by Imagine you have a laser beam trained on a dime that is 200 miles away. Now imagine doing that continuously for 24 hours, while riding a happy ride. It seems difficult? Well, that’s basically what the Hubble Space Telescope does.
After months of technical issues, NASA announced June 4 that Hubble would switch into single-gyroscope mode. This essentially means that the telescope will have to rely on one of the various gyroscopes – devices that measure the orientation of objects in space – that it normally uses to track and follow objects in space.
Named after the astronomer Edwin Hubble, the Hubble telescope launched in 1990 into low Earth orbit. Here, it is above Earth’s atmosphere, which interferes with observations from Earth-based telescopes. During its three decades in operation, it provided us with amazing pictures of distant galaxies and allowed scientists to look closer to the beginning of the universe.
Hubble takes clear, high-resolution pictures of stars billions of light-years away. To collect enough photons – light “particles” – for a high-quality picture, it essentially acts as a very slow-speed camera. It keeps its aperture – that is, the opening in the lens that lets light through – open for up to 24 hours to take a single picture.
Anyone who has taken a photo at a low shutter speed knows how difficult it is to end up with a blurry image. Hubble takes this to an end. It must remain pointed at the same distant point in space with an accuracy within a few milliarcseconds – where one milliarcsecond equals 3,600,000ths of a degree – for up to 24 hours. And it must maintain this accuracy while orbiting the Earth at 17,000 miles per hour (27,000 kilometers per hour) through extreme heat and cold.
To keep track of its target and generate clear pictures, Hubble uses the attitude control systems that aerospace engineers like me have. All spacecraft and aircraft have an attitude control system to help them go in the right direction.
What is a gyro, anyway?
An attitude control system consists of a set of sensors that measure the spacecraft’s orientation, a set of actuators – thrusters, reaction wheels or moment-controlled gyroscopes – that move the spacecraft around, and a flight computer. The flight computer takes the measurements from the sensors and generates the commands for the operators.
A gyroscope is a device that measures an object’s attitude, or orientation in space. In other words, it measures how much the object rotates from some fixed point. In order for Hubble to know where it is pointing to take a picture, it needs to know where it is in space. It requires at least three gyroscopes – one per axis.
Hubble originally had six gyroscopes: three main and three additional ones. But after more than 30 years in orbit, four of the gyros have failed from complications related to aging.
Of the two remaining gyros, NASA has reserved one as a backup, so Hubble is now operating with one gyro. But if you need at least three gyroscopes – one per axis – to know where you are, how can Hubble figure out where it is with one gyroscope?
The clever answer that NASA engineers came up with is actually very simple. You can use other sensors on the telescope, such as magnetometers and star sensors, to make up for the lack of a gyroscope.
Gyro stands
Magnetometers measure the Earth’s local magnetic field, which scientists understand quite accurately. You can use the magnetometers to get a rough idea of the direction of the known magnetic field, much like you use a compass. A three-axis magnetometer can measure the strength and direction of the Earth’s magnetic field as the satellite moves along its orbit to determine its orientation in space.
Or you can use star trackers or sun sensors, which are much more accurate than a magnetometer. These sensors use a map of the sky and align what they see with what is on the map to know where they are pointing.
By combining the star trackers, solar sensors, magnetometers and a single gyroscope, Hubble can maintain a pointing accuracy very close to that of a three-gyro configuration – although a single-gyro configuration will limit how fast Hubble can go tracking objects in space.
Hubble has one of the most accurate pointing attitude control systems ever built, and it has provided humans with amazing pictures of the early universe. But the loss of Hubble’s days is another reminder of all but two gyroscopes.
Hubble’s successor, the James Webb Space Telescope, launched on December 25, 2021. It is located 1,000,000 miles (1,609,344 km) from Earth at a point known as the second Lagrange point (L2).
At this point, the telescope, the Earth and the Sun are always aligned, and the telescope’s protective shield blocks the sun’s rays. This feature allows its infrared camera to work at cold temperatures to provide much better quality pictures.
While Hubble’s long lasting discoveries opened up the universe to astronomers, Webb will allow us to look deeper into the cosmos than ever before.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Panagiotis Tsiotras, Georgia Institute of Technology
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Panagiotis Tsiotras receives funding from NSF, NASA, ONR, AFRL, AFOSR, ARL
