Pluto will soon be coming out of the dark when the New Horizons spacecraft flies past it on July 14, 2015. It will certainly be an exciting year of discovery because so little is known and no detailed photo of Pluto has ever been taken. Even the Hubble Space Telescope can only resolve the tiny disc enough to reveal a few tantalizing dark and bright patches. Old questions will be answered and new ones will be raised during the brief flyby. This will certainly create a yearning to go back to Pluto soon. The next obvious mission to Pluto would be an orbiter that could make continuous observations for months or years, but it is impossible to put a spacecraft in orbit around Pluto with current technology, or is it?
Interestingly, it is not as difficult to put a spacecraft in orbit around Pluto as once thought. The old mantra was that it was impossible because Pluto’s gravity is too weak to capture a spacecraft moving so swiftly. The spacecraft would have to carry huge amounts of fuel in order to slow it down enough to be captured by Pluto. The fuel would weigh down the spacecraft and make it excessively heavy. The cost would be prohibitive and no rocket on Earth would be powerful enough to launch such a heavy spacecraft. An additional problem is that in order for a spacecraft to go into orbit around a planet, it has to follow a long, looping orbit, which is much slower than a flyby mission. The length of time necessary to get to Pluto would not be feasible as the spacecraft could fail along the way and/or the scientists who designed the spacecraft would be retired or deceased before it arrived at Pluto. Technology is finally advancing, which could forever change the way the remote worlds of the Solar System are explored. Miniaturization of equipment has made spacecraft lighter, which makes it possible for the New Horizons flyby mission to get to Pluto in only about 9½ years with the added help of a gravity assist boost from Jupiter as opposed to 15-19 years if it were a large, heavy spacecraft. Without Jupiter’s assistance, a direct shot from Earth to Pluto for a flyby would take about 40 years, and it would be even longer for an orbiter.
The European Space Agency (ESA) has proposed the Pluto Orbiter Probe (POP), which would use emerging nuclear electric propulsion technology using xenon ions coupled with a gravity assist from Jupiter to place a probe weighing no more than about 1800 pounds in orbit around Pluto. The gravity assist from Jupiter would give the required boost to propel the probe to Pluto in a reasonable amount of time using a flyby-like trajectory. The luxury of nuclear electric propulsion is that it can slow down the spacecraft as it nears Pluto just enough for it to be captured into orbit without the need of a decades long, looping trajectory. A series of looping orbits around Pluto aided by properly timed thrusts of the nuclear electric propulsion system would gradually lower the probe into a circular orbit, allowing it to begin photographing Pluto in fine detail immediately. It would also analyze the atmosphere, weather, particulates, any magnetic field, the interaction of volatile surfaces ices with the atmosphere, and global surface temperatures. It would also be able to monitor and measure any geysers if New Horizons should discover them. The Pluto Orbiter Probe would also map and examine the three moons, Charon, Nix, and Hydra, in exquisite detail.
Pluto was discovered by Clyde Tombaugh on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona and is estimated to be 1426 miles in diameter. Charon was discovered by James Christy on June 23, 1978 and is about 744 miles in diameter. Pluto is inclined at an unusual angle of 17 degrees to the ecliptic plane and it crosses the orbit of Neptune for 20 years during its long, 248-year orbit. At that time it actually orbits the Sun closer than Neptune, such as was the case from 1979-99. Methane frost was discovered on Pluto’s surface in 1976 and it was found that Pluto was tipped 122 degrees on its side, which is more than Uranus. Pluto rotates once on its axis every 6.39 days and Charon orbits Pluto once during the same length of time, keeping their same sides perpetually facing each other. The Sun is nothing more than a bright star shining at magnitude -18 at Pluto’s average distance of 3.67 billion miles away, but this is still bright enough to navigate and read by and not safe to look at.
Pluto is a world of highly contrasting patches of bright methane frost and other exotic ices, including carbon monoxide, along with dark patches of water ice and rock. Charon is uniformly gray, mostly covered with water ice and rock, and lacks methane frost. During the occultation of a star on June 9, 1988, an atmosphere was discovered around Pluto composed mainly of methane and nitrogen with traces of carbon monoxide and argon. The Hubble Space Telescope confirmed the mottled surface of Pluto and the pinkish-tan hue that is due to the Sun’s ultraviolet radiation, which damages the methane ice and reddens it. The Hubble Space Telescope discovered two additional tiny moons in 2005, Nix and Hydra, which are each less than 100 miles in diameter. Pluto’s surface temperature currently ranges from -380º to -420º F, but as it slowly moves away from the Sun in its huge orbit, it will grow colder. The atmosphere should be freezing onto the surface and slowly growing thinner, but it is not. Something may be replenishing the atmosphere; one possible source could be geysers. If New Horizons discovers active geysers on Pluto, the desire for a return visit with an orbiter that can make long term, crucial observations at Pluto will be great.
A perfect opportunity to launch the Pluto Orbiter Probe comes whenever Jupiter is in the correct position to allow a slingshot towards Pluto. Such a window of opportunity lasts a few years and the next one occurs later in the decade. A launch date of December 17, 2016 using ESA’s powerful Ariane 5 rocket has been proposed. The probe would fly past Jupiter on June 29, 2018 for a gravity assist and would arrive at Pluto on June 3, 2033 for orbital capture. It would settle into a circular orbit of about 600 miles above Pluto’s surface 316 days later. Detailed global mapping would be under way by June 2034. A lighter spacecraft with less propellant required for orbital insertion makes a Pluto orbiter mission possible.
It is doubtful such a mission will fly during the next Jupiter-assist window of opportunity due to the tight European and U.S. budgets, plus Mars, Europa, Titan, and Enceladus will be priority worlds for future exploration, but it is no longer impossible to launch such a mission in the far distant future. Pluto is thought to be the remains of the building blocks of the Solar System. Getting up close and personal with Pluto will advance our understanding of how the Solar System formed and evolved. Success of the New Horizons flyby in 2015 will make us yearn to return to Pluto soon, and it could commence in 2016, but sadly a great opportunity for a Pluto orbiter in our lifetime will almost certainly slip away with Pluto into the darkness.