On Earth, a compass is a vital tool that has been used for over a thousand years to successfully navigate to the ends of the planet. The first mention of the magnetic attraction of a needle dates back to a book written between the 20s and 100s in China. Since then it has been used to explore our entire planet. But, What happens beyond Earth? Is the compass still useful? And, if so, where would it point?
“A compass in space will measure different things depending on where exactly you are in space,” explains Jared Espley, a planetary scientist at NASA’s Goddard Space Flight Center, in an interview. A compass would still technically work in space, but it wouldn’t necessarily point you back to Earth. Instead, it would point to the north pole of the magnetic field that is strongest, relative to where the compass is located in space.”
On our planet, compasses respond to the Earth’s magnetic field. The compass itself is a magnet, and its north pole naturally aligns with the south pole of our planet’s own magnetic field.. The magnetic field is generated by electrical currents flowing through our planet’s molten metal core, which rotate in a motor called a geodynamo. Earth is the only rocky planet in the solar system with such a strong magnetic field.
This magnetic field arises from the planet and It extends at least 370,000 kilometers, creating a region around the Earth which is known as the magnetosphere. In the hypothetical case that an astronaut wanted to use a compass to return to Earth, they would probably need to be inside this magnetosphere for the compass to register the planet’s magnetic field. However, the magnetic field is not a particularly hard limit.
Evidence from lunar rocks suggests that the Moon once had a magnetic field, but the natural satellite’s inner core has since slowed down and cooled, causing it to lose its magnetic field. And, like the Moon, other celestial bodies in our solar system now lack a strong magnetic field. For example, About 3.9 billion years ago, Mars’ geodynamo mysteriously slowed downdrastically weakening its magnetic field, which ultimately resulted in the loss of its atmosphere.
But even without the planetary magnetic fields of these celestial bodies intact, an astronaut on the Moon or Mars would still pick up some magnetic signals. Although not from Earth.
Of all the planets in the solar system, a compass is most likely to point toward Jupiter. This is because Jupiter’s magnetosphere is huge. According to NASA, Jupiter’s magnetosphere is the largest structure in the solar system, with 21 million kilometers wide. This giant magnetosphere is generated by the planet’s metallic hydrogen core and is currently being studied by the Juno spacecraft to better understand how magnetic fields are created.
But what happens if an astronaut is not inside a planet’s magnetosphere? The next option is the Sun, whose heliosphere or Solar magnetic field extends three times farther than Pluto. Beyond this influence, you will probably find that you have lost your way. Literally.