Last Saturday, on the 4th of July, mission engineers at the Johns Hopkins Applied Physics Laboratory in Maryland scrambled to determine why New Horizons had entered safe mode after briefly ceasing communications with Earth. The spacecraft, only 10 days away from its historic encounter with Pluto, had experienced a critical error while processing previously uplinked commands, and diagnosing the problem was badly hampered by the nine hour round trip time that signals sent between spacecraft and Earth took to traverse the three billion miles of intervening space. Failure to return New Horizons to full operation before the Pluto flyby would mean a near total waste of a ten year mission built on more than two decades of planning.

Communications were restored, and, through frayed nerves, the team was able to restore New Horizons to science activities on 7 July, a week before the encounter. A nearly undetectable timing error in a sequence of commands was the culprit, and mission controllers were confident it would not be repeated.

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The Atlas rocket carrying New Horizons is rolled out to the launchpad, 16 January 2006. Credit: NASA

33 AU and almost 3,500 days earlier, on 19 January 2006, New Horizons sat encased in a cavernous payload fairing on the top of an Atlas 551 rocket, awaiting launch. It had already been delayed by eight days, and faced a less urgent but equally important deadline: the launch window.

No spacecraft had ever been designed explicitly for a target as distant and challenging as Pluto. 33 times as far from the Sun as Earth orbits, a probe in the vicinity of Pluto wouldn't receive enough sunlight to power itself with solar panels; a Radioisotope Thermoelectric Generator, which creates electricity (somewhat appropriately) via the decay of plutonium, would be required. Such a probe would also need thoroughly redundant subsystems to guard against failure over the very long mission duration, and the ability to perform complex operations autonomously, given the long delay in communications with Earth.

Another hurdle was trajectory. Many spacecraft traveling from one body (Earth, usually) to another use what's called a Hohmann transfer orbit, or something approximating it. Simply put, a Hohmann transfer orbit is the most efficient way to get from one orbit to another orbit, requiring the minimum amount of propellant. To get from Earth to Mars using a Hohmann, for example, you design an orbit around the Sun whose perihelion (point of the orbit closest to the Sun) is your point of departure (Earth, in this case), and whose aphelion (point of the orbit furthest from the Sun) intersects with Mars' orbit, timed so that Mars will actually be at that point in its orbit when you reach it.

A basic Hohmann transfer to Mars. The spacecraft's sun-centered orbit is in green. From Earth departure, the perihelion of its transfer orbit, the spacecraft follows the solid green trajectory and intersects with Mars at its aphelion. At that point it performs a braking burn to reduce its velocity and enter orbit around Mars.

Part of what makes this so efficient is Kepler's second law: the further the spacecraft is from the object its orbiting (the Sun, in this case), the slower its velocity with respect to that object. So when the spacecraft arrives at Mars, the aphelion of its orbit around the Sun, it is at its slowest velocity, and will have a much easier time burning its engine and slowing down enough to be captured into Mars orbit. For a Hohmann orbit, it uses just enough propellant to get to its target and no more, and just enough to slow it down once it gets there.

If New Horizons were to use a Hohmann transfer to get to Pluto, 22 times further from the Sun than Mars, it would take over fifty years to reach its destination. Mission planners wanted to reach Pluto as fast as possible, and so they wanted to achieve a trajectory that packed on a lot more velocity.

The first way they went about this was to design a very light spacecraft. The less mass a payload contains, the greater the speed to which a given launch vehicle can accelerate it. Great pains were taken to balance the substantial engineering demands for a very long mission to a very great distance with the need to keep New Horizons trim. They ended up with a spacecraft that massed 478 kilograms, very light by the standards of recent interplanetary probes like Cassini (2,150 kg), MAVEN (2,454 kg), and MESSENGER (1,107 kg). It's lighter even than the previous spacecraft designed for comparably great distances and durations, the 722 kilogram Voyager probes.

To go with their very light spacecraft, planners selected a very powerful launch vehicle -- the Atlas 551 rocket, typically used for launching heavy satellites into low Earth orbit or geosynchronous Earth orbit. The rocket's first stage uses an RD-180 liquid rocket engine, burning kerosene and liquid oxygen, along with solid rocket boosters (5 in this configuration), to lift and accelerate the payload. First the solid rocket boosters separate, and then the first stage booster, and the rocket's second stage, the Centaur, takes over. Centaur is a powerful upper stage that burns the very efficient propellant combination of liquid hydrogen and oxygen, and can be stopped and restarted in space. Consequently it has found frequent use since the 1960s accelerating heavy payloads to their final orbits, often boosting massive surveillance and communications satellites to their higher-energy geosynchronous orbits.

