Humanity has visited the dwarf planet Pluto – the first-ever Kuiper Belt Object visited by a spacecraft – as NASA’s New Horizons probe cruised by the space body that was placed at the center of the debate of planet-hood in 2006 and a planetary object that has sparked significant intrigued and pride for millions since its discovery in 1930.
Completing the Grand Tour of the outer solar system:
When New Horizons swung past Pluto at 11:48 UTC (07:48 EDT in the United States) on 14 July 2015, it not only became the first spacecraft to visit Pluto, but it also completed a goal originally hoped for during the Voyagers’ Grand Tour of the Solar System in the 1970s and 1980s: visiting every planet (as planets were known at the time of the Voyager missions) in the solar system.
When Voyager 1’s course was altered during its approach to Saturn in 1980 to ensure a very close flyby and encounter with the Saturnian moon Titan, the new path put Voyager 1 on an escape trajectory out of the solar system and precluded any possible visit of Voyager 1 to Pluto – as Titian was deemed more important for scientific observation than Pluto because of the moon’s atmosphere.
When Voyager 2 was sent on a trajectory from Saturn to ensure encounters with and flybys of Uranus and Neptune, NASA thus precluded any chance of a Pluto encounter as Uranus and Neptune’s locations and the needed trajectory to get to both of these planets prevented a course alteration of Voyager 2 to Pluto.
Thus, when Voyager 2 completed its planetary observations of Neptune in 1989, Pluto was the only bonafide planet (at the time) left unexplored.
However, with the 2006 International Astronomical Union’s (IAU’s) reclassification of Pluto from planet to dwarf planet status, the decision not to send Voyager 1 or 2 to Pluto and instead focus on the other outer planets meant that the Voyagers had actually visited all of the planets of the solar system as of August 2006.
But Pluto was never forgotten after the Voyager missions, and numerous missions – that would eventually become New Horizons – were proposed to study the curious little object that became curiouser and curiouser in the near-reaches of the trans-Neptunian region of the solar system.
The birth of New Horizons:
Part of NASA’s New Frontiers program, the New Horizons mission rose – as some missions do – from the cancellations of previous Pluto exploration missions, specifically Pluto Fast Flyby and Pluto Kuiper Express.
Specifically, the Pluto Fast Flyby mission was part of NASA’s concerted effort to study Pluto’s atmosphere while the then-planet was in a portion of its orbit that permitted its atmosphere to remain in a gaseous state.
This study was important to understanding key characteristic of Pluto’s nitrogen, methane, and carbon monoxide atmosphere which freezes to the surface of Pluto as Pluto moves into the outer recesses of its orbit of the Sun.
When the Pluto Fast Flyby mission was cancelled, NASA conceived a replacement mission called Pluto Kuiper Express.
This mission would have seen a probe fly by the Pluto-Charon system and at least one other large object in the Kuiper belt – where Pluto and Charon reside.
Unlike the Pluto Fast Flyby mission, which would have reached Pluto in 2010, the Pluto Kuiper Express mission would have reached Pluto in 2012 or 2013.
However, budgetary constraints forced NASA to cancel the mission in 2000, a decision which triggered a three-month concept study in 2001 for additional missions to Pluto.
In total, two mission concepts were studied: New Horizons and POSSE (Pluto and Outer Solar System Explorer).
In November 2001, New Horizons was officially selected for funding under the newly created New Frontiers program for NASA.
Over the following five years, the Southwest Research Institute and the Johns Hopkins Applied Physics Laboratory (APL) built New Horizons for NASA.
Due to the operational needs of the spacecraft in the outer reaches of the solar system, scientists and engineers built New Horizons for limited electronic activity during its short time in the inner solar system and designed interior paint and exterior thermal blanket positions to maximize heat retention in the outer solar system.
Furthermore, because of the distances from the Sun at which New Horizons’ primary mission would occur, the probe could not realistically carry solar arrays.
