efforts for this mission resulted in several design changesespeciallyinthepropulsionsystem.Thepaperdescribesthe

Contamination Effects on the Geostationary Operational Environmental Satellite Instrument Thermal Control System

The instrument thermal control system of the Geostationary Operational Environmental Satellite may have been affected by specie c contamination problems that arose because of the unique conditions and requirements of the spacecraft mission. This paper addresses some specie c contamination effects from the coatings used in the instrument cavities. Contamination control actions that were implemented during ground processing to ensure limited impact on the on-orbit temperature control are described. The selection of thermal coatings is an integral part of the overall spacecraft design. Molecular contamination accretion on thermal coatings may alter the design properties of the surface coatings. In an effort to quantify the molecular contamination effects from material outgassing, an assessment was conducted to address the concerns inside the instrument cavities. The study results prompted an extensive prelaunch vacuum bakeout effort and an on-orbit solar radiation avoidance exercise. In addition, the thermal performance of the instrument radiant coolers could have been detrimentally affected by scattered solar radiation from particulate contamination. To mitigate the impact of the particulate contamination the radiant cooler surfaces were cleaned to an established criteria prior to launch.

Return flux experiment; REFLEX: spacecraft self-contamination

On-orbit, self-contamination of a spacecraft is a concern facing instrument and spacecraft designers. While on the Earth, gases adsorb onto spacecraft surfaces. These gases are later released when placed in the vacuum of space. The rate at which the emitted gases are returned to the spacecraft by collisions with other gaseous molecules is known as the return flux. Models predicting the amount of gas released by a spacecraft that is returned to itself do exist, but these models have had very limited experimental testing. We describe a flight experiment designed to provide a test of these models and the analysis of the data obtained by that experiment. The experiment flew on a 1996 space shuttle mission and provided in-situ testing of the return flux models. Analysis of the limited data obtained by the experiment has determined the return flux is primarily due to collisions with the ambient atmosphere and not collisions with other gases released by the spacecraft. Limited measurements of the ambient atmosphere were also made.

Atomic oxygen erosion of a graphite coating on a TQCM onboard the Return Flux Experiment (REFLEX)

A TQCM coated with graphite was flown aboard a Spartan carrier in January 1996. During a flight of about 46 hours at an altitude of 305 km, the graphite reacted with the atomic oxygen (AO) in the environment and was eroded away. The 15-MHz TQCMs frequency dropped from 6800 to 4000 Hz in about 15 hours of exposure and was shown to be a strong function of the TQCMs orientation to the ram direction. The erosion rates for four different ram angels was measured and found to be both consistent and repeatable. The average graphite volume loss for the 61 degree and -62 degree ram angles was calculated to be about 2 X E-08 cm3/hr and for the 18 degrees and 19 degrees angles to be about 8.5 X E-08 cm3/hr, which is slightly less than previous flight data. The erosion data was then correlated with AO density numbers for the particular times and positions of the spacecraft in orbit. From this analysis, an equation was derived that shoed the carbon volume loss as a function of both atomic oxygen density and ram angle. For example, 1.59 E-07 cm3/hr would be the calculated carbon volume loss for a ram angle of 0- degrees and an AO fluence of 3.52 E+17 atoms/hr. The results of this data and analysis may lead to the development of a sensor capable of monitoring the AO fluence on a spacecraft.

The evaluation of GOES black paint materials

The purpose of this paper is to study the contamination effect of black paint materials on the GOES instrument performance. The GOES spacecraft materials were originally selected for their low outgassing properties. Samples of the materials were tested according to the ASTM E-595 test method to fulfill the total mass loss and collected volatile condensable materials criteria for traditional spacecraft material selection. Due to the instrument design, the cavity will experience high temperatures during operation greater than the specified temperature in the ASTM test. As a result of this high cavity temperature, normally stable paint materials on the painted surface may severely outgas even though they have passed the ASTM test. Further enhancement of the contaminant remaining on the mirror by UV irradiation is also a great consideration. This concern prompted an investigation into the outgassing characteristics of the black paints at the predicted operating temperatures.

Cleanliness Correlation By BRDF And PFO Instruments

At NASAs Goddard Space Flight Center (GSFC), Bi-Directional Reflectance Distribution Function (BRDF) is applied to characterize the scattering properties of optical and thermal surfaces. In addition, the Particle Fall Out (PFO) instrument (currently under evaluation) is used to determine the particulate contamination on surfaces. This paper describes both instruments and correlates the results from their empirical measurements. Both the BRDF and PFO instruments are located in a Class 1,000 cleanroom to minimize contamination. The BRDF instrument is completely automated and controlled by a personal computer. The FF0 is a scattering measurement instrument developed by SAAB and updated by Uramec (Bilthoven/The Netherlands) [1]. The PFO is used to determine the surface cleanliness level on a contaminated plate by measuring the scattered light due to particles on the plate. For this paper, black glass is used as the contaminated sample for both the optical measurements. The PFO and BRDF instruments are then utilized to measure the scattering of the contaminated sample. Results are compared and related to the surface cleanliness level as well as obscuration factor.