The public is enamored of images returned to Earth by space probes. Whether they show the moons of Jupiter, the rings of Saturn, or the icy surface of Pluto, these captivating images garner loads of attention.
In addition to cameras, robotic probes always carry an array of scientific instruments that get far less attention from the media, but these devices provide critical data that space scientists use to expand our knowledge of the solar system and the universe.
Among the most important instruments are magnetometers. As their name suggests, they are devices that detect and measure magnetic fields. But how do magnetometers work, and how is the information they gather useful?
Invisible forces all around us
Many planets and moons, and all stars, generate magnetic fields. Earth’s magnetic field, for example, is created by the movement of molten iron and nickel in the planet’s core and helps to protect Earth from charged particles in the solar wind. If unchecked, such particles would destroy our atmosphere. The Sun also generates a magnetic field because of the motion of charged particles in the plasma of its interior. Jupiter’s magnetic field is so strong that it’s second only to that of the Sun in intensity.
The region around a body where its magnetic field influences space is known as its magnetosphere. Jupiter’s magnetic field is so vast that its magnetosphere extends out as far as the orbit of Saturn!
Why we study magnetic fields
The presence, strength, and orientation of a body’s magnetosphere can tell scientists a great deal about a planet, moon, or star’s composition and interior (and can often be measured from a significant distance). Magnetic fields themselves cannot be imaged with normal cameras (although anyone who played with magnets as a child can understand that they have definite shapes and variable strengths). When humans began to send probes out into space, they needed to invent new tools to study unseen phenomena, including magnetic fields.
How magnetometers work
When designing a magnetometer, scientists must decide what aspect of a magnetic field they want to study. Some magnetometers measure the strength of a magnetic field, while others detect its orientation. Still others can measure the rate of change in strength and orientation over time. Robotic space probes can carry more than one type of magnetometer, with many subtypes of these devices existing as well.
Magnetometers can work in different ways. The simplest use coils of wire which, when passed through a magnetic field, generate an electrical current. The voltage of that current provides information about the surrounding field. Other types of magnetometers calculate how magnetized a material becomes or by detecting the change in an object’s electrical resistance after passing through a field. And some magnetometers use ionized (energized) gases to measure changes in magnetic field strength and direction.
Early spacecraft typified some of these differences. The magnetometers carried by the Voyager 1 and 2 spacecraft consisted of two different devices: a high-field magnetometer (to measure very strong magnetic fields) and a low-field magnetometer (to measure weak magnetic fields). These devices measure magnetic field intensity along three mutually orthogonal axes simultaneously, and were used at different phases of the mission as the spacecraft drew closer to, and then more distant from, a particular target. Launched in 1977, at the time of writing these magnetometers are still functioning and have provided extensive information about the magnetic fields of many moons and planets, as well as the space beyond them.
Many other spacecraft, including most of the Mariner probes, Pioneers 10 and 11, GOES, Europa Clipper, Cassini, and Juno — to name just a few — have carried one or more magnetometers into the depths of space.
The catch
Space probes are, by and large, made of metals with many electronic parts. As such, the craft themselves generate a magnetic field. It’s critical that any magnetometer placed on a spacecraft be unaffected by the spacecraft itself. What this means in practical terms is that, with rare exception, the magnetometer needs to be placed as distantly from the core of the spacecraft as possible.
Many space probes make use of long, rigid structures called booms to keep the magnetometer as far as possible from the main spacecraft bus (or body). These booms feature prominently on Pioneer 10 and 11, Voyager 1 and 2, Cassini, and other spacecraft. While primary magnetometers are often placed at the tip of a boom, scientists often place a second or even a third magnetometer at different points along the boom so that the magnetic fields generated by the spacecraft itself can be measured and subsequently factored out of measurements.
Magnetometer booms can be very long. The booms on the Voyagers are nearly 43 feet (13 meters) long and 9 inches (23 centimeters) wide, and the boom on Cassini stretches 36 feet (11 m) long. While spacecraft that only require short booms can use rigid rods for their magnetometer booms, longer booms tend to be elaborate, latticelike structures that can be compressed into small canisters to fit inside the launch shroud of a rocket nose cone. Once free of their launch vehicles, these booms are extended to their full length and become rigid (so cannot be retracted).
More than cameras
Cameras aboard space probes are critically important, as they can show our eyes the wonders of the cosmos. But while scientific instruments, including magnetometers, are far less glamorous and garner a lot less attention, it is these instruments enhance our knowledge and understanding of space in countless ways, and are often the unsung heroes of space missions.