Instead, New Horizons was built with one Radioisotope Thermoelectric Generator (RTG) to provide power – 250 W at launch and 200 W during the Pluto-Charon system encounter – for its systems and all-important scientific instruments.
Given the nuclear power aspect of the probe, the engineers building New Horizons built-in numerous protections around the plutonium-238 oxide pellets (New Horizons’ “fuel”), covering them with iridium and then encasing them in a graphite shell to prevent any accidental nuclear contamination of Earth’s atmosphere or launch site in the event of a launch failure.
The U.S. Department of Energy performed several pre-launch studies of a potential launch failure and estimated the chances of a launch accident resulting in the release of radiation into the atmosphere to be 1 in 350.
Worst-case scenario estimates of radiation exposure from a launch failure stood at 80% of the average annual dosage in North America received from background radiation spread over an area of 105 km (65 miles) from the point of a potential launch accident.
Once in space, engineers designed New Horizons’ attitude control system with a spin stabilization for cruise stage and a three-axis stabilization for scientific modes – both controlled through 16 hydrazine monopropellant burning thrusters (with four thrusters providing 1.0 lbf and 12 thrusters providing 0.2 lbf).
Additionally, New Horizons’ designers included two star cameras for fine attitude control during spin and three-axis stabilization modes and two Sun sensors for overall attitude control based on an angle to the Sun and measurements of spin rate and clocking.
Branching from these propulsion and attitude requirements, scientists chose an X-band communications system for New Horizons to allow the probe to achieve a communication rate of 38 Kbit/s during its Jupiter flyby and 1 Kbit/s during its Plutonian system encounter.
But above these requirements, the science equipment needed to achieve the mission’s objectives was the most important aspect of pre-mission design and planning.
In all, the seven scientific instruments chosen for New Horizons were the Long-Range Reconnaissance Imager (LORRI); the Pluto Exploration Remote Sensing Investigation (PERSI) platform, consisting of two instruments: the Ralph telescope and Alice; the Plasma and High-Energy Particles Spectrometer Suite (PAM), consisting of two instrument: SWAP (Solar Wind At Pluto) and PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation); the Radio Science Experiment (REX); and the Venetia Burney Student Dust Counter (VBSDC).
Launch and cruise to Jupiter:
New Horizons arrived at its launch site on 24 September 2005 for final launch preparations, which at that time targeted a launch date of 11 January 2006 at the opening of a 23-day window to allow for a gravity assist flyby of Jupiter.
Back-up launch opportunities were secured for February 2006 and February 2007.
However, those February launch dates would preclude a flyby of Jupiter and add two to four years travel time for the probe to Pluto. Thus, the 11 January launch date at the opening of the Jupiter gravity assist window was consider unmissable.
However, the launch date soon slipped to 17 January to allow for inspections of the Atlas V launch vehicle’s kerosene fuel tank.
When inspections of the Atlas V’s kerosene tank revealed no anomalies, launch operations proceeded toward 17 January but were foiled due to unacceptable high winds at the pad.
The second launch attempt on 18 January 2006 was subsequently scrubbed due to low clouds and a power outage at the Applied Physics Laboratory.
Finally, and with a great sense of pride and relief, New Horizons successfully launched on the Atlas V 551 variant on 19 January 2006 at 14:00 EST from Launch Complex 41 at the Cape Canaveral Air Force Station, Florida.
The five solid rocket motors and core stage of the Atlas V in cooperation with the Centaur second stage successfully boosted New Horizons onto its initial trajectory before the Centaur second stage reignited at 14:30 EST to send New Horizons into an Earth-and-Solar escape trajectory.
An added third stage burn followed the escape burn and boosted New Horizons’ velocity to 58,536 km/h (36,373 mph).
With this third stage burn, New Horizons set a record for the highest launch speed of any human-made object from Earth, taking only nine hours to reach the moon’s orbit.
Toward the end of January 2006, with New Horizons safely on its way, mission controllers performed the craft’s first trajectory correction maneuver. This maneuver was so successful that the second trajectory correction maneuver was cancelled.
