NEAR Magnetometer
Educational Outreach
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  • On Earth a compass helps us find our direction so we can navigate our cars, ships, and planes. How does a compass work?
  • A compass needle is a small magnet with two poles that we call north and south because that is the way they point. As you know, the Earth is also a magnet, huge and very strong. The motion of the liquid core of the Earth causes this magnetic force.
  • What do magnets have to do with spaceflight and why do we want to measure magnetic fields with NEAR?



The Earth's Magnetic Force Field Lines

To picture the Earth's magnetic force field, just remember that lines of the magnetic force field leave one end (or pole) of a magnet and enter the opposite pole. The arrows on the lines indicate the direction of the magnetic force. Where the lines are close together, the Earth's magnetic field is strong, and where they are apart, the field is weak.

So how does this make a compass work?
To understand how compasses work on the Earth, all you need to remember is that the compass tends to line up with the Earth's magnetic force field - with the compass north pole end pointing along the direction of the force field. Because the Earth's magnetic force field generally points to the geographic north pole, compass needles line up with their north magnetic poles pointing to geographic north.

How does the compass work?
The lines of the Earth's magnetic field cause the magnetic field lines of the compass needle to line up so that all the arrows are pointing in the same direction. Because the lines lined up with the magnetic field of the Earth, one can always know the direction of geographic north by using a compass. That's cool!

Magnetic Field Lines with Several Compasses

So what does this have to do with spacecraft?
Can you see that the compass is actually measuring the direction of the magnetic field? It is a simple magnetic field "meter". By collecting lots of compass readings, one can tell the strength of the Earth's internal magnet and how it is pointed. Scientists use instruments that work a lot like compasses, only better, to measure the magnetic force fields of the Earth, our Moon, planets and now with NEAR, an asteroid.


Spacecraft Orbiting Earth, Moon, Jupiter, and Eros

By measuring the magnetic fields of planets and asteroids, one can tell a lot about what these things are made of. That's because things made of iron tend to have strong magnetic fields but things made of rock, like sandstone or granite, have very weak magnetic fields. The shape of the magnetic field also tells you how things are organized inside.

To measure the magnetic fields of planets and asteroids, spacecraft are equipped with magnetic field detectors called magnetometers. Magnetometer just stands for "magnetic" and "meter". Think of it as a fancy compass. The NEAR spacecraft has a magnetometer mounted on its main antenna.



The NEAR Magnetometer

The NEAR Magnetometer does what a compass does but without moving parts, and it has special electronics and a computer hooked up to it that pick out the magnetic field signals, turn them into data, and send them to the NEAR radio dish (antenna) to be sent back to Earth.

Why does the NEAR Magnetometer have to be different than the compass?
Think about these questions:

  • Can you see the needle on the compass your friend holds up across the room?
  • Can you read your friend's compass if you are in your house and he is in his house?
  • Can you read your compass if all the lights in the house are turned off?

You know that in space objects are far from the Earth so we can't look at the compass with our eyes to see its measurements. That's why we need to fly an automatic/electronic compass that can radio its readings back to Earth.


NEAR Radios Signals to Earth

Faraday

How does the NEAR Magnetometer work?
A scientist named Faraday discovered something cool about magnets. He discovered that the electric current produces a magnetic field. When you connect a wire loop to a battery, an electric current moves through the wire. This current produces a magnetic field that encircles the wire like this.

But there is more. . .

If the wire is laid out in circular loops rather than in a line, the magnetic field inside the loops is straight - right down the middle of the loops.

Solenoidal Set of Loops
Imagine that we have a compass telling us the direction of a magnetic field, say on the Earth. Now take a coil and place it around the compass so that the center of the coil is lined up with the way the compass points. Something astonishing happens if we turn on the current in the loop. If we adjust the amount of electric current in the coil to just the right value, we can exactly cancel the magnetic field at the compass. (We only cancel it at the center of the coil.) When we cancel the field then the compass needle will not turn to point down the center of the coil. In fact, we could turn it any which way and it will just stay the way we leave it. That's because by adjusting the current we've cancelled the field in the coil where the compass is. The compass won't turn to point along the coil because there's no field!

So what? The "so what" is that the amount of electric current we need to make the compass completely free is a very accurate measure of the strength of the magnetic field. The current is a meter of the magnetic field - we've invented a magnetometer!

In fact, this is the basic principle that the NEAR magnetometer uses. Because magnetic fields can point in any of the three directions (up, forward, or sideways), we need three sets of coils each with an electric compass. The electronics runs the compasses and adjusts the electric current in each coil until the compass in each of the three coils reads zero field. The computer then reads these current values and sends the readings to the spacecraft radio to be sent to Earth.

(Here's a technical detail we just glossed over. For the three axis measurement, the current is adjusted until the compass needle points perpendicular to the coil axis. That's because each coil only cancels the field along its axis so in general there will still be a magnetic field perpendicular to the coil axis inside the coil. The principle is the same, its just a little more complicated.)

Coils, Compasses, Meters

Of course, the NEAR magnetometer does all this electronically and automatically, but that really is what it does. By examining the data from the magnetometer, scientists can tell what the magnetic field is millions of miles from Earth, without ever looking at a compass. When NEAR gets to the asteroid Eros, the magnetometer will help us learn whether it is iron or something else.

Now that you know what a magnetometer does you are ready to look at what the real NEAR magnetometer data look like as the NEAR spacecraft whizzes past the Earth on January 23, 1998.


Contact:
 Brian Anderson, Instrument Science Lead

This page was created by the Technical Publications Group, JHU/APL
Copyright © 1998 The Johns Hopkins University Applied Physics Laboratory