Cassini done another close flyby of Enceladus

After the successful March flyby of Enceladus, Cassini did it again on August 11, flying 50 km above the surface of Enceladus at a speed of 18 km/sec.

Enceladus is of much interest to scientist because it is geologically active. Not many objects in our Solar System are active, other examples being Jupiter’s moon Io and Neptune’s moon Triton. Most of the objects in our Solar System are “dead”, such as our Moon, so an active object definitely will catch astronomers’ attention.

Although we now know that the “tiger strips”, or formally called “sulci”, on the south pole are busy spewing icy plumes, there are still a lot of unanswered questions: what is driving the geysers? Is there any near-surface water? Why or how such a small moon can presently have such active geology?

Tiger Strips on Enceladus. Click to enlarge.

Mosaic of Saturn’s moon Enceladus. The four most prominent sulci (from top to bottom: Damascus, Baghdad, Alexandria and Cairo) appear as generally horizontal fractures near lower right, and they extend into the moon’s night side. Credit: NASA/JPL/Space Science Institute.

In the previous flyby the Ion and Neutral Mass Spectrometer (INMS) onboard Cassini took centre stage and determined the composition of the icy plumes that emanating from Enceladus’ south pole. This time, however, the objective is to obtain high resolution images of the south polar vents, along with detailed maps of the composition and temperature of the geologically active region.

And this is not easy.

Just imagine, the spacecraft is moving at a whopping speed of 18 km/sec, at a distance of only 50 km above the surface. This means that Cassini will be zipping through the moon, and this will cause motion blurring in the images, making taking a sharp, unsmeared image very difficult. The challenge is, described by the Cassini Imaging Team, as equivalent to trying to capture a sharp, unsmeared picture of a roadside billboard about 1.5 km away with a 2000 mm telephoto lens held out the window of a car moving at 80 km/hr.

From Cassini’s point of view, Enceladus was streaking across the sky so quickly that the spacecraft had no hope of tracking any feature on its surface. The best option was to point the spacecraft far ahead of Enceladus, spin the spacecraft and camera as fast as possible in the direction of Enceladus’ predicted path, and let Enceladus overtake at a time when we could match its motion across the sky, snapping images along the way. This special technique came up by the imaging team is called “skeet shooting“.

And they did it!

Cassini spacecraft has pinpointed precisely where the icy jets erupt from the surface of Saturn’s geologically active moon Enceladus.

Click to enlarge.

Cassini shot past the surface of Enceladus on Aug. 11, 2008, acquiring a set of seven high-resolution images targeting known hot spot locations on the moon’s “tiger stripe” fractures, or sulci. Five of those images are presented in this mosaic.

One highly anticipated result of this flyby was to pinpoint previously identified source locations for the jets that blast icy particles, water vapor and trace organics into The yellow circles on the mosaic indicate source locations I and V identified in “Enceladus Jet Sources.” Credit: NASA/JPL/Space Science Institute.

Click to enlarge.

Another two of those seven high-resolution images are presented in this mosaic.
Here, Damascus Sulcus runs across the center, from left to right. The yellow circles on the mosaic indicate source locations II and III identified in “Enceladus Jet Sources”. Credit: NASA/JPL/Space Science Institute.

More skeet shoots of Enceladus.

There will be two more flybys of Enceladus this year, on October 10 and October 31, which may bring the spacecraft even closer to the moon.


~ by thChieh on August 16, 2008.

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