The ESA Rosetta
Mission to a Comet – A Talk at University of Alberta by Matt Taylor (Part 2)
On Tuesday Oct 4,
2016, Dr. Matt Taylor, the Project Scientist of ESA’s Rosetta Comet probe did a
talk at the University of Alberta. This,
of course, is the fascinating mission to comet 67-P/Churyumov-Gerasimenko,
which was launched in 2004 and rendezvoused with 67-P in 2014. It has just recently (late Sept 2016) been
decommissioned, via flying it into the comet. For simplicity, I will generally
refer to it as 67-P or “the comet”, much as Matt Taylor did during the lecture.
The talk was
enormously well attended and well received.
Here are some of my notes from the presentation, in point form.
I have broken the
blog into two parts, since there is a lot of information. Part 1, published last week described an overview of the mission and the trip to
the comet. Part 2 focuses on the mission
once the craft had caught up with the comet.
The Speaker
·
Dr. Matt Taylor has a PhD in space
physics from the Imperial College of London.
His undergrad was in physics, from the University of Liverpool.
·
He has been with the ESA
(European Space Agency) since 2005, working on the Cluster project and the
ESA-China Double Star mission.
·
The Cluster mission is based on
a set of 4 satellites flying in formation around the Earth, measuring the solar
wind and its effect on the Earth.
·
He became involved with the
Rosetta mission in 2013 and has been with it since then. As he noted on several occasions, he was just
one part of a large team of professionals of all sorts, that were jointly
responsible for the success of the mission.
·
As he noted, the Rosetta
mission has been quite different from the Cluster mission. For one thing, it is easier to explain to his
mother (and most of the public) – after all, flying a spacecraft around a
comet, then landing a probe on it, is more intuitive for the average person
than mapping the solar wind and its interactions with the Earth.
·
He is an excellent speaker, and
livened up the talk with some comments about:
o
The various conspiracy theories
generated by the mission.
o
Rock band Queen’s member Brian
May’s (who is also an astrophysics PhD) following of the mission.
o
The metal band Napalm Death has
also given shout-outs to the Rosetta mission.
o
He also made many amusing
allusions to SF pop culture’s intersection with Rosetta, particularly the Star
Wars saga (but others, such as Star Trek, Battlestar Gallactica and Blade
Runner got some mentions too) .
Part 2
The Journey with the Comet
From ESA website and Matt Taylor’s talk
- Rosetta’s target was first
discovered in 1969. It has visited the
inner solar system several times since then (every 6.5 years).
-
- The idea of the mission was to rendezvous
in the colder regions of the solar system so that mapping could go forward, and
the lander could look at the surface up close.
-
- As the spacecraft caught up
with the comet (Jan to May 2014), more details about its shape and surface were
revealed. It became clear that the comet
is very un-smooth, for example.
-
·
- The detailed structure of the
comet was then mapped and named (summer of 2014), with features names being
drawn primarily from Egyptian mythology).
-
- It is now mostly a
left-over nucleus, though there was still substantial off-gassing of the comet
during the mission, especially during the latter stages.
-
- After catching up with the
comet (August 2014), mission scientists and planners only had a few months to
determine where to put down the lander Philae.
-
·
- As noted above, Philae had
several docking mechanisms, to enable it to stick to the comet.
-
- Touchdown was successful
(November 2014), but the harpoons didn’t work, and thus the lander “bounced” to
a different location on the surface.
-
- The touchdown speed was about 1
km per second, a walking pace. However,
due to the low surface gravity, the “bounce” was over a kilometer.
-
- Matt Taylor thought that this
might have been a bit of luck, as it ended up in a more interesting spot than
its target locations was, so much was learned, even though the time was
shorter.
-
- The lander took measurements
for about 60 hours, then went to sleep.
Contact with the lander was lost in July 2015.
-
- The orbiter made several close
approaches to the comet after that, through 2015 and into 2016. However, it couldn’t stay very close for very
long, as the dust coming off of the comet interfered with the star-tracker
mechanism that lets the spacecraft know where it is in space.
-
- The dust is also highly
electrically charged. The spacecraft had
the same (negative) charge, as a result of its interaction with the space
environment (plasma, solar particles), so this effectively slowed the dust down
when it collided with Rosetta. Thus, the
collisions were low speed and not very dangerous (averaging about 1 to 3 km per
second).
·
- There was both compact dust and
“fluffy” dust. The fluffy dust is thought to be primordial, similar to
interstellar dust, that was accreted by the comet in deep space.
-
- In addition, boulders as large
as one meter across were observed being emitted by the comet. Matt Taylor informed us that conspiracy
theorists had fun with that, comparing it to a Battlestar Gallactica Ceylon
fighter.
-
- The lander seemed to go into
shadow, so further contact was lost. It
couldn’t recharge its batteries, as the solar cells were not getting sunlight.
