The year just passed saw the end of the Cassini satellite. It had a great run and ended in spectacular fashion, with both great drama and great science. It ended by going deep into Saturn, where it made some great measurements, then melted away.
It is worth remembering that Cassini was controversial when it was launched in 1997. It used 72 pounds of plutonium-238 as a power source. Some people objected to having radioactive material on a rocket, which they claimed could be dangerous. Careful NASA studies determined that the danger was very minor. Any accident would be indistinguishable from natural radiation, for example.
The cost of not using a radioactive energy source would be to basically abandon all exploration of the outer planets. For the four planets closest to the sun, such as Venus and Mars, satellites can use solar panels for power. For the outer planets, however, the sun's light is too dim for solar panels to be very effective. Fossil fuels are not useful for many reasons. Radioactive material is light, long lasting, reliable and the only reasonable option.
Back to the science. One of the final goals of the Cassini mission was to better estimate when the rings of Saturn formed. Broadly speaking, there have been two different ideas on this issue: that the rings formed about the same time as Saturn – about 4.5 billion years ago – or that the rings formed about 200 million years ago. Those are very different numbers!
Part of the process of determining the rings' age was to figure out the rings' mass. If the rings had a relatively large mass, more than, say, the mass of Saturn's moon Mimas, then the rings probably would need to be old. If the rings had less mass, the rings probably would need to be younger.
There are several reasons for a connection between mass and age. One is that the rings, which are mostly frozen water, are slowly being contaminated by rock and carbon from the very outer solar system. This process of contamination makes the rings darker over time. Based on how dark they appear today and how much mass the rings have, we can estimate how long they have been collecting rocks and carbon. If the mass is small, then the length of time for contamination has been short. If the mass is large, then the length of time for contamination has been long.
So one key was to measure the mass of the rings. Most astronomy is done by looking at the light that reaches the Earth. Unfortunately, the rings pose a special problem. The rings are solid material. Light doesn't usually pass all the way through solid material as it can through thin gas. Without getting light from the insides of all the solid chunks in the rings, it was hard to determine the mass precisely enough to estimate the age.
A more reliable, but also more difficult, way to measure the rings' mass is to use gravity. Astronomers aimed to measure the rings' gravitational pull on something. Unfortunately, this cannot easily be done from a large orbit, such as Cassini had for the past 13 years. The problem is that the bulge of Saturn, which we only kind of understand, can create an effect similar to the rings' gravity. Up until the final Cassini measurements, measurement uncertainties of 50 percent were common.
To solve this problem, NASA had to guide Cassini in between the rings and Saturn. That is a relatively close orbit and requires a lot of fuel to undo if you want to stop the Cassini satellite from also falling inside Saturn.
So, the logic was as follows. We want to figure out when Saturn's rings formed. A key measurement to make is the rings' mass. Light doesn't work very well, so we send a satellite to Saturn. The goal was to measure the gravity of the rings, which allows us to calculate the mass. However, Saturn's bulge and rings have a similar effect unless a satellite is very close. Being very close is dangerous because it requires a lot of fuel to get the satellite back into a normal orbit.
Bottom line: We wait until the satellite is near the end of its mission, then send it into a death spiral between the rings and the planet. By very, very carefully measuring how the satellite moves between the rings and planet, we can determine the gravity, which then allows us to determine the rings' mass.
And the measurement result? The rings are about 40 percent of Mimas' mass, which probably means they were formed about 200 million years ago. Mission accomplished. Thanks, Cassini.
Christer Watson, of Fort Wayne, is a professor of physics at Manchester University. Opinions expressed are his own. He wrote this for The Journal Gazette, where his columns appear the first and third Tuesday of each month.