How do you measure a comet's weight?

Scientists have calculated that the comet at the centre of the Rosetta mission weighs 10 trillion kilograms.

How the figure was worked out seems almost as amazing as the fact we spent a decade chasing the comet 67P/Churyumov-Gerasimenko in the first place.

Onboard kit is sending back a flood of data about the comet to a team of scientists on the ground.

Artist’s impression of ESA Rosetta approaching comet 67P (Spacecraft: ESA/ATG medialab; Comet image: ESA/Rosetta/NAVCAM)

But the unmanned spacecraft, which is now travelling in sync with the comet, can also generate stats about how it is moving because of the comet.

As the spacecraft passes close to comet 67P/C-G, it is pulled slightly off course by the comet’s gravity, resulting in tiny changes to the spacecraft’s velocity.

These small deviations are recorded in the spacecraft’s radio signals as they are beamed back to Earth, and can subsequently be translated into measurements of the mass and density structure of the comet.

67P as photographed on August 15 from Rosetta at a distance of about 91km (ESA/Rosetta/NAVCAM)

For the Rosetta mission, this task comes under the umbrella of the Radio Science Investigation (RSI).

Using 80 hours worth of tracking data between August 6, when Rosetta completed its rendezvous, and August 9, the RSI team made a first estimate of the comet’s mass as approximately 1×10^13 kg +/-10%, or about 10 trillion kilograms.

Let’s put that in context: you could give every person on Earth (about 7 billion Earthlings) one kilogram, and you wouldn’t even be close to the total. Every single one of us would have to hold 1,428.5kg and a bit more.

The Rosetta team caution that there is still work to do with analysing errors on the value, not to mention refining the value as the spacecraft gets closer in the coming weeks.

The early measurements were made when Rosetta was 100km away from the comet and it’s now en route to a distance of 50km.

Eventually, the comet’s density will be revealed, and assumptions regarding the comet’s interior will also be able to be made.