Scientists performed the first initial onboard tests of three science instruments on 20 February 2006, resulting in a clean bill of health for those three instruments.
Mission controllers then performed the third trajectory correction maneuver on 9 March, and by 7 April 2006, New Horizons sailed beyond the orbit of Mars, moving away from the Sun at a relative velocity of 76,000 km/h (47,000 mph).
As New Horizons cruised toward the asteroid belt by mid-2006, control teams scanned the spacecraft’s trajectory to see if it would, by complete chance, pass close enough to an asteroid to allow for observation.
On 13 June 2006, New Horizons passed close to asteroid 132524 APL at a minimum distance of 101,867 km (63,297 miles).
The incidental encounter allowed mission controllers and the scientific teams to use the Ralph telescope to make observations of the asteroid as well as test New Horizons’ observational and tracking systems ahead of the planned Jovian encounter in 2007 and arrival at Pluto in 2015.
As New Horizons cruised through the asteroid belt in August 2006, the probe’s upcoming encounter with Jupiter took on a different significance than had originally been expected when the probe was launched seven months prior.
On 24 August 2006, the IAU officially defined “planet” for the first time.
According to IAU, a planet is an astronomical object orbiting a star or stellar remnant that: (1) is massive enough to be rounded by its own gravity, (2) is not massive enough to cause thermonuclear fusion, and (3) has cleared its neighbouring region of planetesimals.
Under this definition, only eight bodies in the solar system meet the definitional requirements for planet status: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
However, the IAU went a step further, recognizing and formalizing a new class of celestial bodies that are planetary-mass objects that are neither planets nor a natural satellite, that are in direct orbit of the Sun, that are massive enough for their shape to be in hydrostatic equilibrium (round) under their own gravity, but (crucially) are not massive enough to clear the neighborhood around their orbit.
This new classification was formally recognized as dwarf planet, of which five solar system bodies currently meet the criteria: Pluto, Ceres, Haumea, Makemake, and Eris.
Thus, as of 24 August 2006, Pluto has been a dwarf planet, not a planet, and the solar system is now defined as having eight planets and five dwarf planets.
Thus, New Horizons’ upcoming encounter with Jupiter meant that Jupiter would be the only officially recognized planet of which New Horizons would visit.
Encounter with the Jovian system: A practice run for Pluto
At the beginning of September 2006, New Horizons’ teams activated the LORRI and commanded it to take its first long-range photographs of Jupiter at a distance of 291 million km (181 million miles).
Formal observations, proximity operations, and experiment package testing with Jupiter began in January 2007.
During proximity operations with the Jovian system, New Horizons’ scientists and mission controllers used Jupiter as a stand-in for Pluto to test New Horizons’ systems and the spacecraft’s ability to perform automated maneuvers during planetary flyby operations.
Since New Horizons was outfitted with state-of-the-art instruments that far exceeded the instrument capabilities of the Jupiter-dedicated Galileo mission of the 1990s, New Horizons returned significant scientific data about the Jovian system.
This, coupled with the probe’s still close proximity to Earth (allowing for faster and higher bandwidth communications), meant that New Horizons actually returned more scientific data about Jupiter than it is expected to return about Pluto.
Of the data returned about Jupiter, New Horizons observed atmospheric condition, analyzed the structures and composition of Jupiter’s clouds, and discovered debris from recent collisions within Jupiter’s rings while also searching for new rings and new moons around Jupiter.
The search for new rings and moons revealed no previously undiscovered objects.
However, during New Horizons’ observations of the Jovian moons Io and Callisto, the probe captured 11 volcanic eruptions on the surface of Io, including one eruption that sent ejecta material to an altitude of 330km above the moon’s surface.
Moreover, observations of Callisto further defined how lighting and viewing conditions affect infrared spectrum readings of surface water ice on the moon.
