But overall, Dr Smith has calculated that the Earth - including the sea and the atmosphere - is losing mass. He points to a handful of reasons.
For instance, the Earth's core is like a giant nuclear reactor that is gradually losing energy over time, and that loss in energy translates into a loss of mass.
But this is a tiny amount - he estimates no more than 16 tonnes a year.
And what about launching rockets and satellites into space, like Phobos-Grunt? Dr Smith discounts this as most of it will fall back down to Earth again.
But there is something else that is making the planet lose mass. Gases such as hydrogen are so light, they are escaping from the atmosphere.
Smith concluded that the Earth loses 50,000 metric tonnes each year.
Link -via @BrainPicker | Photo: NASA