In the first of a series of installments written exclusively for Sky & Telescope, New Horizons principal investigator Alan Stern offers his behind-the-scenes perspective on the navigational effort needed to get the spacecraft to Pluto.

It sounds like science fiction, but it's not: New Horizons is on final approach to the Pluto system! After 111 months in flight, the fastest spacecraft ever launched is now less than 100 days from its destination.

Closest approach of the flyby occurs on July 14th at 11:49:58 Universal Time, at a carefully selected miss distance just 7,800 miles (12,500 km) from Pluto's surface.

Alan Stern awaits New Horizons's launch
Alan Stern, principal investigator for NASA's New Horizons mission, checks out the launch site prior to the spacecraft's departure on January 19, 2006.
Alan Stern

In this first "insider" blog about the mission for Sky & Telescope, I want to give you a look at an under-appreciated but crucial part of the approach to Pluto: navigating and homing in on the target point of the flyby. This activity involves the New Horizons team at the Johns Hopkins Applied Physics Laboratory in Maryland; our primary navigation team at KinetX Aerospace in California; and our independent navigation team at NASA's Jet Propulsion Laboratory, also in California.

The work done by these groups is absolutely critical to mission success at Pluto because, unless the proper aim point at closest approach is achieved, several mission objectives in the exploration of Pluto and its system of moons will not be accomplished. Among these are the critical solar and Earth occultation observations that will probe Pluto's atmosphere and search for an atmosphere around Charon. Their success depends critically upon hitting a narrow window of space at closest approach. If that window is missed, the spacecraft will not end up where it needs to be to cross the occultation shadows of Pluto and Charon a few hours later — and it will therefore be impossible to observe the Sun and Earth rising and setting above Pluto and Charon to observe how their atmospheres are structured and composed.

Also at risk will be a good fraction of our close-approach geological and composition mapping of Pluto and Charon. Why? If the aim point window is missed, the spacecraft's instruments might take many images and spectra of blank sky instead of Pluto or Charon.

70-m Goldstone tracking antenna
This 230-foot (70-m) radio dish is the "big gun" at NASA's Goldstone (California) Deep Space Network site.

How do we navigate and home in on the aim point? First, we must determine where the spacecraft is — and where it's going — to high accuracy. To achieve this, we combine radio tracking of the spacecraft's trajectory (using NASA's Deep Space Network of tracking antennas) with a technique called optical navigation, in which New Horizons uses an onboard imager called LORRI to record the precise positions of Pluto and its satellites against background stars.

We've been taking these images for years, and once radioed to Earth they're analyzed to determine how far off course the spacecraft is — or how far off Pluto is from its expected position. (After all, we've never been there, and its orbit is less well know than those of planets closer to the Sun.)

The pace of these positional determinations will increase in the days and weeks leading up to the encounter. From those calculations, our two navigation teams — after cross-checking one another — compute the timings and durations for a series of six trajectory-correction rocket maneuver opportunities that occur in the flight plan between May 15th and July 1st. (We might not use all six, depending on how close the spacecraft is to its planned trajectory.)

Trajectory of New Horizons past Pluto and its moons
Timing will be everything when New Horizons dashes past Pluto and its moons on July 14, 2015.

After July 1st, engine burns are no longer feasible due to other on-board activities associated with the approaching encounter. But the navigation teams will continue to use radio-tracking and optical-navigation data to compute how far off we are from the planned arrival time at closest approach. That's the most critical parameter. Knowing that, they can then upload timing offsets to the spacecraft that precisely compensate for the estimated timing error. In effect, we'll shift the flight plan's carefully orchestrated sequence of events forward or backward to keep the timeline synched to the spacecraft's intended position relative to Pluto and its moons.

The expert engineers working on this portion of the New Horizons team are some of my heroes on the project. Imagine: they are driving New Horizons to thread the imaginary eye of a needle in space above Pluto — by remote control from Earth — that's 3 billion miles and 4½ light-hours away. Without them, much of the science of the Pluto flyby would be lost. Thanks to their work, we expect to arrive at Pluto closest approach to within just 8 minutes of the expected time after a journey of 9½ years (that's 5 million minutes) and less than 500 miles of the expected position error after traveling all those 3 billion miles.

