BIO Space Sciences
After 10 years and a journey of four billion miles, the European Space Agency’s Rosetta spacecraft arrived at its destination on Wednesday for the first extended, close examination of a comet.
A six-minute thruster firing at 5 a.m. Eastern time, the last in a series of 10 over the past few months, slowed Rosetta to the pace of a person walking, about two miles per hour relative to the speed of its target, Comet 67P/Churyumov-Gerasimenko.
“It is like driving a car or a bus on a motorway for 10 years,” said Andrea Accomazzo, the flight director, at a post-rendezvous news conference. “Now we’ve entered downtown. We’re downtown and we have to start orienting ourselves. We don’t know the town yet, so we have to discover it first.”
Over the coming months, Rosetta and its comet, called C-G for short, will plunge together toward the sun.
In November, a small 220-pound lander is to leave the spacecraft, set down on the comet and harpoon itself to the surface, the first time a spacecraft will gently land on a comet.
At this point, the comet and its shadowing spacecraft are more than 330 million miles from the sun (more than three times as far out as Earth), traveling at 35,000 miles per hour. With the final firing of the thruster, Rosetta was a mere 60 miles from the comet’s surface.
“This morning, we hit a milestone, an important milestone of this mission,” said Laurence O’Rourke, a member of its science team.
“But this mission isn’t just about arriving at a comet,” he went on. “It’s about studying the comet. It’s about placing a lander on a comet, but again, the mission does not end there. The science continues. We’re trying to follow this comet all around its orbit.”
Launched in March 2004, Rosetta — a boxy structure roughly nine by seven feet, powered by two 47-foot-long solar panels — followed a circuitous route through the solar system, using flybys of the Earth and Mars to fling itself into the same orbital path as Comet C-G.
It is still not close enough to be captured by the comet’s gravity, but instead will be flying a triangular path in front of the comet as it maps the surface. It will eventually move within 6.2 miles of the surface and enter orbit around the comet.
Comets, made of ice, dust and rock, are frozen leftovers from the formation of the solar system. Rosetta is named after the Rosetta Stone, the engraved block that was crucial in deciphering Egyptian hieroglyphics, and scientists hope the spacecraft’s observations will offer important clues to how the solar system came together 4.5 billion years ago.
Photographs have revealed a surprisingly irregular shape for the two-and-a-half-mile-wide comet, possibly an amalgamation of two icy bodies or a result of uneven weathering during previous trips to the inner solar system. From a distance, the blurry blob looked something like a rubber duck; as it came into focus, it began to bear a closer resemblance to a knob of ginger flying through space.
At the news conference, Holger Sierks, principal investigator for Rosetta’s high-resolution camera, revealed the latest images, pointing to cliffs, deep shadows and also flat areas with boulders sitting on the surface. “We’ll learn in the coming months what this is telling us,” he said.
Credit ESA/Rosetta/MPS for OSIRIS Team
The spacecraft had earlier measured the flow of water vapor streaming off the comet at a rate of about two cups a second, which would fill an Olympic-size swimming pool in about 100 days. As the comet accelerates toward the sun, its surface will warm, and the trickle will grow to a torrent a hundred or a thousand times that size, contributing to the long tail that is characteristic of comets.
An unsolved mystery of Earth is where the water in the oceans came from; some suggest it came from comets. The water in comets from the distant Oort Cloud, far beyond Pluto, does not match the water on Earth, but the water in nearer comets may.
C-G is one of the nearer comets: Its orbit extends not far beyond Jupiter. Scientists should now be able to get a better idea of its composition by measuring temperatures at its surface and a few inches below, and in the gases streaming off the comet, along with the weight of water molecules streaming off it.
Much of the work in the next three months is to find a safe place for the lander, Philae, to set down. Once released from Rosetta, the lander will be pulled down by the comet’s gravity and will strike its surface at a couple of miles per hour, like someone walking into a wall. “It’s hurting, but it doesn’t kill you,” said Stephan Ulamec, head of the consortium that built the lander.
The harpoons and a thruster will help keep it from bouncing off, though Dr. Ulamec said recent photographs suggested a dusty surface more like cigarette ash or newly fallen snow than ice. “But actually, we do not know it yet,” he said. “We will only find out when we land there.”
Designed to operate through 2015, Rosetta and Philae will make observations as the comet makes its nearest approach to the sun a little more than a year from now, at 115 million miles, still outside the orbit of Earth. The comet will remain too dim to be seen by the naked eye.