You wouldn’t remember this now, but readers who picked up a copy of Smithsonian magazine exactly 11 years ago may have noticed my article “All Aboard for Comet Wirtanen,” In brief this explained how and why the spacecraft Rosetta would rendezvous with this particular chunk of space debris. I did a fair amount of mumbling and grumbling to myself as the article went through the editing process prior to publication. But the editor saved me (and perhaps the magazine) from the danger of a chorus of boos and jeers, simply by inserting five magic words at the start of the first sentence: “If all goes as planned this month, an Ariane 5 launch vehicle will take off from Kourou, French Guiana, lifting into space an unmanned remote-controlled device the size of a sport utility vehicle, known to scientists as Rosetta.”
In fact the European Space Agency’s (ESA) mission to Wirtanen never happened because a previous Ariane launch had failed and the problem could not be identified and fixed in time for Rosetta’s launch.
The good news is that Rosetta lives, and later this month (if all goes as planned) will break out of a two-year “deep space hibernation” and proceed to a May rendezvous with Comet 67P/Churyumov-Gerasimenko. It will be then at a point more than five times Earth’s distance from the Sun. On command a robot passenger will detach from Rosetta and like a great insect land on the comet’s surface. Its three legs will damp out most of the shock of landing, and a harpoon will deploy to keep the box-like lander in place. The legs can rotate, lift or tilt to return the lander to an upright position. For as much as a year, as it zooms around the Sun back toward the orbit of Jupiter, the piggyback robot and its companion “mother ship” will record the comet’s decomposition – the loss of material that is carried away in the solar wind as a luminous tail. It is equipped to drill more than 20 centimeters (8 inches) into the comet to collect samples for analysis by its on-board laboratory so that we will learn more about the makeup of the comet’s nucleus. Fantastic!
It’s a busy, exciting season for the ESA, which launched its Gaia mission on December 19 “to study a billion suns” and “create the most accurate map yet of the Milky Way.”
Churyumov-Gerasimenko, which repeats its elliptical journey around the Sun every 6.45 years, is named for the two astronomers who located it from an observatory in Kazakhstan in 1969. It originated in the Kuiper Belt, debris left over from the Sun’s early eons that rings the solar system as a kind of comet nursery. Comets are knocked from the nursery and into solar orbit when they collide with another space object or even when disturbed by gravitational waves from passing galaxies.
On arrival it’s expected that Rosetta will find a comet that is roughly 2 by 2.5 miles across – a nucleus surrounded by a gaseous coma (“cloud”) that is swept back by the solar wind into the characteristic cometary tail. The treasure is the nucleus, for logically it is made up of materials that were present when the solar system was formed – such as loosely packed rock, ice, and/or water.
Rosetta is the latest of nine robotic examinations of the comets that have been mounted so far. For example, Halley’s Comet was the subject of its first fly-bys when ESA’s Giotto and the Russian Vega probes visited in 1986. NASA’s Galileo spacecraft made the first near encounter with a main-belt asteroid, Gaspra, on its way to Jupiter in 1991. Earth-based telescopes including the Hubble Space Telescope have contributed additional valuable information. So why go to all this trouble to visit a chunk of debris that is admired most for its tail? First, there is a general curiosity, not limited to scientists, about how the Earth and the rest of the solar system formed. And there’s more. We’d like a more clear-cut explanation of the difference between asteroids and comets, for the most distant asteroids seem to be suspiciously like cometary nuclei. Is Chiron, an asteroid near Saturn, really a giant cometary nucleus? Will short-period comets — those with relatively smaller and faster orbits – turn into drab asteroids when their tails have burned out?
I have another reason for celebrating Rosetta’s wake-up call. Humankind’s ability to send relatively tiny packages of delicate high-tech gear to a very small destination in the vastness of space is a prodigious feat that required four gravity-assist maneuvers to gain additional energy, following a circling path that at one point reached about 600 million miles from Earth. And then, like a van easing into a delivery bay, Rosetta must deliver its cargo to a precisely selected spot on the comet’s surface. The technical, human skill to accomplish such missions, which started decades ago when spacecraft were first sent to the Moon, Mars, and Venus, is truly phenomenal. The need to document the amazing accomplishments of men and women to these achievements, from computer scientists to astronauts, prompted me to write my most ambitious book about space science and technology, Solar System, in 2002.
When the Rosetta lander weighs anchor on Churyumov-Gerasimenko I’ll lift a glass of vodka to the space technologist – and the two astronomers — who found a comet for Rosetta. Maybe I’ll invite the neighborhood.