After three years of viewing enormous geysers spew from a distant moon, the Cassini spacecraft is attempting an unprecedented manoeuvre on Wednesday, flying along the edges of the mysterious plumes in hopes of learning more about what causes them.
Wednesday's flyby will take the unmanned probe to within 50 kilometres of the surface of Enceladus, one of the dozens of moons orbiting the planet Saturn, and also skirt close to the edge of the geysers erupting from fractures on its south pole.
Writing on a Cassini blog hosted on NASA's website, Cassini scientist Amanda Hendrix said at 1:40 p.m. ET that the flyby sequence had officially started. It should reach its closest approach at about 4 p.m. ET.
Since the probe will pass behind the moon, which is considered to be one of the brightest objects in our solar system, Hendrix said the scientists would be out of contact with Cassini until 10 p.m. ET.
The geysers have attracted broad interest in the scientific community since they were discovered in 2005, with a number of astronomers suggesting they pointed to both volcanic activity and the possibility of liquid water on the moon.
Approach a risky manoeuvre
Scientists always pay special attention to planets and moons that show signs of liquid water and a stable heat source, both considered important preconditions for the development of life on Earth.
But this is the first time the probe will have gotten close enough to "touch" the edges of the icy plume, which can jet forth from the moon to a height of about 200 kilometres.
Cassini scientist John Spencer, also writing on the Cassini blog, said while the close view gives NASA scientists a chance to study the plume particles from a short distance, the manoeuvre is potentially dangerous.
"Plume particles are wonderful things to study, but it's possible to have too much of a good thing. At the speed that Cassini is going, particles as small as a millimeter in size could cause serious damage to the spacecraft if we ran into one. So, the decision to enter the plume was not taken lightly," wrote Spencer.
Spencer said the close flyby could be the first of many if the spacecraft is successful on its voyage.
"If Cassini reports 'come on in, the water's fine!', as we expect, we'll dip our toes in a little further next time," he wrote.