Gravitational waves may be one of the biggest scientific discoveries in recent years, but pinpointing their sources isn't easy. To help with this, NASA is turning to a surprising source – lobsters. The space agency's Goddard Space Flight Center in Greenbelt, Maryland is studying the tasty crustacean's eye as a model for new space-based instrument designed to seek out the cosmic events that produce the enigmatic phenomena.
First predicted by Einstein's general theory of relativity, gravitational waves are generated when massive objects accelerate and collide. These collisions create ripples in the very fabric of spacetime that propagate through space at the speed of light. These waves can only be detected by the most sensitive of instruments, but they are of extreme interest to scientists. Not only do they provide vital clues for theoretical physicists in understanding the fundamental nature of the universe, they are also a powerful tool for astronomers because gravity waves can pass through anything – making them a sort of universal X-ray.
The tricky thing about studying gravitational waves is that they are generated by rare, highly energetic phenomena, like colliding black holes or exploding supernovae. This means that astronomers need to find the source before they can know where to look for the waves. To help with this, NASA is doing a feasibility study of the Transient Astrophysics Observatory on the International Space Station (ISS-TAO) as well as two similar experiments for the Explorer Mission of Opportunity, so the space agency can in 2019 choose which of the three to build and send into space.
The idea is to create an instrument than can be sent to the ISS or aboard a satellite that can monitor the entire sky for signs of X-rays and gamma rays associated with the type of high-energy events that generate gravitational waves. Preferably, this should be over a broad range of the electromagnetic spectrum because each wavelength carries different information about the source's properties, including its composition, temperature, and velocity. The instrument would not only seek transient energy emissions, but precisely locate the X-ray signatures of gravitational waves sources to allow other observatories to join in the study.
"LIGO and Virgo (a recently upgraded interferometer facility in Pisa, Italy) form the advanced network of gravitational-wave observatories," says Goddard Principal Investigator Jordan Camp. "They will alert us to the most exciting candidates, like the final moments of a compact binary system. Although these facilities can detect the ripples in spacetime, they can't focus gravitational waves and instead achieve their source localization by timing of noisy signals. Thus, they can't precisely locate their sources."
The new NASA instrument, called the X-ray Wide-Field Imager (WFI), is different in that it will use optics based on the eye of the lobster that will allow it to scan large swatches of the sky and pinpoint X-ray locations. Lobsters are hunters that live in extremely dangerous environments where they face constant threat from not only larger predators, but from other lobsters who are both territorial and cannibalistic. For this reason, they have evolved an eye architecture with long narrow cells that reflect small amounts of light from any given direction that gives them a very wide field of vision focused as a single image and the ability to see in near total darkness.
NASA's approach is to create new optics for the WFI that rely on thin, curved slabs of material dotted with tiny tubes across the surface called microchannel plates that are similar to the cells in a lobster's eye. According to the agency, X-rays entering these tubes from various angles are focused by a process called grazing-incidence reflection where X-rays skip along the surface of the square tubes. This may seem complicated, but it's necessary because X-rays can't be reflected or refracted like visible light.
Camp says that if ISS-TAO is selected for an Explorer Mission of Opportunity, it could be ready to launch by 2022.
"This mission is more relevant today than ever before," says Camp. "The detection of gravitational waves in late 2015 was a watershed event. "Gravitational waves are so different, so new. We want a way to connect conventional electromagnetic astronomy with this emerging science."