[meteorite-list] Rosetta's Target: Comet 67P/Churyumov-Gerasimenko

[Ron Baalke]

http://sci.esa.int/rosetta/14615-comet-67p/

Rosetta's target: comet 67P/Churyumov-Gerasimenko
European Space Agency
December 9, 2013

There are hundreds of comets flying around the Solar System, each of them 
a potential target for ESA's comet-chasing Rosetta mission. As the mission 
took shape, the science team was faced with the difficult task of sifting 
through these candidates until they identified a handful of suitable objects.

Of particular interest were comets that had been observed over at least 
several orbits of the Sun, and which were known to be fairly active. Ideally, 
they had to follow orbital paths near the ecliptic plane, so that a rendezvous, 
prolonged survey and landing would be easier to achieve. Furthermore, 
the comet's flight into the inner Solar System had to coincide with the 
mission timeline of Rosetta, so that they both arrived in the right place 
at the right time for the historic rendezvous.

The favoured target for Rosetta was the periodic comet 46P/Wirtanen, but, 
after the launch was delayed, another regular visitor to the inner Solar 
System, 67P/Churyumov-Gerasimenko, was selected as a suitable replacement.

Like all comets, Churyumov-Gerasimenko is named after its discoverers. 
It was first observed in 1969, when several astronomers from Kiev  visited 
the Alma-Ata Astrophysical Institute in Kazakhstan to conduct a survey 
of comets.

On 20 September, Klim Churyumov was examining a photograph of comet 32P/Comas 
Solá, taken by Svetlana Gerasimenko, when he noticed another comet-like 
object. After returning to Kiev, he studied the plate very carefully and 
eventually realised that they had indeed discovered a new comet.

Comet 67P is one of numerous short period comets which have orbital periods 
of less than 20 years and a low orbital inclination. Since their orbits 
are controlled by Jupiter's gravity, they are also called Jupiter Family 
comets.

These comets are believed to originate from the Kuiper Belt, a large reservoir 
of small icy bodies located just beyond Neptune. As a result of collisions 
or gravitational perturbations, some of these icy objects are ejected 
from the Kuiper Belt and fall towards the Sun.

When they cross the orbit of Jupiter, the comets gravitationally interact 
with the massive planet. Their orbits gradually change as a result of 
these interactions until they are eventually thrown out of the Solar System 
or collide with a planet or the Sun.

Churyumov-Gerasimenko reflects the steplike process of how encounters 
with Jupiter push a comet further into the inner Solar System. Analysis 
of its orbital evolution shows that, up to 1840, its perihelion distance 
- closest approach to the Sun - was 4.0 AU (four Sun-Earth distances or 
about 600 million km). This was too far from the Sun’s heat for the ice-rich 
nucleus to vaporise and for tails to develop. This meant that the dormant 
comet was unobservable from Earth.

That year, a fairly close encounter with Jupiter caused the orbit to move 
inwards to a perihelion distance of 3.0 AU (450 million km). Over the 
next century, the perihelion gradually decreased further to 2.77 AU. Then, 
in 1959, another Jupiter encounter reduced the comet's perihelion to just 
1.29 AU - which has changed little ever since. It currently completes 
one orbit of the Sun every 6.45 years.

The comet has now been observed from Earth on seven approaches to the 
Sun - 1969 (discovery), 1976, 1982, 1989, 1996, 2002 and 2009. Like all 
comets, it has a fairly small, solid nucleus which is thought to resemble 
a dirty snowball.

The density of the nucleus seems to be much lower than that of water, 
indicating a loosely packed or porous object. Like other comets, its nucleus 
is generally blacker than coal, indicating a surface layer or crust of 
carbon-rich organic material.

We still know very little about the surface properties of the nucleus, 
so the selection of a suitable landing site for the Philae probe will 
only be possible after the arrival of Rosetta in August 2014, followed 
by a detailed survey from close quarters.

Observations indicate that, if the activity of 67P is consistent from 
orbit to orbit, then Rosetta is likely to return images of an active nucleus 
when it rendezvous with the comet at a solar distance of about 3.5 AU. 
Approaching from the sunward side of the comet's orbit, the spacecraft 
should encounter less dust, with a low probability of being disabled by 
a large impact.

As it moves towards the Sun, the ice in the nucleus begins to sublimate 
and the comet begins to eject increasing amounts of dust. Ejection of 
micron-sized grains starts at about 4.3 AU, but millimetre-sized grains 
are more likely to appear between 3.4 and 3.2 AU. This leads to the development 
of a coma (a diffuse cloud of dust and gas surrounding the solid nucleus) 
and subsequently a tail of dust that trails away from the  Sun.

During the 2002/2003 apparition, the tail was up to 10 arc minutes long 
as seen from Earth, with a bright central condensation in a faint extended 
coma. Seven months after perihelion the tail continued to be very well 
developed, although it subsequently faded rapidly.

As is the case with most comets, activity is not evenly distributed on 
the surface of the nucleus and the coma of 67P is fed by several dust 
jets - at least three prominent active areas were identified during the 
2009 apparition. In general, a rapid increase in cometary activity could 
be a problem for Rosetta, so the mission team plans to move the spacecraft 
further from the nucleus as the level of activity increases beyond an 
acceptable level.

Even at its peak of activity about one month after perihelion, the comet 
is not very bright, with a typical visual magnitude of around 12, meaning 
that it will require a telescope to see it from Earth.

Comet 67P is classed as a dusty comet, with a dust to gas emission ratio 
of approximately 2:1. The peak dust production rate in 2002/03 was estimated 
at approximately 60 kg per second, although values as high as 220 kg per 
second were reported in 1982/83.

Sixty-one images of comet 67P/Churyumov-Gerasimenko were taken with the 
Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST) 
on 11-12 March 2003. The HST's sharp vision enabled astronomers to isolate 
the comet's nucleus from the coma. The images showed that the nucleus 
measures roughly five by three kilometres and has an approximately ellipsoidal 
(rugby ball) shape.

These results were independently confirmed by some of the largest ground-based 
telescopes, including the European Southern Observatory's  Very Large 
Telescope, by observing the comet when it was inactive at large distance 
from the Sun.

Changes in its light curve appear to be closely linked with the effective 
radius of the nucleus as it rotates, rather than with variations in its 
surface albedo (brightness). These observations indicate that it spins 
once in approximately 12 hours.

Its axial inclination, orientation and direction of spin are still uncertain, 
although recent observations suggest that the axis is tilted about 40 
degrees. This means that, as the comet approaches the Sun, its northern 
hemisphere is illuminated while part of the southern hemisphere is in 
darkness. During that period, no jets are visible.

The Sun is overhead at the comet's equator about 120 days before perihelion. 
If the comet behaves as in 2003 and 2009, the main jets should become 
visible a month before perihelion, i.e. mid-July 2015. This late onset 
of major activity will be good news for anyone concerned with the safety 
of Rosetta and its Philae lander.

Comet 67P/Churyumov-Gerasimenko

Diameter of nucleus - estimated (km)
3 x 5

Rotation period (hours)
~12.7

Orbital period (years)
6.45

Perihelion distance from Sun (million km)
186 (1.243 AU)

Aphelion distance from Sun (million km)
849.7 (5.68 AU)

Orbital eccentricity
0.640

Orbital inclination (degrees)
7.04

Year of discovery
1969

Discoverers
Klim Churyumov & Svetlana Gerasimenko