New Horizons • Mission Plan

Designing a spacecraft and mission plan to explore the vicinity of Pluto is made difficult by many factors. Not least of these is the time it will take to get there .. without playing orbital tricks and stealing energy from other objects orbiting the Sun, getting to Pluto would take nearly twenty years.

Interplanetary flights, until the last few years, have relied on giving a spacecraft a speed boost as it leaves the the Earth. That extra velocity allows the spacecraft's path to diverge from the Earth’s, either drifting outward to Mars, Jupiter, Saturn, etc, or drifting inwards to Venus or Mercury.  It is surprising, perhaps, that getting an object to Mercury requires extraordinary measures to slow it down enough to get there. But New Horizons had the other problem, gaining energy to raise its orbit so it would reach forty times further from the Sun than the Earth, and still be traveling fast when it got there. In fact, New Horizons, like the long-range Pioneer and Voyager spacecraft before it, will never return.

Even with the most powerful rocket available, the trip to Pluto would take too for long for electronic and mechanical systems to remain reliable by the time Pluto was close enough to observe, but celestial mechanics give us a gift of time. If launched at the highest possible velocity in the last two weeks of January 2006, New Horizons could pass Jupiter just in time to use that planet's massive gravity to accelerate it to a speed that would hurl it to Pluto's orbit in about nine years. That favorable alignment happens every year if all you want to do is travel 35-45AU in a decade; if you want that trajectory to put you there when Pluto is nearby, that chance comes only a very few times every 250 years. The next "launch window" opened in early February 2007, but that delay would add three years to the flight time.

So on January 19, 2006, New Horizons was launched on an augmented Atlas-Centaur three stage rocket (NASA's most powerful at the time), and was the fastest spacecraft ever to leave the Earth. It passed the distance of the Moon in a nine hours (most flights to the Moon last more than three days), and it reached Jupiter in thirteen months.

The science instruments, cameras, fields and particle detectors, nine instruments in all, are the reason to fly, but the engineering systems to deliver them to the right place, power them, point them in the right direction, keep them warm, and transmit their data back to the home plant are pretty critical too. Spacecraft design and construction is a constant well-managed battle between science and engineering! New Horizons weighed about 480Kg at launch; about 30Kg was science instruments.

Fuel for course correction and pointing accounts for about 75Kg of that; It carries a 210cm dish antenna on its back and, since it will be too far from the Sun for solar cells to work, it also carries a 200watt radioisotope thermoelectric generator (RTG). At Pluto its fastest data transmission rate will be 700 bits/second, and it will take nine months to dump the entire "Pluto Encounter" dataset back to Earth.

Once past Jupiter, New Horizons was placed in 'hibernation mode' which it mostly remained in (woken every year for a system check) till three weeks ago when it was woken to start preparations for Pluto. Pluto "far encounter" starts on January 15, when distant imaging of Pluto against the starry background (for navigation), and other measurements of the interplanetary environment are begun.

New Horizons • Pluto

2015 will be a special year for interplanetary exploration. By the end of the 1970's, humans had sent spacecraft to every planet in the Sun's family, except one.  Missions to orbit Mercury, Venus, Mars, Jupiter and Saturn, and to land four rovers on Mars, have followed. More recently, many of the lesser objects in the solar system have been studied up close: asteroids, the moons of Jupiter and Saturn, and even comets.

In 1930, Pluto was discovered by Clyde Tombaugh, working at the Lowell Observatory in Flagstaff, Arizona. Much smaller than the Earth's Moon, Pluto was found nearly one and a half billion Kilometers beyond Neptune. Pluto has always been been an emigma, its orbit dips inside Neptune's for part of its year and is tipped out of the plane of the other planets, its polar axis is tilted by nearly 90 degrees, its biggest moon, Charon, is about the same size as it is.

In fact, Pluto isn't a planet like the others, it's a member of the Kuiper Belt of objects which has strayed down into the outer regime of the regular planets. The Kuiper Belt represents a cloud of material that never joined in the serious planet making beginnings of the solar system .. some clumping of rocky and icy material happened, but probably none of the violence of melting, bombardment, destruction and collisions that created the inner planets, or the immense gravitational influences that pulled Jupiter, Saturn, Uranus and Neptune into gaseous giants. The Kuiper Belt has been called the dust bunny collection of the solar system.

Following the discovery of more Kuiper Belt objects (KBO's) by Hubble and other powerful telescopes, Pluto joined the new designation of 'dwarf planets' in 2006. Arguments revolve about this reclassification but Pluto is now in a class containing more named objects than there are classical planets (and there are hundreds more observed candidates for the class awaiting confirmation of their orbits and naming).

But Pluto remains unexplored, and enigmatic, to this day. Telescopic instruments have improved since Tombaugh's day; but even the Hubble Space Telescope can magnify the faint point of light that is Pluto barely enough to image it as a fuzzy disk. The HST has also observed four small moons (Nix, Hydra, Kerberos and Styx) in addition to Charon. Pluto is rocky and has a thin atmosphere (mostly nitrogen) that freezes, falling like snow, for the Plutonian 'winter'.

[Pluto takes about 250 years to orbit the Sun. It swings in to about 30AU (closer than Neptune) for some of that time getting close enough to heat its atmosphere into a gas, then back out to about 50AU for a century and a half cold spell. This extreme orbital swing has more impact on Pluto's temperature that its seasons .. because its rotational axis is so tilted, Pluto's "arctic circle" is, essentially, its equator!  Pluto's south pole saw the sun for the first time in 120 years in 1987. Just to completely remove it from your list of vacation spots, the temperature of the surface is 35-55 degree above absolute zero.] -- 1AU (Astronomical Unit) is the distance from the Sun to the Earth ~ 150 million Km.

Planning for a mission to Pluto began in 1989, and the New Horizons spacecraft was launched in January 2006. It will fly though Pluto's space (about 10,000Km above its surface) on July 14, 2015. Recently, after a long search, another KBO was found by Hubble, far beyond Pluto, but reachable, with little expenditure of fuel, by New Horizons in 2019.