Update: NASA says Dawn successfully
Land ho! After nearly four years of interplanetary travel, NASA鈥檚 Dawn spacecraft is about to be captured into orbit around the giant asteroid Vesta. Meanwhile, a new paper argues that Vesta and Ceres, another huge asteroid set to be visited by Dawn, are the main stumbling blocks to predicting the long-term fate of the solar system.
At 530 kilometres across, is one of the biggest denizens of the asteroid belt, the junkyard of leftover planetary building blocks found between Mars and Jupiter.
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is expected to enter orbit around it at 0500 GMT on Saturday (2200 PDT on Friday). Its cameras and spectrometers will study Vesta鈥檚 topography and chemical composition. That could reveal clues about the era of early planet formation, since Vesta is thought to have finished growing long before Earth and the other planets.
Chaotic orbits
After a year in orbit, Dawn will head to Ceres, the solar system鈥檚 biggest asteroid. While Vesta, like Earth, is divided internally into a metallic core and an outer rocky mantle and crust, Ceres seems to contain a lot of water ice. Scientists hope data collected by Dawn will help them understand how the two large asteroids ended up so different in composition.
Although Vesta and Ceres are tiny compared to Earth, they have a surprisingly large influence on our planet鈥檚 orbit.
A study published this week says that because of chaotic interactions between Vesta and Ceres, astronomers will never be able to calculate Earth鈥檚 trajectory more than 60 million years in the future, or rewind it more than 60 million years in the past.
Vesta and Ceres have frequent close passes with each other that change their orbits. The result is that their orbits are chaotic, changing in ways that are impossible to predict more than about 400,000 years into the future.
Event horizon
Gravitational tugs from Vesta and Ceres in turn affect the orbits of Earth and the other planets. The effect of these small tugs build up over time, making it impossible to calculate the positions of the planets more than 60 million years forwards or backwards in time.
Astronomers will more precisely measure the positions of Vesta and Ceres with the Dawn spacecraft, but this will hardly matter for long-term predictions.
The chaotic interactions between Vesta and Ceres will quickly amplify even the tiniest of measurement errors, foiling any attempt to predict planet orbits beyond the 60-million year horizon, says the study鈥檚 lead author of the Observatoire de Paris, France.
鈥淸This] appears to be an absolute limit that will not be improved in the future,鈥� he says.
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