Australian Sky & Telescope — July 2017

26 AUSTRALIAN SKY & TELESCOPE July 2017

COMET:

ESA / ROSETTA / MPS FOR OSIRIS TEAM; CREW: ESA / J. MAI

quickly. Crucially, Rosetta’s images show that the two lobes

are layered — and that the layers in one lobe do not align

with those in the other. The densities of the two lobes might

be different as well. These differences suggest that the lobes

formed independently and then became stuck together.

Comet 67P’s density is so low, about half that of water ice

or about the density of flour, that it would float in water. The

porosity is very high, about 70%, but only on relatively small

scales. The portion of the interior probed by the CONSERT

experiment, which sent radio signals back and forth between

Philae and Rosetta through the nucleus, is uniform on scales

larger than 10 metres. So there don’t seem to be any large

cavities inside the part of the nucleus probed by radio waves.

Watching the comet change

By measuring the gravitational field’s effect on Rosetta as it

orbited, we could measure the comet’s mass very precisely. It’s

9.982 trillion kilograms. And we were able to determine that

the nucleus lost about 0.2% of that (18 billion kilograms)

over just two years. This mass was lost as heat from the

months-long close exposure to the Sun penetrated the

nucleus to sublimate (go from ice to gas) buried ices. The

escaping gases also dragged out dust, creating the classic gas

and dust tails well known to comet watchers.

We also found that 67P’s rotation period decreased by

about 21 minutes during Rosetta’s extended visit. A day on

the comet lasted about 12.4 hours before perihelion (August

13, 2015) and was around 12 hours afterward. Apparently

the nucleus spins faster with every 6½-year-long loop around

the Sun. Most likely, escaping gas exerted asymmetric forces

due to the comet’s unique shape and the strikingly different

‘seasons’ that each lobe experiences.

The axis of rotation is orientated as if the comet were

spinning sideways. That is, one side of the ‘duck’ is the northern

STHE LAST WORD Paolo Fermi, ESA’s head of mission operations,

sends the final command to Rosetta on September 30, 2016, as mission

manager Patrick Martin (white shirt) and others look on.

WCOMET ENCOUNTER Rosetta’s view of Comet 67P/Churyumov-

Gerasimenko, taken on January 31, 2015, shows a misshapen

nucleus packed with features and a strong jet of gas and dust.

Discovery September 11, 1969

Discoverers Klim Churyumov, Svetlana Gerasimenko

Orbital period 6.45 years

Perihelion 186 × 10^6 km (1.24 a.u.)

Aphelion 851 × 10^6 km (5.69 a.u.)

Orbital eccentricity 0.64

Orbital inclination 7.04°

Obliquity (to orbit) 52°

Overall size 4.3 × 2.6 × 2.1 km

Large lobe 4.1 × 3.5 × 1.6 km

Small lobe 2.5 × 2.1 × 1.6 km

Volume 18.7 km^3

Mass 9.982 × 10^12 kg

Density 0.53 g/cm^3

Rotation period 12.40 hours (June 2014)

12.06 hours (September 2016)

Surface temperature –93° to 53°C

Water vapor output 300 g/s (June 2014, August 2016)

300 kg/s (August 2015)

Dust-to-gas ratio 4 ± 2 (by mass)

Comet 67P/Churyumov-Gerasimenko

at a glance

ROSETTA RESULTS