Pluto explorer passes milestone distance

Saturday evening at 8:42 p.m. EDT, New Horizons passed 50 astronomical units (AU) from the Sun, or about 4.6 billion miles.  Just the fifth spacecraft to reach this great distance, it joins Voyagers 1 and 2 and with Pioneers 10 and 11 in the "50 AU club".

Launched in 2006, the baby grand piano sized spacecraft spent the next 9.5 years traveling through the solar system for a flyby of Pluto which produced spectacular images of the frozen planet.

New Horizons took on a secondary mission on New Years Day January 2019 flying past 486958 Arrokoth, a 22-mile long snowman shaped Kuiper belt object.

“One thing that makes this distance tangible is how long it takes for us on Earth to confirm that the spacecraft received our instructions. This went from almost instantaneous to now being on the order of 14 hours. It makes the extreme distance real.” said Alice Bowman, the New Horizons mission operations manager at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland.

Think of the solar system like a really long neighborhood street. The Sun is the house next door, Mars is 2 houses down the street, and Pluto is 34 houses away.  New Horizons is 50 houses down the street.

Voyager 1 is in the next neighborhood, 152 houses away.

4.6 billion miles is a long way away, but New Horizons still well within the solar system.

Even at its current speed of over 31,000 mph, New Horizons won't reach the heliopause, or the region of space where the solar winds from other stars are stronger than the solar winds from our star, for another 25 years.

Launched in the late 1970s, both Voyager 1 and 2 remain active with about half their instruments still functional. Those instruments confirmed the missions reached interstellar space in 2012 and 2018 respectively.

When Pioneer 10 and 11, launched in the early 1970s, are probably next to reach that interstellar milestone, but exactly when is up for debate.  Scientists haven't heard from either spacecraft since 2003 so direct measurements aren't possible. 

NASA's Interstellar Boundary Explorer (IBEX) mission is working out not just how far out the Sun's influence extends but the resulting shape of the solar system.  Its not a perfect circle as the heliosphere name might suggest.

While scientists have traditionally thought of the heliosphere as a comet shape, with a rounded leading edge, and a long tail trailing behind, new research suggests another explanation.