Aperture Synthesis in Space: Challenges and Prospects

Introduction

Aperture synthesis in radio astronomy involves the use of two or more widely separated receivers to achieve high-resolution imaging. While the concept has been pioneered on Earth, the feasibility and challenges in implementing this technique in space are significant but intriguing. This article explores the complexities and potential benefits of conducting aperture synthesis in space, focusing on key technical challenges and recent developments.

Understanding Aperture Synthesis

In the classic approach to aperture synthesis, pioneered by Martin Ryle at Cambridge in the 1960s and 1970s, signals from multiple receivers are combined, preserving phase information. Phase information is crucial for determining the precise location of interference fringes on the source in the sky, effectively measuring a Fourier component of the brightness distribution. Without accurate phase information, one can only infer angular scales with some structure but not their exact distribution.

Challenges on Earth vs. in Space

On Earth, the physical limitations and environmental factors complicate the process of phase preservation and interference pattern analysis. However, in space, the absence of an atmosphere significantly reduces phase variations, which can be a major source of interference when working at very short wavelengths. This provides a potential advantage for space-based aperture synthesis, especially for signals above several GHz and with radio telescopes of kilometer scale.

Techniques for Phase Closure

To address the random phases introduced by each dish in very long baseline interferometry, a technique called phase closure has been developed. With 10 dishes, there are 50 independent interferometry baselines but only 9 unknown relative phases. Using statistical methods and a priori knowledge of the likely brightness distributions, reliable phase estimates can be obtained to reconstruct probable images. This method is exemplified by the Event Horizon Telescope (EHT), although the technical complexities are formidable.

Applications in Optical Interferometry

Optical interferometry also employs similar techniques, leading to the production of image reconstructions of the surfaces of red giant stars. The VLT (Very Large Telescope) is an example of successful implementation of these concepts. However, the process requires extremely precise measurements and continuous adjustments to compensate for atmospheric turbulence.

Space-Based Interferometry: Feasibility and Challenges

Extending the concept to space involves several challenges. Multiple satellites are needed, and their relative positions must be precisely monitored. The system would need a central correlator to synchronize signals, a highly complex device ensuring each optical path has the same total optical length. The station-keeping requirements are immense. Despite these challenges, the absence of atmospheric phase variations is a significant advantage.

Technological progress, such as the abovementioned plans for gravitational wave detectors, demonstrates the feasibility of extremely precise distance monitoring among spacecraft. However, the cost of such ultra-flagship space missions means they are rare, with one expected per decade. Consequently, smaller scale experiments in space are more likely to prove the necessary technologies.

Conclusion

While the challenges of implementing aperture synthesis in space are significant, the potential benefits of high-resolution imaging in an environment devoid of atmospheric interference are promising. The techniques used in space-based astronomy, such as phase closure and the use of multiple satellites, are complex but hold the key to unlocking new frontiers in radio and optical astronomy.

Keywords

Aperture synthesis Radio astronomy Interferometry in space