# How do we really know that Saturns mass is 5.6846 × 1026 kg?

I’ve always been interested in planets and space exploration and spend a lot of time researching facts about planets in and outside our solar system. It seems that there is very specific information on very distant planets, such as what percentage of the said planets atmosphere is helium, etc. How could earthlings possibly have such detailed information for a planet that is so far away? Do they have special equipment that can measure these things? Obviously, they do, but how exactly?

Saturn’s mass affects the orbits of its moons, so by measuring their oribital period and radius we can work out the mass of Saturn. From its mass and size we can work out its density, and from spectroscopy we can work out its most prominent elements. There is a simple discussion here

Cool. Thanks. I did a quick search earlier today, but didn’t find anything because I was apparently not using the correct terminology. Thanks for the URL, very helpful…

Right, the major point is that the mass of Saturn could be worked out from Newton’s laws of physics, which means that it has been known for a long, long time, and required nothing more sophisticated than a low-powered telescope.

As others have said, watching Saturns moons orbit pretty much gives the precise number.

Lets say a moon orbits in 10 days. Thats about 40 orbits a year. 400 in ten years. 4000 in a hundred years.

Now, lets say you can measure the moons position accurately to 1/10 its orbit (you could do WAY better than that, but its a good “eyeball” level measurement).

Now, that means you can measure the mass of Saturn to 1 part in 40,000. That accuracy gets you plenty of decimal places just due to a good deal of time passing. No need for super accurate measurents either.

Okay, so we know what Saturn’s weight is.

Other than being a nice piece of trivia, what would be the practical applications for this information?

…and once the constant G was measured.

Well, when you fly probes byit and put probes into orbit it around it, knowing its mass does help a little

It’s nice to know that it would float if it ever came crashing down into our oceans.

Edit: More seriously, it’s important to know for plotting the course of space probes and whatnot.

Nitpick:

That’s “5.6846 × 10[sup]26[/sup] kg”, or, for situations where you can’t put in a superscript, something along the lines of “5.6846 × 10^26 kg” or “5.6846 × 10e26 kg”. Please don’t mush together the 10 and the exponent 26.

I see this happen a lot when superscript formatting gets lost during a cut and paste, and “ten to the power twenty six” turns into “one thousand and twenty six”.

Just a reminder from your friendly local technical writer with the semi-astronomical name…

Nitpick, but we know what Saturn’s mass is.

Nitpick: 10[sup]26[/sup] = 1e26. And the way to write that amount in that notation would be 5.6846e26. The e in that context means “times ten to the power of.”

Anyways, the ^ notation is probably more well known, so stick with that, OP.

So until recently, masses of moonless planets like Venus were known with much less precision? Interesting (if useless!) factoid!