A Project Science Retrospective On 25 Years Of Chandra

Douglas Swartz (USRA/MSFC), Steven Ehlert, and Steve O’Dell (NASA/MSFC)

Introduction

The Project Science team is pleased to provide this brief retrospective of its role in the formulation, development, calibration, and operation of the Chandra X-ray Observatory. Our hope is to help provide today’s science community with a well-grounded perspective on the past and to help place our shared current challenges into a broader context, as we mark twenty-five years of Chandra scientific achievements.

The Project Science team has always served as the interface between the science community and the project to help ensure the scientific integrity of the mission. We believe Chandra's outstanding record of scientific and technical success is because all components of the project—scientists, engineers, and management and both industry and science institutions—worked together towards the common objective of building and operating the world’s greatest X-ray astrophysics facility.

Formulation And Development

The Project Science story began decades before the launch of Chandra, when NASA assigned project management responsibility to the Marshall Space Flight Center (MSFC) for a bold X-ray mission initiative proposed in 1976 by a team from the Smithsonian Astrophysical Observatory (SAO), led by Riccardo Giacconi and Harvey Tananbaum. The first Advanced X-ray Astrophysics Facility (AXAF) Project Scientist, Martin C. Weisskopf, was selected and arrived at MSFC by 1977 and began building the Project Science team by 1980. From the beginning, MSFC was responsible for managing AXAF and for preliminary scientific systems engineering studies while SAO provided additional scientific and technical support. By 1985, Project Science became a well-defined collaboration among the Project Science team at MSFC (led by Martin), the SAO Mission Support Team (led by Harvey), and the Telescope Scientist (the late Leon Van Speybroeck, also at SAO).

The first major milestone occurred when the Decadal Survey report for the 1980s recommended AXAF as the top priority major national space observatory, specifically recognizing the importance to the entire astronomy community of a Flagship X-ray observatory. High-resolution X-ray mirrors were soon identified as the highest technological challenge to the project. This led to the Project Science team overseeing the immediate development of the Technology Mirror Assembly (TMA)—a 2/3-scale version of the AXAF innermost mirror pair, having a 6-meter focal length—and testing it in the existing X-ray Calibration Facility (XRCF) at MSFC. Testing was performed in 1985 and again after additional figuring and polishing of the optic in 1989. The success of these tests gave the project confidence that the biggest technological hurdles could be overcome.

Despite these and other successes, Congress mandated that continued mission development funding required demonstration of sub-arcsecond X-ray imaging capability with the largest AXAF mirror pair before the end of fiscal year 1991. Confirmation of the requisite performance was made by testing the so-called Verification Engineering Test Article (VETA) in the refurbished XRCF, thereby avoiding outright cancellation of AXAF. At about the same time, the 1991 Decadal Survey re-affirmed AXAF as the highest priority large program for astronomy and astrophysics. In addition, SAO was competitively selected to operate the AXAF Science Center, what would later be called the Chandra X-ray Center.

Still, major challenges abounded. Critically, NASA directed an extensive restructuring in 1991 and 1992 to reduce costs and to help secure continued congressional support. This was a major undertaking for Project Science and for the entire project, involving numerous trade studies to maximize scientific return within project resource constraints. The result was a split of the mission into two satellites: one, called AXAF-I, would have an emphasis on high-resolution imaging, but with only 4 of the original 6 mirror pairs; the other, called AXAF-S, would carry the microcalorimeter high-resolution spectrometer as its only science instrument. In 1993, Congress canceled AXAF-S, relegating the microcalorimeter to eventually fly on JAXA’s Suzaku mission over a decade later. AXAF-I, what is today the Chandra X-ray Observatory, did survive with a new, lighter weight, its present complement of instruments, a non-serviceable, highly-elliptical-orbit, and the remaining 4 HRMA mirror pairs now with Ir coatings.

Fortunately, funding stabilized after 1994 and remained relatively steady throughout the rest of development, calibration, and deep into science operations by successfully overcoming numerous key decision points, independent and semi-independent reviews and assessments (including all 7 Senior Reviews of Operating Missions), and many other programmatic and technical hurdles.

