The New Horizons spacecraft has officially entered Pluto's neighborhood. NASA mission specialist Alex Parker tweeted today that the probe has entered the area of space where Pluto exerts more gravitational influence than the Sun does. That means New Horizons has entered into Pluto's sphere of influence — also known as its Hill sphere.
New Horizons has entered Pluto's Hill sphere. We are now in Pluto space.— Alex Parker (@Alex_Parker) July 10, 2015
The Hill sphere of a planet — or in this case, a dwarf planet — depends on the planet's size and distance from the Sun. The greater the mass, the greater the sphere of influence. However, the farther away a planet is from the Sun, the larger the Hill sphere grows, since the Sun's pull isn't as strong. So while Jupiter may be the largest planet in our Solar System, Neptune actually holds the prize for the largest Hill sphere of all the planets.
We've officially entered Pluto's neighborhood
Earth's Hill sphere extends nearly 1 million miles outward into space, and it's within this region that objects can orbit our planet indefinitely. The Moon is well inside this area at 238,900 miles away from our planet.
As for Pluto’s Hill sphere, it's actually five times larger than Earth’s. That’s because even though the dwarf planet is one-sixth the size of Earth, it’s so far from the Sun that the sphere of influence is much bigger.
Parker noted that New Horizons will only be mildly affected by Pluto's gravity. The spacecraft is flying so fast that its trajectory will hardly be altered as it makes its way toward the dwarf planet for its July 14th flyby. As New Horizons approaches, NASA continues to release images of Pluto taken by the spacecraft, including the latest crystal-clear image above of Pluto and its moon Charon.