Now that's precision engineering!


Image of

April 3, 2015 at 2:15 pm

The excitement I feel about this upcoming encounter with Pluto reminds me of the Voyager flybys of Jupiter back when I was in high school. I've been waiting for a mission to this distant world ever since I read that it was a target for the original TOPS (Tour of the Outer Planets) mission 44 years ago. I can't wait to see what New Horizons finds 🙂

You must be logged in to post a comment.

Image of

April 3, 2015 at 6:03 pm

My thoughts exactly, Drew. My first TV memories are of the BBC documentary just before the Viking landings. My first astronomy book had newly released Voyager pictures from Jupiter, then I waited with bated breath for the Saturn/Titan encounter, after which I created a countdown calendar for the Uranus & Neptune encounters as soon as I got the dates. I've waited expectantly for every major flyby and orbital insertion (Cassini!) ever since.

You must be logged in to post a comment.

Image of Hadyn-Butler


April 5, 2015 at 11:51 am

Alan Stern, Dear Sir,
Undoubtedly, there will be a revival of the dwarf-planet debate when the popular press discovers that New Horizons is nearing Pluto. The reason is, of course, that the press thinks that it needs controversy to survive. There are too many varieties of media and an on-going need for flashy imagery (e.g., Neil deGrasse Tyson stuck unfortunately with a gravitationally unstable version of the “asteroid belt” in the latest version of Cosmos) and, dare I say it, hyper-active and imagined time restraints as well as restricted budgets that rarely allow carefully crafted media reports. Only print media seem to retain any notion of reasonable scientific discussion (Sky and Telescope, for instance). Television news appears to have a limited time for reflection and TV journalists give the impression that they are experts on most topics – such are the personnel, time and budget restrictions in that medium – it’s not really their fault that most topics end up incomplete. I believe that it is time for planetologists to assert their prerogative in the dwarf-planet debate. In the foreseeable future a sufficient population of extrasolar planets and moons will be discovered so that planetary classes will fall out naturally in properly constructed relational databases. At that point, the dwarf-planet debate will become moot.
Best Regards,
Hadyn Butler

You must be logged in to post a comment.

Image of

April 6, 2015 at 6:43 am

I can certainly understand the value of Pluto's (or Charon's) occultation of the sun, in terms of acquiring useful data/images.

The value of the occultation of Earth, however, is less clear to me.

Does this involve the use of radio waves?

If so, "who" is the transmitter for "radio-wave occultation/bending" experimentation, New Horizons or Earth (or both)? [As high-gain as Earth-based dishes may be, there's got to be a pretty wide "beam" by the time Pluto is reached. However, that's probably irrelevant.]


You must be logged in to post a comment.

Image of

April 8, 2015 at 8:27 am

Yes, it involves radio waves, and the transmitter is indeed Earth. Google these search terms:

tyler linscott new horizons radio science experiment

for a paper about the REX instrument.

You must be logged in to post a comment.

Image of

April 6, 2015 at 7:40 am

I'm quite curious about the reasons for the limitations on when course corrections may be performed.

6 opportunties were mentioned, as well as a July 1 "cut-off" date.

Why can't a correction be performed at any time?

Is this partially due to scheduled access to DSN resources?

I don't know how frequently navigation data, whether radio tracking, or downloaded images, is updated.

Is there a significant time required for the spacecraft to prepare itself for a "burn"? If so, why?

I imagine the needed delta-v's are very small, requiring only a few seconds of thrust.

Also, why the July 1 "cut off"?

Certainly, you want the craft focused on "science", not "engineering", at that point.

However, why isn't a course correction something that can be done in a matter of minutes*, at any point?

What is involved in performing a course correction?

I can understand it taking some time to perform the two analyses (radio, optical), and "integrate" the results.

I can also understand that there is little point in performing corrections" close together".

However, this would not limit course corrections a few "fixed" opportunites.

Nor would computational difficulties.

Can someone enlighten me, and/or refer me to material on this subject?


* By minutes, I mean, from the spacecraft's perspective. A 10-second "burn" may involve days of data acquisition and analysis.

You must be logged in to post a comment.

You must be logged in to post a comment.