Hello, Helene!

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On June 18, 2011, the Cassini spacecraft performed a flyby of Saturn’s moon Helene. Passing at a distance of 6,968 km (4,330 miles) it was Cassini’s second-closest flyby of the icy little moon.

The image above is a color composite made from raw images taken with Cassini’s red, green and blue visible light filters. There’s a bit of a blur because the moon shifted position in the frames slightly between images, but I think it captures some of the subtle color variations of lighting and surface composition very nicely!

3D anaglyph of Helene assembled by Patrick Rutherford.

At right is a 3D anaglyph view of Helene made by Patrick Rutherford from Cassini’s original raw images … if you have a pair of red/blue glasses, check it out!

Cassini passed from Helene’s night side to its sunlit side. This flyby will enable scientists to create a map of Helene so they can better understand the moon’s history and gully-like features seen on previous flybys.

(When Cassini acquired the images, it was oriented such that Helene’s north pole was facing downwards. I rotated the image above to reflect north as up.)

Helene orbits Saturn at the considerable distance of 234,505 miles (377,400 km). Irregularly-shaped, it measures 22 x 19 x 18.6 miles (36 x 32 x 30 km).

Helene is a “Trojan” moon of the much larger Dione – so called because it orbits Saturn within the path of Dione, 60º ahead of it. (Its little sister Trojan, 3-mile-wide Polydeuces, trails Dione at the rear 60º mark.) The Homeric term comes from the behavioral resemblance to the Trojan asteroids which orbit the Sun within Jupiter’s path…again, 60º in front and behind. These orbital positions are known as Lagrangian points (L4 and L5, respectively.)

Read more on the Cassini mission site here.

An irregular crescent: Cassini's flyby of Helene on June 18, 2011.

Images: NASA / JPL / Space Science Institute.

Deep Impact

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Deep Impact is the name of a NASA space mission whose primary objective was to study Comet Tempel 1 (a.k.a. 9P/Tempel). It was launched on 12 January, 2005, and the smart impactor crashed into the comet on 4 July, 2005.

Oh, and yes, Deep Impact is also the name of a movie … but the two have no connection (the science team came up with their name independently of the Hollywood studio), other than that they both concern a comet!

Comets had been the focus of several space probes before Deep Impact, perhaps the most famous of which is the ESA’s Giotto flyby of Comet Halley. However, flybys could not, and cannot, tell us much about what’s beneath the cometary surface; in particular, what the relative amounts of ices and dust is, how porous the comet body is, and so on. The Deep Impact mission was designed to address many of these unknowns.

The space probe consisted of two parts, a 370 kg copper Smart Impactor – that smashed into the comet – and the Flyby section, which watched the impact from a safe distance. In addition, many ground-based telescopes – including those of thousands of amateurs – and some space-based ones, watched the event from an even safer distance.

The mission was a great success in that the heavy copper section did, in fact, smash into the comet, and the other section did observe the impact up-close-and-personal, but safely. A great deal was learned about this comet – its composition and mechanical strength, etc – and comets in general. However, the plume which resulted from the impact was much denser than expected, so the Flyby did not get any images of the impact crater itself.

After the encounter with Comet Tempel 1, an extended mission for the Flyby was designed and implemented, called EPOXI, after its two objectives: the Extrasolar Planet Observation and Characterization (EPOCh) and the Deep Impact Extended Investigation (DIXI) … hence Extrasolar Planet Observation and Deep Impact Extended Investigation. The former uses the larger telescope on the space probe to look for exoplanet transits; the latter is a flyby of another comet, Hartley 2, now expected on 11 October, 2010.

There are several official Deep Impact websites, including NASA’s, JPL’s (Jet Propulsion Laboratory), and the University of Maryland’s on EPOXI.

The Deep Impact mission resulted in lots of Universe Today stories, far too many to mention here. Some of the best are Deep Impact Smashes into Temple 1, What the Ground Telescopes Saw During Deep Impact, Deep Impact Turns Up Cometary Ice, and Deep Impact Begins Searching for Extrasolar Planets.

Comets, our Icy Friends from the Outer Solar System is a good Astronomy Cast episode which gives a good background on comets.

Source: NASA