Sensor Could Help Detect Gravity Waves in Orbit

This article is part of our exclusive IEEE Journal Watch series in partnership with IEEE Xplore.

The first direct detection of gravitational waves was a major scientific feat, but now researchers are looking beyond that 2015 achievement and beyond what current ground-based detectors can offer. Now, they’re turning their eyes to the skies as they design the first generation of space-based gravitational wave detectors.

In a recent advance, a team of researchers in China describe a novel capacitance sensor with six degrees of freedom, which they hope will one day become part of the TianQin Project, a project to develop a gravitational wave observatory in space. They describe the sensor in a study published 6 June in IEEE Sensors Journal.

Hong Ma is a professor at the School of Electronics and Information Engineering and School of Physics at the Huazhong University of Science and Technology, in Wuhan, China. He notes that studying gravitational waves—which are ripples in the fabric of space-time—hold “immense scientific significance” for understanding the universe, including investigating the evolution of dense binary star systems, verifying the existence of massive black holes and understanding their formation, learning about the origins of the early universe, and testing theories related to gravity.

However, ground-based gravitational detectors can only sense relatively high frequency ripples in space-time, on the order of dozens of hertz. This is in part because these detectors are sensitive to ground vibrations and confounding factors related to Earth’s own gravity.

These issues will be mitigated with space-based detectors, allowing detectors to pick up on a broader range of frequencies. “The spaceborne gravitational wave detectors will be more sensitive to gravitational waves in the mid-to-low frequency band than the ground-based ones,” Ma explains.

The space-based TianQin detector projected to launch in the mid 2030s, will involve three identical satellites that will be deployed in a nearly perfect, equilateral triangle, about 100,000 kilometers above Earth. Critically, the exact distance between each satellite must be known in order to correctly identify when a gravitational wave is detected.

Real-time measuring of the distance between any two satellites will be done with lasers reflected by a free-floating cube made of a gold and platinum alloy within the satellite’s inertial sensors called a test mass. If there are any fluctuations in the distance of the laser beam connecting the internal test masses of the inertial sensors of two, perfectly distanced satellites—bingo, a gravitational wave is detected. A highly sensitive sensor is needed to detect these minute fluctuations, which is where Ma and his colleagues’ research comes into play.

Their capacitance sensor has six independent capacitive sensing channels to detect the translational and rotational displacement variation of the sensor’s test mass in six directions. Importantly, the device’s controlling unit was designed to counteract the tiny, non-gravitational disturbances caused by other factors, such as vibrations in the spacecraft, solar wind, and solar radiation pressure.

The research team also developed novel software to process the data from the sensing circuit. As a result, Ma says, “The circuit could offer performance advantages in terms of noise and channel consistency, and does not require analog demodulation nor the analog local oscillator, significantly improving the circuit’s stability and reliability.”

In their study, Ma’s team conducted some initial tests on their novel sensor, finding it capable of detecting capacitance in the millihertz band, which Ma says may be the most important performance requirement for this type of sensor. At this frequency, scientists will be able to detect gravitational waves from the early universe and massive black holes up to tens of millions of times the mass of the Sun.

However, Ma also notes the sensor will need additional work before it is deployed, in particular to ensure the six different channel capacitive sensing circuits work harmoniously together. Ma says this will be important for the long-term functionality of the detector in space, and his team plans to work on this issue in future research.

Ma’s team still has plenty of time to complete that additional research before TianQin’s launch in the mid-2030s. Elsewhere, the European Space Agency is in the midst of developing its own novel detector, the Laser Interferometer Space Antenna (LISA), with a similar anticipated launch date.