Chandra X-ray Observatory

The Chandra X-ray Observatory (CXO) is one of NASA’s most advanced space telescopes, specifically designed to observe the universe in X-ray wavelengths. Launched on 23 July 1999 aboard the Space Shuttle Columbia (STS-93), Chandra is named after the renowned Indian-American astrophysicist Subrahmanyan Chandrasekhar, who made pioneering contributions to the understanding of stellar evolution and black holes.
It is part of NASA’s Great Observatories Programme, along with the Hubble Space Telescope, Compton Gamma Ray Observatory, and Spitzer Space Telescope. Chandra has revolutionised high-energy astrophysics by revealing the hidden, energetic, and violent processes of the cosmos that are invisible to optical telescopes.

Background and Purpose

Prior to Chandra, astronomers relied on balloon and satellite missions with limited resolution to detect cosmic X-rays. Since Earth’s atmosphere absorbs X-rays, such radiation can only be observed from space.
Chandra was conceived to fill this gap by providing:

• High-resolution imaging of X-ray sources.
• Spectroscopic studies of high-energy phenomena.
• Deep-field observations of distant galaxies and cosmic structures.

The observatory enables scientists to study objects and regions emitting X-rays, including black holes, neutron stars, supernova remnants, galaxy clusters, and the hot interstellar and intergalactic medium.

Launch and Orbit

• Launch Date: 23 July 1999
• Launch Vehicle: Space Shuttle Columbia (STS-93 mission)
• Deployment Orbit: Highly elliptical orbit — perigee at 16,000 km and apogee at 133,000 km from Earth.
• Orbit Duration: 64 hours (2.67 days)

This elongated orbit allows Chandra to spend about 85% of its time above Earth’s radiation belts, providing long, uninterrupted observations of cosmic X-ray sources without background interference.

Structure and Design

Chandra’s design combines sophisticated optics and detectors to focus and record faint X-rays from distant celestial sources.
Key Components:

1. High-Resolution Mirror Assembly (HRMA):
   • Comprises four pairs of nested, cylindrical mirrors coated with iridium for high reflectivity.
   • Designed using grazing incidence optics since X-rays cannot be focused by normal mirrors — they would penetrate instead of reflecting.
   • Provides angular resolution better than 0.5 arcseconds, the highest of any X-ray telescope.
2. Science Instruments:
   • Advanced CCD Imaging Spectrometer (ACIS): Captures detailed images and measures the energy of incoming X-rays for spectral analysis.
   • High Resolution Camera (HRC): Delivers the finest spatial resolution, ideal for imaging faint or compact X-ray sources.
   • High Energy Transmission Grating (HETG) and Low Energy Transmission Grating (LETG): Disperse X-rays into their component energies for high-resolution spectroscopy.
3. Spacecraft Bus:
   • Provides power, communication, and thermal control.
   • Equipped with solar arrays, star trackers, and gyroscopes for stable pointing and navigation.
4. Data Transmission:
   • Scientific data are sent to Earth via NASA’s Deep Space Network (DSN) for processing and public release.

Scientific Objectives

The Chandra X-ray Observatory was designed to address key questions in high-energy astrophysics, including:

• The nature and structure of black holes and neutron stars.
• The mechanisms behind supernova explosions and stellar evolution.
• The distribution and behaviour of hot gas in galaxy clusters.
• The role of dark matter and dark energy in cosmic structure formation.
• The chemical enrichment of galaxies through stellar deaths and cosmic feedback processes.

Major Discoveries and Contributions

Over more than two decades of operation, Chandra has made groundbreaking discoveries that have reshaped our understanding of the universe:

1. Black Hole Physics:
   • Provided direct evidence of supermassive black holes in the centres of galaxies.
   • Observed high-energy jets emitted from black holes, revealing how they influence galaxy evolution.
2. Supernova Remnants:
   • Produced iconic images of the Crab Nebula and Cassiopeia A, showing the aftermath of stellar explosions.
   • Traced the distribution of heavy elements created in supernovae, confirming models of nucleosynthesis.
3. Galaxy Clusters and Dark Matter:
   • Mapped the hot X-ray-emitting gas in galaxy clusters, which constitutes most of their visible mass.
   • Provided compelling evidence for dark matter through observations of the Bullet Cluster, where visible matter and gravitational mass were spatially separated after a collision.
4. Star Formation and Stellar Death:
   • Studied high-energy processes in young star-forming regions, revealing interactions between stellar winds and surrounding gas clouds.
5. Cosmic Feedback and Evolution:
   • Showed how black holes regulate the growth of galaxies by heating surrounding gas and preventing star formation.
6. High-Energy Phenomena:
   • Detected mysterious X-ray flashes, gamma-ray burst afterglows, and the diffuse X-ray background of the universe.

Achievements in Collaboration

Chandra’s findings are often combined with data from other observatories such as:

• Hubble Space Telescope (optical)
• Spitzer Space Telescope (infrared)
• Very Large Array (radio)

This multi-wavelength approach enables astronomers to form a comprehensive understanding of astrophysical phenomena, from low to high energy.

Technical Specifications

Legacy and Ongoing Operations

Originally designed for a five-year mission, Chandra has exceeded all expectations and continues to function well beyond its design life, with operations extending into the 2020s. Regular software and hardware updates from NASA’s Marshall Space Flight Center and Smithsonian Astrophysical Observatory (SAO) ensure optimal performance.
Chandra remains one of the most productive X-ray observatories ever built, contributing thousands of peer-reviewed scientific papers and high-resolution images that have deepened our cosmic understanding.

Challenges and Maintenance

While Chandra remains operational, it faces typical challenges of aging spacecraft:

• Thermal stress and degradation of onboard systems.
• Reduced efficiency of gyroscopes and solar panels.
• High background radiation exposure due to its orbit.

NASA periodically places the observatory into safe mode for calibration and maintenance but continues to extend its mission as long as data quality remains excellent.

Significance

The Chandra X-ray Observatory has transformed astrophysics by unveiling the high-energy universe. It has revealed the invisible processes shaping galaxies, stars, and black holes, offering insight into the most energetic and exotic events in nature.
Chandra’s unmatched sensitivity and resolution have made it a cornerstone of space science, providing humanity with a window into cosmic phenomena that operate at energies far beyond the reach of visible light.