[LINK] On the imminent encounters of probes with dwarf planets Ceres and Pluto (and Charon, too)

[rfmcdonald]

Universe Today's Elizabeth Howell observed yesterday that the Dawn spacecraft has begun to take photographs of fast-approaching dwarf planet Ceres.

And today (Dec. 1) comes a special day for Dawn — when it turns its cameras to Ceres to capture the world, which will appear about nine pixels across. The reason? Besides scientific curiosity, it turns out to be a perfect calibration target, according to NASA.

“One final calibration of the science camera is needed before arrival at Ceres,” wrote Marc Rayman, the mission director at the Jet Propulsion Laboratory, in a recent blog post.

“To accomplish it, the camera needs to take pictures of a target that appears just a few pixels across. The endless sky that surrounds our interplanetary traveler is full of stars, but those beautiful pinpoints of light, while easily detectable, are too small for this specialized measurement. But there is an object that just happens to be the right size. On Dec. 1, Ceres will be about nine pixels in diameter, nearly perfect for this calibration.”

This isn’t the first picture of Ceres by Dawn — not by a long-shot — but it sure will loom bigger than you see in the image at left, which was taken in 2010. Dawn hadn’t even arrived at Vesta at the time, the blog post points out, and the spacecraft was about 1,300 times further from Ceres then as it is now. Translating that into visual magnitude, the new pictures of Ceres will show an appearance about as bright as Venus, from Earth’s perspective.

In October, the Dawn blog said that more pictures of Ceres are planned on Jan. 13, when Ceres will appear 25 pixels across. This won’t be quite the best view ever — that was taken by the Hubble Space Telescope, which you can see below, — but just wait a couple of weeks. The mission planners say that by Jan. 26, the images will be slightly better. On Feb. 4, they will be twice as good and by Feb. 20, seven times as good.

The New Horizons spacecraft, as Wired's Marcus Woo noted, is preparing for its first encounter with dwarf planet Pluto (and its co-orbital partner Charon).

The spacecraft’s systems are programmed to start up again on Dec. 6 at 12:00 p.m. PST/3:00 p.m. EST. An hour and a half later, it will send a signal back to Earth confirming that it’s awake. But because it’s so far away, it will take more than four hours for the message to reach mission control—around 6:30 p.m. PST/9:30 p.m. EST. Mission controllers will then take six weeks to check all of the spacecraft’s systems and prepare its approach toward Pluto, which starts in earnest on January 15, 2015.

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At the time of launch, Pluto was known to have three moons: Charon, discovered in 1978, and Nix and Hydra, spotted in 2005. Then in 2011 and 2012, scientists found two more, Kerberos and Styx, respectively, giving New Horizons even more places to explore. One of the mission’s goals is see whether Pluto has any more companions, and if it has a ring system. Astronomers using the Hubble Space Telescope haven’t seen anything yet, but that doesn’t mean there aren’t moons and rings too small and faint to detect.

More moons and a ring system would certainly be exciting. But they could also be bad news, says Simon Porter, a planetary scientist at the Southwest Research Institute in Tucson, Arizona, who’s on the New Horizons science team. If there are smaller, yet-to-be-detected moons, then they likely have been struck by all sorts of other tinier objects, like baseball-sized space rocks. Those collisions would have kicked up dust that could escape the gravity of its moon, but not the Pluto system. That means there could be a lot of dust floating around, posing a hazard to New Horizons.

From the spacecraft’s point of view, the millimeter-wide dust particles would be space bullets, zipping by at almost 30,000 miles per hour with enough force to do some major damage.

The New Horizons team is especially worried because the spacecraft itself will be chock full of exciting data. As it flies by Pluto, it will save all of its images and measurements onboard before sending them back to Earth (there will be so much data that it will take until late 2016 to finish transferring). If something happens to the spacecraft, all that information could be lost.

Fortunately, Porter and his colleagues have been scoping out the Pluto system. In addition to analyzing Hubble images, they’re running computer simulations to assess the potential dangers posed by hypothetical moons placed in various orbits. So far, they don’t see anything that could threaten New Horizons. But the worry is in the unexpected. “The concern is from dust from satellites that we don’t know about,” he said. New Horizons won’t be close enough to Pluto to really assess the threat until late April. But even if there are unknown moons, the spacecraft might still be safe because its current trajectory takes it through areas that shouldn’t be too dusty based on the physics of the system, Porter explains.