Hyperion, a moon of Saturn so porous and oddly shaped it almost looks like a sponge, is about to get a "last close look" from NASA's Cassini probe, the space agency says.

The spacecraft, a joint effort of NASA, the European Space Agency and the Italian Space Agency, has been programmed to make close flybys of several of the ringed planet's moons over the course of the coming years, scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., explained.

Cassini will fly past Hyperion at a distance of around 21,000 miles on May 31, taking images that should arrive on Earth within 24 to 48 hours, they announced.

The goal is to reveal parts of Hyperion's surface not previously captured in earlier flybys, something made difficult because of the moon's tumbling, chaotic action as it orbits Saturn.

Hyperion's odd, "spongy" appearance is attributed to the fact that, for such a large cosmic object, it has extremely low density, about half that of water.

That makes the moon unusually porous, with weak surface gravity, and any material blown off its surface during collisions or impacts never settles back but is lost into space, JPL says.

After the Hyperion approach, Cassini's next moon "look-in" will be June 16, when it will pass just 321 miles above icy Dione. Then in October the spacecraft will make two close passes by Enceladus, with its active jets of icy spray, coming within 30 miles of it.

Cassini's last flyby of Hyperion will not be the closest the probe has ever come to the moon; that happened on Sept. 26, 2005, when it passed at a distance of 314 miles.

Near the end of this year, Cassini will move out of Saturn's equatorial plane - and thus away from the planet's moons - to prepare for its final years of science, a JPL scientist said.

During the finale, the spacecraft will repeatedly speed through the space between Saturn's surface and its closest rings, controllers say.

Cassini launched in 1997 and entered orbit around Saturn in 2004.

On the way there it made flybys of Earth, Venus and Jupiter.