On the Approach to Pluto

What’s New Horizons Doing This Week?

Take a close look at these animations from the New Horizons mission. They show the first good views of the two moons Nix and Hydra, taken from a distance of between 201 million to 186 million kilometers (125 to 115 million miles). The spacecraft’s camera snapped these between January 27 to February 8th. It spotted Charon (Pluto’s largest moon) in 2013, and is expected to see the rest of the system in the next few weeks.

What amazes me about these pictures is that the two moons are really quite small: Nix is somewhere between 23 and 68 kilometers across (14 to 42 miles), while Hydra is about 30 to 48 kilometers across (18 to 30 miles). That’s like spotting a city on Earth from a point roughly outside the orbit of Mars. Imagine what we’re going to see in a few weeks, as New Horizons gets ever closer to the Pluto system!

Last week the spacecraft entered Phase 2 of its approach to Pluto. Right now it’s busily studying the dwarf planet, as well as the solar wind environment in the Kuiper Belt (the region of the solar system that stretches out beyond the orbit of Neptune). The space plasma instruments onboard New Horizons—called SWAP and PEPSSI—are assessing the space weather environment there, and will be particularly focused on the effect of the solar wind as it gets nearer to Pluto. Interestingly, the spacecraft is experiencing a pretty low level of solar wind—the weakest ever measured since the beginning of the space age.

The solar wind is a stream of charged particles, mainly protons and electrons, but also contains trace amounts of ions of helium and oxygen. The instruments aboard New Horizons can detect and measure the intensity of the wind and the density of particles. Typical solar wind speeds at Pluto’s orbit distance range from 350-500 kilometers per second (about 750,000-1 million miles per hour). The usual solar wind density near Pluto’s orbit is about 6,000 particles per cubic meter, which is about a thousand times less than the density of the solar wind at Earth.

Information about the solar wind at the spacecraft’s (and Pluto’s) location in the solar system is extremely important to the mission scientists. A weaker solar wind could change the interaction between Pluto’s escaping atmosphere and the Sun’s stream of charged particles. It could also mean some interesting data results as the spacecraft crosses through the interaction boundary between Pluto and the solar wind, which could be a scientific bonanza for studies of the composition and escape rate of the dwarf planet’s atmosphere.

Over the next three months, New Horizons draws ever closer to the Pluto system. Very soon now, it should be able to start showing us more details of Pluto, Charon, and some tantalizing glimpses of the other moons in the system. Because of the long transmission time between Earth and the spacecraft (and back), images and data will not be instantaneous. But, when the information does arrive at Earth, humanity will (at long last) have a close-up look at this King of the Kuiper Belt Objects.

I’m pretty jazzed about the Pluto flyby, but I’m also quite excited about what happens after that. The spacecraft will next study at least two other Kuiper Belt objects, recently discovered by the Hubble Space Telescope. It will be the first (and perhaps only) mission ever to pass by other objects out in this distant realm of the solar system. What it finds at those objects will tell planetary scientists a LOT about the objects that exist so far away from the Sun.

Want to know more about Pluto and the New Horizons mission? Check out the mission Web site for the latest updates and events connected to the mission. And, mark your calendars: Pluto Day is not far away!