Why New Horizons’ closest approach to Pluto will be just 8 minutes long

New Horizons made its successful Pluto flyby July 14. 


For 9-1/2 years, NASA’s New Horizons spacecraft has sped through space at more than 30,000 mph on a one-way trip to Pluto. But when it finally gets to its distant destination, it has no plans to slow down.

“We are just flying by, so the spacecraft does not go into orbit,” said New Horizons principal investigator Alan Stern. “We do a deep dive into the Pluto system and then we’ll go further out in the solar system to explore some more.”

The time of closest approach, when the spacecraft will be just 7,800 miles from the surface of Pluto, is approximately 4:50 a.m. PDT Tuesday.


“‘Closest’ is instantaneous,” said Jim Baer of Ball Aerospace and the lead optical engineer of the Ralph camera on board the spacecraft. “But at three minutes before or after closest approach the distance is only 2% greater.”

“If your ‘tolerance’ for minimum distance was 10%, one could say that closest approach lasts about 8 minutes,” he said.

Eight minutes still doesn’t sound like a whole lot of time after a 9-1/2-year journey across 3 billion miles of space, but Stern points out that New Horizons was able to start collecting data on Pluto well in advance of the Tuesday rendezvous.

“We’ve already been taking pictures for months, and we’ll keep taking pictures as we fly away,” he said.

There are a few reasons why New Horizons won’t linger in Pluto’s neighborhood. A straightforward flyby makes for a simpler and more cost-effective mission, Stern said.

And while it will result in less data than an orbiting mission would get, it gives scientists a chance to determine which instruments will be most effective on future missions to the Kuiper Belt object without guesswork ahead of time.


Also, if New Horizons did slow down at Pluto, it would lose the momentum it gained from blasting off from Earth in January 2006 and the gravity assist it got from Jupiter 13 months later. That would make it harder for it to explore other objects in the Kuiper Belt.

Stern added that flybys are not what they used to be.

After noting that Wednesday marks the 50th anniversary of the day the Mariner 4 mission flew by Mars in 1965, he said that New Horizons will collect 5,000 times as much data as that mission did.

“For a first fly-by reconnaissance mission, we are going to knock your socks off,” he said.

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