A Centaur upper stage being lifted on the way to mating atop an Atlas rocket. The single engine bell is an Aerojet Rocketdyne RL-10 liquid hydrogen/liquid oxygen engine. The spheres attached to the bottom are tanks containing helium and hydrogen peroxide, which are pumped into the main propellant tanks, providing pressure to keep the propellants settled in microgravity. This facilitates engine restart in space. Credit: NASA

An Atlas rocket thusly configured, according to its technical specifications, is capable of lifting 18,850 kilograms of payload to Earth orbit. Employing it for a payload that massed only 2.5% of that would allow mission planners to give New Horizons the sort of launch velocity they were looking for. Not only would the Centaur be able to accelerate the light New Horizons far beyond the stage's usual geosynchronous destination, but they could also add a third stage, to provide still a further kick after the Centaur burnt out.

For this purpose they selected the Star 48 solid rocket motor. The Star 48 is essentially a sphere packed with solid rocket propellant with a small engine attached. Its usual purpose was to "kick" a small payload that was already in low Earth orbit up to a higher orbit. It was often used on Shuttle missions, in which a satellite with a Star 48 attached would be deployed from the Shuttle's payload bay. Upon reaching a safe distance from the Shuttle, the Star 48 motor would ignite and accelerate the satellite to a higher energy orbit, usually geosynchronous. In New Horizons' case, the Star 48 would simply ignite after burnout of the Centaur, adding yet more velocity to the spacecraft's trajectory.

Left: Star 48 motor (silver sphere with black engine bell) attached to the bottom of a satellite being deployed from the Space Shuttle. Right: Star 48 motor (olive green sphere) attached to New Horizons prior to the spacecraft's encapsulation in the payload fairing. Above the Star 48 is an adapter connecting it to New Horizons, and below it is an adapter that would connect it to the Centaur stage once the rocket was fully assembled. Credit: NASA

Final configuration of the Atlas V 551 rocket which launched New Horizons, showing the spacecraft and its Star 48 kick motor, the Centaur upper stage, the first stage rocket, and its strap-on solid boosters. Note the size of New Horizons (gold) compared to the entirety of the hardware required to get it to its high energy trajectory. Credit: NASA

All of these elements of the design meant that New Horizons would have the fastest initial velocity of any spacecraft ever launched. It was greater than the escape velocity of the Sun, meaning that New Horizons would eventually leave our Solar System, like Voyager I did, never to return. But it was still not quite what mission planners wanted. Consider throwing a ball straight up into the air. The instant it leaves your hand, Earth's gravity begins pulling at it, reducing the ball's velocity as it rises. In the same way, as soon as New Horizons was launched away from the Sun towards Pluto, the Sun's gravity would begin to pull at it, slowing it down as it trekked further out into space. Consequently, any mission launched directly towards Pluto in the 2006 to 2008 timeframe wouldn't arrive until 2019 at the earliest.

New Horizons had now maximized the abilities of the Atlas V launch vehicle, but there was an opportunity for a big velocity boost en route.

True color mosaic of Jupiter taken by the Cassini spacecraft en route to Saturn in 2000. Credit: NASA/JPL/Space Science Institute

Many interplanetary spacecraft, when needing to add or subtract a lot of velocity without sufficient propellant for the necessary burn, will make use of a gravitational assist. Every planet orbits the Sun with a certain amount of velocity. Having mass and traveling in a roughly circular path gives them angular momentum, which is the product of their mass times their velocity. The bigger they are and the faster they travel in their orbit, the more angular momentum they have. By flying close to a planet and allowing its gravity to alter their trajectory, spacecraft can steal some of this angular momentum for themselves. Since they're comparatively tiny in mass, it's a tiny, almost undetectable loss for the planet, but makes for a potentially enormous increase in velocity for the spacecraft. A commonly used analogy to understand the effect is throwing a tennis ball at an oncoming train: you throw the ball at the train with a velocity of 50 miles an hour, and, in bouncing off the massive, fast-moving train, some of the train's enormous momentum is transferred to the ball, markedly increasing its velocity.

Since the goal of a gravity assist is to borrow a planet's momentum, and momentum is dependent on mass, Jupiter, the most massive planet in the solar system, is the mother of all gravity assist targets. A close fly-by of Jupiter to boost New Horizons' velocity was the mission plan that ensured the earliest arrival at Pluto, in 2015. A great deal of care had to be taken to get the timing and trajectory correct. The team wanted the spacecraft to fly close enough to Jupiter to get a big boost in velocity, but far enough away that it wouldn't be exposed to the high amounts of radiation in the belts created by Jupiter's magnetosphere. They also had to ensure that the new trajectory and velocity that resulted from the fly-by would cause an encounter with Pluto.

When addressing complex trajectory problems like this, mission planners simplify the problem by defining what's called the b-plane, a 2 dimensional plane that is perpendicular to the incoming spacecraft's trajectory and lies at the point of closest approach for the spacecraft. Think of the b-plane as taking a giant piece of graph paper and placing it at the target planet, where the spacecraft will shoot through it. It allows mission planners to visualize their target as a point on that graph paper, specified by 2 coordinates (X,Y is probably what you're familiar with, but in practice T and R are used). Target ellipses can be designed and drawn on the b-plane such that, if aimed for and successfully hit, the spacecraft will be in the right position to perform a maneuver, in this case a gravity assist, and get the correct results.

