Space Sunday: Pluto – the history of a brief encounter

Pluto (right) and Charon, as captured by the LORRI instrument aboard NASA's New Horizon's probe on July 8th, 2015. The colour of Pluto has been obtained by combining the image with data gathered by another instrument on the spacecraft, called Ralph
Pluto (right) and Charon, as captured by the LORRI instrument aboard NASA’s New Horizon’s probe on July 8th, 2015. The colour of Pluto has been obtained by combining the image with data gathered by another instrument on the spacecraft, called Ralph

Tuesday, July 14th promises to be a major day in the annals of space exploration, as the deep space probe New Horizons hurls through the Pluto-Charon system, making its closest approach to both, allowing us to gain our best views yet of this binary pairing of dwarf worlds and their little nest of moonlets.

The mission is already fast approaching the 10th anniversary of its launch (January 19th, 2006),  with the overall mission (from inception to the present day) already  almost 15 years old – although the planning for a Pluto mission goes back a lot further than that. Getting to the Pluto-Charon system has been a remarkable feat.

Originally, Voyager 1 had been provisionally scheduled to make a Pluto flyby as a part of its half of the “grand tour” of the solar system, using its encounter with Saturn to swing the probe on to a rendezvous with Pluto in 1986. In the end, Saturn’s Mighty moon Titan was considered a more valuable target for study, and the laws of celestial mechanics meant that a study of Titan and a swing-by of Saturn suitable to send the mission on to Pluto were mutually exclusive.

In the 1990s various missions to Pluto were proposed, ranging in size from the huge Mariner II mission, utilising an update on NASA’s veritable Mariner class probes, weighing two tonnes, down to the tiny Pluto 350, a comparatively tiny vehicle massing just 350 kilogrammes (772 pounds). These evolved, through short-lived programmes such as the Pluto Fast Flyby mission and the Pluto-Kuiper Express mission to eventually become New Horizons in 2001, a mission conceived and operated by the Applied Physics Laboratory, which often operates in partnership with NASA’s Jet Propulsion Laboratory.

At launch, New Horizons became the fastest spacecraft ever launched, using an Atlas V booster with no fewer than five strap-on solid rocket boosters. In addition, a high-powered third stage was used to boost it directly onto a solar escape trajectory – something which required the vehicle to achieve a velocity of over 16 kilometres per second (56,000 km/h or 37,000 mph) following launch. To put that in perspective, such was New Horizons’ velocity that it had passed beyond the orbit of the Moon (an average of 384,400 km / 238,900 miles from Earth) less than nine hours after launch.

The nuclear-powered (RTG) New Horizons - one of the fastest man-made craft ever made to date, now closing on the Pluto-Charon system
The nuclear-powered New Horizons – one of the fastest man-made craft ever made to date, now closing on the Pluto-Charon system The RTG system which provides electrical power through the radioactive decay of plutonium, can be see in the upper right of the vehicle in the main image, alongside the inset image of New Horizons under construction

Just under 3 months after launch, and travelling at over 21 kilometres a second, (76,000 km/h; 47,000 mph), New Horizons passed beyond the orbit of Mars, heading onwards for Jupiter, and a manoeuvre referred to a gravity assist.

Reaching the Jovian system in September, 2006, New Horizons was able to stretch its scientific legs, when it started observing Jupiter and its moons from a distance of 291 million kilometres (181 million miles). Over the next 6 months, the craft continued to close on Jupiter, gathering a huge amount of data along the way to add to our understanding of the biggest planet in the solar system, its complex weather systems and atmospheric composition, and its ever-growing system of smaller moons, many of which perform a vital role is “shepherding” Jupiter’s thin ring system.


This was the first real opportunity to observe Jupiter and its moons since the end of the Galileo mission in 2003, and New Horizons did so spectacularly well, passing within 2.3 million kilometres of the planet and using its gravity to further increase its speed by 14,000 km/h (9,000 mph), shortening the journey time to Pluto by some 3 years.

Following the Jupiter mission, the vehicle went into a hibernation mode, allowing it to reduce the power drain on its nuclear “battery”, the radioisotope thermoelectric generator (RTG) which provides the vehicle with all its electrical power (and which itself was the back-up unit for the Cassini mission which is still in operation around Saturn, 18 years after its launch).

