Sky and Telescope - April 2015 - 18

Cosmic Relief
David Grinspoon

Farewell to a Friend
After a tremendously successful mission, Venus Express has orbited its last.
It's time to say goodbye,

and thanks, to a
friend who has served us well. For me the friendship began
in 2006 with a letter from the European Space Agency (ESA)
informing me that I had been selected as a
member of the Science Team for the Venus
Express mission. This was a dream come
true. I had been fascinated with "Earth's
twin" at least since the 5th grade when I read
Isaac Asimov's novel Lucky Starr and the
Oceans of Venus, which described epic battles
and exotic aquatic creatures.
I soon learned that this fantasy had been
obsolete since 1961, when Mariner 2 - the
ﬁrst successful mission to another planet -
had proved Venus far too hot to host oceans or surface life.
The harsh reality dashed Venusian water-world fantasies
but raised delicious new questions: What happened to take
these twin worlds down such diﬀerent paths? What could
we learn from this about the life stories of Earth-like planets? Further spacecraft results hinted that Venus was likely
once cooler and wetter. We began to see it as a place where
planetary climate had gone oﬀ the rails, into the hot zone.
The plucky European spacecraft arrived in April 2006
and orbited for eight great years, providing our ﬁrst
continuous, detailed observations of the cloud-shrouded
planet. It revealed a vibrant world of constantly shifting
cloud patterns, immense tornado-like vortices dancing
chaotically around the poles, intense bursts of lightning,
and seemingly active volcanism.
We knew we were living on borrowed time. Venus
Express had long since exceeded its originally expected
mission lifetime, and all last year it had been running low
on the fuel to power its thrusters. The last phase of the
mission was focused on dynamic and variable phenomena, both on the surface, where we used the infrared
spectrometer to scan for volcanic activity, and in the upper
atmosphere, where we monitored the changing abundance of sulfur dioxide above the clouds.
Doing science to the end, we tried some risky maneuvers we would not have dared attempt earlier in the mission. In June and July of last year we performed a series
of "aerobraking" experiments, lowering the orbit to an
altitude of 80 miles (130 kilometers), well into the thin

April 2015 sky & telescope

uppermost atmosphere. Changes in spacecraft motion
allowed us to compute the density of the air, which
we found was surprisingly variable and more strongly
aﬀected by time of day than expected. New
data like these help us build better climate
models for Venus - and for Earth.
We had hopes that the mission might last
longer, but the spacecraft's orbit had been
decaying. We planned a series of 10 daily
rocket burns for the last week of November
to raise the orbit. These could have kept
the spacecraft safe until February, when
another series of burns could have kept it
going until June. The suspense came from
the fact that we didn't really know how much fuel was
left. It was like running on fumes with your gas tank's
warning light on, except we knew when we ran out there
would be no roadside assistance.
On November 28th, during one of these rocket burns,
the spacecraft stopped communicating normally and suddenly seemed unable to maintain proper orientation with
its antenna focused on Earth. Venus Express was still
alive, still sending out telemetry, but we never established
a good communication link, and it soon became clear
that its fuel had run out, and there was nothing more to
be done. On December 16th, ESA announced that the
mission was over. Sad but proud emails darted among the
science and engineering teams, with congratulations on
an amazing mission and reminders of the years of data
analysis still to be done.
It really is like saying goodbye to a friend after so many
years of making plans, sending instructions, receiving
photos and data, losing and regaining contact, worrying
over problems, crossing our ﬁngers, and rejoicing when
everything is okay. After years of this, you get attached.
Sometime around the end of January the spacecraft
was expected to plunge into the atmosphere, falling in
pieces, corroding and melting, toward the searing surface.
Goodbye, Venus Express. Please tell Venus that we'll
be back. ✦
David Grinspoon is an astrobiologist and author at the Library
of Congress. Follow him on Twitter at @DrFunkySpoon.

Sky and Telescope - April 2015

Table of Contents for the Digital Edition of Sky and Telescope - April 2015