Nix and Hydra, two of Pluto’s small moons, appear in glorious sharpness in close-up pictures released this week by a New Horizons team member.
Nix is shown in high-resolution black-and-white and lower resolution color. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
Nix and Hydra were discovered in 2005 – shortly before New Horizons launched in 2006 – by a team of astronomers using the NASA/ESA Hubble Space Telescope.
These moons are around 5,000 times fainter than Pluto, as seen from Earth, and are about two to three times farther from Pluto than its largest moon, Charon.
Nix and Hydra are much smaller than Charon and have irregular shapes.
Nix is estimated to be 26 miles long and 22 miles wide, while Hydra is 34 miles long and 25 miles wide.
“Nix is the second-largest of Pluto’s small moons and was the closest to New Horizons during the flyby, so we got better imaging of it than any of the other small moons,” said Dr Simon Porter, a member of the New Horizons team and a planetary scientist at the Southwest Research Institute in Boulder, Colorado.
“So far, we’ve been able to download close-up pictures of Nix taken at three different times by New Horizons’ LORRI high-resolution camera, but the best image is still on the spacecraft’s digital recorders waiting to come to Earth.”
Nix is viewed at three different times during the New Horizons flyby. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
“We knew that Nix’s brightness regularly changed over time, and therefore it was probably elongated. However, the first image really surprised us, because Nix appeared to be round – not at all elongated,” Dr Porter said.
“The next image showed Nix looking far more elongated, but with one great surprise in it: a big crater.”
The last Nix image the New Horizons team has so far was taken right after the spacecraft passed Pluto and started to look back on its crescent.
“Because Nix has no atmosphere, it isn’t as spectacular as the images looking back at Pluto, but measuring the brightness of that little crescent of light can help tell us about what the surface of Nix is made of, and whether its surface is smooth or covered in boulders,” Dr Porter explained.
New Horizons also imaged Hydra and has sent some of these images to Earth.
This image, taken by New Horizons’ LORRI camera on July 14, 2015, shows Hydra. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
“Unfortunately, Hydra was on the opposite side of Pluto from New Horizons at closest approach, so the images of Hydra are from farther away and therefore are at lower resolution than the Nix images we have,” Dr Porter said.
“Because Hydra’s orbit was still somewhat uncertain, the mission planners designed this observation to be a mosaic of six slightly-overlapping shots.”
“As it turns out, we hit the jackpot and Hydra fell right at the intersection of four of those six shots, meaning we got two full and two half-views of Hydra for the price of one.”
“The composite of these images shows that Hydra has a much more complicated shape than Nix and looks a bit like a much bigger version of Comet 67P/Churyumov-Gerasimenko, which is currently being orbited by the Rosetta spacecraft.”
“As some have proposed for 67P/Churyumov-Gerasimenko, it is possible that Hydra is the result of a low-speed collision of two older moons.”