During the vehicle’s hibernation, things were changing with regards to Pluto. Until the 1990s, it had always been classified as a planet – albeit one with an unusual orbit, which is both sharply inclined to the plane of the ecliptic in which the other planets of the solar system orbit, and highly elliptical, bringing it closer to the Sun than Neptune during certain periods.

Eris and Dysnomia (bright spot, lower left) imaged by the Hubble Space Telescope in 2007.
Eris and Dysnomia (bright spot, lower left) imaged by the Hubble Space Telescope in 2007.

Both of these factors, coupled with Pluto’s relatively small size, suggested that it was more of a “captured” object from the Scattered Disc, a region of the Solar System between Neptune and the Kuiper Belt  that is sparsely populated by icy minor planets (Pluto’s orbit around the Sun actually sits within the Scattered Disc).

In 2005, while New Horizons was sleeping,  astronomers at Mount Palomar Observatory imaged Eris, a Scattered Disc object, complete with a moon of its own (Dysnomia), which is some 27% more massive than Pluto. This discovery, coupled with the fact that the Scattered Disc may be the home of other objects of similar size, caused the International Astronomical Union to officially define the term “dwarf planet” in 2006, and downgrade Pluto’s status to match – although not without a certain amount of controversy and protest.

New Horizons crossed the orbit of Saturn on June 8th, 2008, and Uranus on March 18th, 2011. Observations of Neptune’s trailing L5 Lagrange point, which may host hundreds of small bodies, had been considered during the mission; however, when the time came, the mission team opted not to wake-up the vehicle with orders to make observations, as preparations for the Pluto encounter – and particularly the threat of collisions with unexpected bodies orbiting Pluto-Charon were considered to be of a higher priority,  particularly given that both Styx Kerberos, two of the system’s tiny “moons” were not discovered until after New Horizons had launch, leading to concerns there might be more objects in the system which might pose a threat to the craft if not identified in advance.

Observations of Pluto and Charon actually commenced in July 2013, when the LORRI imaging system on New Horizons was, for the first time, able to resolve these tiny worlds as separate objects. A year later, and a year to the date prior to closest approach, a course correction was made, and LORRI commenced a series of ultra-long range observations of Pluto from about 429,000,000 km (267,000,000 miles) to 422,000,000 km (262,000,000 miles).

In January 2015, the vehicle entered into its “distant encounter” phase of observations, although it wasn’t until May 2015 that the craft was able to start returning images of a higher resolution that could be obtained via the Hubble Space Telescope.

A composite image of 48 10-second exposures taken with New Horizons’ Long Range Reconnaissance Imager (LORRI) camera on June 26th, 2015, showing the Pluto system, with Pluto and Charon towards the centre and the four identified additional moons of Styx, Nix, Hydra and Kerberos. All are seen as bright / dark pairs of dots due to the nature of the exposures, taken 105 minutes apart. The other dots seen in the picture are imaging artefacts and not objects within the Pluto system

On July 4th, the mission had something of an upset, when New Horizons unexpectedly shifted into a “safe” mode. Such activities are not uncommon on missions, and are expressly designed to prevent a vehicle doing itself serious damage should something unexpected happen. In this case, and despite a very dramatic recounting of the situation by The Washington Post, the safe mode switch-over was caused by a scheduling flaw in instructions sent to New Horizons which resulted in the primary computer trying to write the new information to its flash memory while at the same time attempting to compress a rich set of existing data from that memory for transmission to Earth – something the system isn’t designed to do.

Once diagnosed, the problem was rectified on July 7th, and the vehicle resumed normal operations. Estimates suggest that around 6% of the data gathered between the 4th and 6th July had been lost, which represents less than 1% of the overall science data returned as New Horizons approaches Pluto.

On July 8th / 9th, New Horizons reached a point where surface features previously hinted at by patches of light and shadow were starting to come into focus. In particular, attention has been drawn to a band of four evenly spaced rings along the equator, each one around 480 km (300 miles) across.