The primary Project Science role during the formulation and development stages was overseeing development and testing of TMA and VETA at XRCF and performing modeling and analyses to support mission trade studies during restructuring. These studies included effective area trades for different mirror configurations and mirror coatings and the formulation of science requirements and optics designs for the AXAF-S mission. After restructuring, Project Science team efforts at MSFC concentrated on detailing science requirements that flow down to technical specifications, establishing HRMA particulate and molecular contamination requirements, identifying and suppressing potential sources of background noise (such as baffling UV-to-IR stray light), and shielding against charged particle and X-radiation, among many other studies.

Calibration

MSFC Project Science had primary scientific responsibility for the Chandra ground calibration effort at the XRCF. This was a large undertaking culminating in a 6-month-long, 24/7 exercise beginning late in 1996. The calibration test sequence included calibration of the HRMA flight optics (alone and in conjunction with the LETG and HETG assemblies) and of the HRC, the ACIS-2C (a custom-built two-CCD surrogate ACIS with one BI and one FI chip), and finally ACIS itself, first with the mirrors and gratings and then alone after the HRMA was sent to final assembly and integration. Flight instrument calibration was preceded with calibration rehearsals in the summer of 1996 using the Au-coated TMA coupled with TOGA—the TMA Objective Grating Assembly, a custom grating assembly populated with flight-type LEG, MEG, and HEG grating facets—and a large suite of ground support hardware.

Project Science, with inputs from SAO and the instrument teams, wrote the calibration requirements document that specified the types of tests and the X-ray support equipment required for test implementation. Project Science also co-led the team that planned and executed the calibration and coordinated efforts to resolve issues in calibration data analysis, while the AXAF Science Center had primary responsibility for data ingestion, analysis, and documentation in collaboration with the science instrument teams and the Telescope Scientist.

Operations

Since the launch of Chandra and the start of science operations in 1999, Project Science has primarily held an advisory role supporting the MSFC Program Manager and the CXC leadership. Project Science has actively participated in resolving several anomalies in the scientific performance of the instruments, the following two being among the most notable:

(1) The rapid degradation in CCD performance during initial operations, with the Project Science team helping identify the cause as radiation damage by low-energy protons scattered off the X-ray mirrors onto the focal plane; supporting formulation of a subsequent radiation management program; and developing with the MSFC space environments group the Chandra Radiation Model, which is used to estimate proton fluence throughout Chandra's orbit.

(2) The loss in ACIS low-energy efficiency, with the Project Science team helping establish the cause as accumulation of molecular contamination onto the Optical Blocking Filters; supporting an extensive CXC-led investigation of the anomaly and how to mitigate it, including simulating a bakeout of the instrument; and supporting calibration efforts to quantify and to model the ongoing time-dependent change in efficiency.

Importantly, Project Science has worked to maintain a strong knowledge base by conducting scientific research—especially with Chandra—throughout the science operations phase. This research work helps us to gain a user’s perspective that enables Project Science to better identify issues impacting General Observers and to ensure fair and equitable use of Chandra by its increasingly diverse and evolving science community.

The first peer-reviewed scientific contribution from Project Science was, not surprisingly, an analysis of Crab Nebula spatial and spectral structure, led by the long-time Project Scientist Martin Weisskopf. Now, just within the past several months, Project Science team members at MSFC have authored or co-authored (1) an investigation of the effect of AGN heating on the disruption of cooling cores in a large sample of clusters, (2) the discovery with Chandra of Fe emission line complexes, extending to several arcsecond scales in a nearby Compton-thick AGN (where 1″~ 200 pc), and (3) research showing a newly-detected XRB can power an outflow in a nearby luminous compact galaxy, enabling Lyman-continuum emission to escape the galaxy.

Summary

Project Science has served as an interface between the science community and the Chandra project for nearly five decades. Assisting individuals and organizations involved in the project helps to serve the needs of the thousands of scientists who utilize Chandra for astrophysical research and contribute to Chandra's outstanding record of success. As we celebrate twenty-five years of Chandra operations, Project Science stands committed to helping preserve Chandra for the future benefit of the world’s astrophysics community.