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 Sceince 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 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 save 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 pieces, corroding and melting, toward the searing surface.
Goodbye, Venus Express. Please tell Venus that we'll be back.
This Cosmic Relief column first appeared in print in the April 2015 issue of Sky & Telescope.