Pluto slowly reveals itself: a July 9th image from LORRI showing surface features on the planet.
Pluto slowly reveals itself: a July 9th image from LORRI showing surface features on the planet. This image was captured at a distance of some 5.4 million kilometres (3.3 million miles) from Pluto

Close to these rings are a further series of linear and other features that have caused much interest and speculation among the space science community. On July 11th, New Horizons took what amounts to a “last look” at these surface features prior to them slipping out of view as the probe’s trajectory through the system takes them out of sight.

Pluto and Charon share a number of unusual properties which make them an interesting study. The first is that the barycentre between them  lies outside of Pluto, so that rather than Charon orbiting Pluto in the same way as the Moon orbits the Earth, both are orbiting a common point between them. This, and the relative size of Charon compared to Pluto, technically makes them a binary system, with the remaining four asteroid-sized moons orbiting both of them.

A closer image of Pluto, captured by New Horizons on July 11th, and the last time the “rings” and linear features on the surface will be seen in daylight

Another unusual feature is that Charon and Pluto are tidally locked to one another: Charon always presents the same face to Pluto and vice-versa. When seen from any position on either body, the other is always at the same position in the sky, or always obscured.

Pluto is effectively lying on its “side” in terms of its rotation, with an axial tilt of 120°. This means that seasonal variations are extreme; at its solstices, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness.

New Horizons will reach its point of closest approach to Pluto at 11:49:59 UTC (12:49:59 BST; 07:49:49 EST; 04:49:59 PDT), at which time it will be some 12:400 km (7,80 miles) from Pluto and around three times that distance from Charon. The flyby will carry the space probe over both the “day” and “night” sides of the dwarf planet, which will hopefully allow for the broadest range of data and images to be captured.

One particular thing scientists will be looking for as New Horizons passes over the night side of Pluto is any evidence of snowfall. Such is the temperature difference between the day and night sides of the planet, it is thought that much of the nitrogen-rich atmosphere falls as snow on the night side. Indeed, when Pluto is furthest from the Sun, it is believed all of its atmosphere may “freeze out” as surface snow.

“Winter is coming…” It has been night at Pluto’s south polar region for round 20 years. It will be another 80 before the sun rises once more. The only sunlight to reach the surface of the planet is that which is reflected by Charon, which is so large compared to Pluto, it appears seven times larger in the night sky than our own moon. This reflected light, 1,000 times dimmer than sunlight seen from Earth, will be the only illumination New Horizons will have to help it image surface features on Pluto’s night side (image: NASA artist’s impression)

The night-side pass, with the surface of Pluto Illuminated by sunlight reflected by Charon, may additionally yield more information about the ring and other formations which have been fascinating the science team. In addition, the probe will be keeping an eye out for possible “ice volcanoes” on Pluto, and evidence (via radar) of a possible subsurface ocean. With Charon, the team will be on the look-out for any signs that it as has an atmosphere – or even if it is possible syphoning  the upper layers of Pluto’s own tenuous atmosphere.

During the close approach, Pluto’s surface will be imaged by LORRI and Ralph at a resolution of 80 metres per pixel, with both able to capture images up to 80 kilometres across. At the time of its close flyby, New Horizons will be travelling at 49,600 Km/h (30,800 mph).

New Horizon's journey thus far (click for full size)
New Horizon’s journey thus far. Planets and distances obviously not to scale (click for full size)

All told, the close approach phase of the mission will last a total of 12 days, with July 14th actually marking the half-way point.  However, it is liable to be many months before all the data is returned to Earth. For one thing, it takes radio signals over 4.5 hours to reach Earth from New Horizons. Very high-resolution images also take a long time to transmit, so it is likely there will be quite a wait for some of these, as other, more accessible data is transferred to Earth and then processed / crunched.

So this just isn’t a real-time event where we can watch the data, wait a couple of hours and then see the pictures; at best it will be at least 24 hours before we start to see some of the images captured by New Horizons, and official estimates suggest it will take around 16 months to return all of the data gathered by the craft.

But from what Pluto has revealed so far, the wait, however long, is likely to be worth it.

All images courtesy of NASA / John Hopkins University / APL, unless otherwise stated. With thanks to Ziki for The Washington Post pointer