Diagram of the b-plane for orbital insertion targeting of the 2001 Mars Odyssey mission. The grey circle is Mars. The ellipse labeled OD015 represents the spacecraft's trajectory before TCM-2 (Trajectory Correction Maneuver 2), an engine burn performed en route to Mars, designed to put the spacecraft on the right path for orbital insertion. The ellipse labeled OD027 is the target ellipse for the TCM-2 maneuver. It represents where they're aiming the spacecraft to be at the time of closest approach to Mars. It is over the north pole because they wanted to insert 2001 Mars Odyssey in to a polar orbit. "TCM-2 Target," right in the middle of the ellipse, was the ideal target for the maneuver. "TCM-2 Reconstruction" is the actual point the spacecraft was on course for after the maneuver was performed. As you can see, it's still well within the target ellipse, meaning the maneuver was successful. Note the danger: a small portion of the target ellipse for the maneuver passes under the "Mars Impact Radius" circle, representing the range of points at which Mars' gravity and atmosphere would've combined to cause the spacecraft to crash into the planet. Credit: Deep Space Craft by Dave Doody, adapted from NASA/JPL data

B-plane targeting of Jupiter fly-by for the New Horizons launch. As with the Mars example above, the b-plane, represented by the grid of squares here, is perpendicular to the trajectory of the incoming spacecraft (imagine the New Horizons shooting through the screen at the target point). As mentioned above, this target was selected to achieve the correct added velocity and altered trajectory to get to Pluto more quickly, and avoid the intense radiation closer in to Jupiter. Credit: Yanping Guo/Robert Farquhar/Johns Hopkins Applied Physics Laboratory

Top-down view of the New Horizons Jupiter fly-by as actually achieved. Note that it passes outside the orbit of even the most distant Gallilean satellite, Callisto. Credit: Yanping Guo/Robert Farquhar/Johns Hopkins Applied Physics Laboratory

As planned, the Jupiter gravity assist would give New Horizons an additional 3.83 kilometers per second of velocity at no cost of additional propellant. But it imposed a relatively tight window on the launch of New Horizons. The Jupiter gravity assist would only be possible while Earth, Jupiter, and Pluto were in just the right alignment to enable it, and that window would close on 3 February 2006. If New Horizons didn't launch before then, it would have to take a direct route to Pluto, and would not arrive until 2020.

Fortunately, on 19 January, all the kinks were worked out, and the Atlas V 551 rocket lifted into the sky at 2 PM:

Launch occurs at 1:00. Separation of solid boosters occurs at 2:45. Click here for a more complete but lower quality video of the launch.

4 minutes and 30 seconds after launch, the first stage burnt out and separated, and the Centaur second stage ignited to continue accelerating the payload. 10 minutes and 8 seconds into the launch, the Centaur shut down, leaving New Horizons in a temporary "parking orbit" around the Earth. After a brief coasting period, waiting until the craft was at precisely the right point for injection into a heliocentric orbit, the Centaur stage re-ignited, and accelerated the payload to Earth and then solar escape velocity, and beyond. After 10 minutes, the Centaur was finished its second burn and separated from the craft. Shortly thereafter, the Star 48 kick motor ignited and increased the probe's velocity still further. About 2 minutes later, it was out of propellant.

This is not the actual launch profile of New Horizons, but a similar, prototypical one, that shows the parking orbit coast phase and Centaur/Star 48 injection burn. Credit: Yanping Guo/Robert Farquhar/Johns Hopkins Applied Physics Laboratory

A full 47 minutes after launch, New Horizons separated from the Star 48 motor and was on its way. Its velocity after launch was 16.2 kilometers per second (some 36,238 miles per hour), the fastest spacecraft ever following launch. It passed the orbit of the moon just nine hours later, something that took the Apollo missions 3 days of transit. The launch had been amazingly accurate in hitting the Jupiter gravity assist b-plane target. Planners had budgeted about 100 meters per second of delta V to allow for corrections in the trajectory due to launch error; only 18 meters per second was actually required to refine the trajectory. Physics now had the wheel -- New Horizons was on course for a 14 July 2015 date with Pluto.

For New Horizons, as with all interplanetary missions, experts in a multitude of disciplines in physics, engineering, and astrodynamics came together to develop a mission where precision in mathematics, astronomy, and orbital mechanics was of the utmost importance. An operation with countless moving parts and variables, from the engineer who serviced the turbopump on the Atlas V RD-180 engine, to the trajectory specialist who worked out the mathematics of the Jupiter gravity assist, was flawlessly coordinated and executed. This is no less than what spaceflight demands, and why it compels humans to muster the absolute best of themselves. The reward is always commensurate with the incredible effort. On 14 July, when we're treated to a truly new discovery of an old and well-known companion in our solar system, we will all share in that reward.