-
- Signals from the lander were
regained for a short time in June, though they were lost again. We don’t know why contact wasn’t
re-established after that short contact though, as the orbiter was in a
position to receive another signal.
-
- In August 2016, the lander was
located. We got a very nice shot of the
lander then.
-
-
-
- The orbiter was “landed” (or
crashed, if you like) on the duck’s head on September 29, 2016. It went down into a pit in the region known
as “Ma’at”, which is an Egyptian reference, but also suspiciously like the name
of the project scientist. Here’s an
image from about 1.2 km.
·
- ·
The last image received was
from about 20 meters above the surface.
·
- The decision to crash the craft
into the orbit was based on several lines of reasoning:
-
- o
Power was running out, and the
situation wouldn’t improve, due to the orbit of the comet moving away from the
sun.
-
- o
The spacecraft was old now, and
had some mechanical issues. It seemed unlikely
that it would last through another hibernation period.
-
- o
It was getting difficult to
control the craft’s orientation in space, so contact with Earth might well have
been lost, even if the craft was still operative.
-
- o
Costs were no doubt a factor,
though Matt Taylor didn’t explicitly mention this, as far as I can recall.
The Science of the Comet
·
Some highlights of the analysis
of the data:
o
It became clear that the comet
has “seasons”, as different aspects of it are sunlit or in shadow, due to the
comets rotation access remaining fairly constant.
o
The north (the duck’s head)
tends to be more dusty.
o
The surface is quite fractured,
probably due to thermal stresses as the comet rotates. This causes heating and cooling cycles,
whereby the temperature can vary in a range of about 100 degrees Celsius, over
3 to 4 hours.
o
There are also smooth areas, which can include “sand dunes”, though
there is no wind to generate them, as there is on Earth. That seems to be due to sublimation of
gasses.
o
There a pits and voids,
somewhat like Karst topography on Earth, though obviously not caused by the
same processes.
o
Surface ice is visible. When it melts, it can cause the collapse of
cliffs.
o
The off-gassing includes a lot
of gases that wouldn’t be pleasant to smell.
That helps to tell what the comet was originally made of though.
o
A very significant finding was
that of molecular oxygen, along with molecular nitrogen and noble gases. Molecular oxygen was unexpected.
o
These findings lend support to
the idea that the comet was born in a very cold region of the protoplanetary
nebula. It also appears to have formed
quite slowly.
o
The deuterium level in the
hydrogen indicates that the comet’s water is not chemically the same as Earth’s
water. So, the Earth’s water doesn’t
seem to have come from comets.
o
About one million tons of water
per day are lost.
o
At perihelion, it appears that
about one percent of the comet’s mass is lost.
o
So, the comet will get smaller
and smaller over time. There will
probably be very little left of it within 5000 years or so.
o
It is thought that the comet
was not that big (maybe 10 km or so in diameter) when it formed. However it spent most of its existence in the
outer solar system, before some passing object perturbed it gravitationally,
and it got kicked into the inner solar system.
o Recent orbital modelling work by researchers at Western University (in Canada) suggests that the comet probably migrated into the inner solar system about ten thousand years ago, probably from the Kuiper Belt, beyond Neptune.
o
At least one amino acid was
discovered, along with some other organic chemistry compounds.
o
What causes the duck
shape? It is probably because of two
objects colliding at low speed and being jammed together.
o
The two halves of the comet
seem different. There may be a density
difference, but there is recirculation of dust between the north and south,
which makes it difficult to be sure about these differences.
o
The nucleus is very
porous. It may be as much as 80% void
space.
o
The pits have considerable
structure, as does the comet overall.
Some features seem to replicate at various scales (i.e. the structures
look similar at long distances as they do when you get closer).
o
The boundary where the magnetic
field of the sun is screened by the outflowing gas of the comet was
substantially larger than was expected, and existed farther from the sun than
was expected.
And here is where Rosetta was at the end of Sept 2016 (on the comet's surface). By year's end it will be closer to Jupiter, as both the comet and the planet are heading in the same general direction:
Sources
·
Notes from Matt Taylor’s talk
at the University of Alberta, Oct 4, 2016.
=================================================================
Science
Fiction to Read
Now that you
have read some real science (astronomy and astrophysics), you should read some
science fiction.
Kati of
Terra
How about
trying Kati of Terra, the 3-novel story of a feisty young Earth woman, making
her way in that big, bad, beautiful universe out there.
The Witches’ Stones
Or, you might
prefer, the trilogy of the Witches’ Stones (they’re psychic aliens, not actual
witches), which follows the interactions of a future Earth confederation, an opposing
galactic power, and the Witches of Kordea.
It features Sarah Mackenzie, another feisty young Earth woman (they’re
the most interesting type – the novelist who wrote the books is pretty feisty,
too).