Closest encounter with Jupiter occurred at 05:43:40 UTC on 28 February 2007 when New Horizons passed within 2.3 million kilometers (1.4 million miles) of the largest planet in the solar system.
The probe’s trajectory during the flyby was meticulously designed to send New Horizons through a gravity assist slingshot maneuver to maintain the probe’s trajectory while increasing its speed by 14,000 km/h (9,000 mph), thereby accelerating the probe to a total velocity of 83,000 km/h (51,000 mph) relative to the Sun and shortening the probe’s journey to Pluto by three years.
Jupiter to Pluto:
Following the probe’s encounter with Jupiter, New Horizons’ control teams commanded the spacecraft into hibernation mode to preserve power as well as the onboard scientific systems.
From 2007 through 2014, New Horizons spent a majority of its time in hibernation, awakened by its flight control team periodically for a series of tests to ensure that its systems were still operational.
In terms of travel milestones, New Horizons crossed the orbital boundary of Saturn on 8 June 2008 and the orbit of Uranus on 18 March 2011.
Following the crossing of Uranus’ orbit, astronomers studying the Plutonian system announced the discovery of two previously unknown moons of Pluto.
(The two moons were eventually named Kerberos and Styx. The last two previously discovered moons of Pluto, found in 2005, were named Nix and Hydra — both for their connection in mythology to the god Pluto and also because their names began with the first two letters of the New Horizons mission name.)
However, the discovery of Kerberos and Styx, which had not been detected by previous observations of the Plutonian system by the Hubble Space Telescope, caused concern among mission operators about the possibility of New Horizons running into unseen debris in the Plutonian system.
To this end, numerous Earth-based telescopes as well as the Hubble Space Telescope where aimed at Pluto to determine the possibility of a catastrophic collision with debris or dust within the system.
The observations yielded a 0.3 percent chance of a catastrophic collision with an object in the Plutonian system.
By 20 July 2014, during a non-hibernation phase of its cruise stage, New Horizons, for the first time, successfully imaged Pluto and Charon as two distinct bodies from a distance of 2.8 AU. (An AU – Astronomical Unit – is defined as the average orbital distance of Earth from the Sun.)
Following this observation, New Horizons’ team placed the spacecraft into its final hibernation phase.
On 25 August 2014, New Horizons successfully surpassed the orbital distance of Neptune.
Distance observations of the Plutonian system:
On 6 December 2014, NASA awakened New Horizons from hibernation and began regular operations and preparations for the probe’s arrival at the Plutonian system.
Distance operations officially began on 4 January 2015.
Between 25-30 January, New Horizons’ onboard cameras captured an orbital dance photographic series of Pluto and Charon – which were still far too distant to be discerned in any clarity – which distinctly showed the two objects orbiting each other with the barycenter (the center of mass of two or more bodies that are orbiting each other, or the point around which they both orbit) of their orbits outside each of them.
At this time, the remaining four Plutonian moons were still too small and faint to be seen.
Finally, in earlier February, New Horizons captured its first images of Nix and Hydra.
It was not until 25 April that New Horizons’ cameras finally captured Kerberos and Styx.
Prior to Kerberos’ and Styx’s imaging, New Horizons captured evidence for a possible polar cap on Pluto on 15 April.
By 11 May, New Horizons’ team commanded the spacecraft to perform its own hazard search of the Plutonian system – to be used in conjunction with already established hazard photographs from the Hubble Space Telescope and numerous Earth-based telescopes.
The hazard scan was also designed to identify any previously unknown objects, such as rings or smaller moons, that had previously avoided detection. To date, no such items or hazards have been identified.
On 15 May, New Horizons’ cameras began providing higher resolution images of the entire Plutonian system than are capable of being produced from Hubble.
Furthermore, in addition to direct observations of the Plutonian system during this time, New Horizons also performed observations of Kuiper Belt object VNH0004 when it was just 0.5 AU away.
Proximity Ops and close-encounter with Pluto:
Following a small glitch on 4 July (which temporarily placed New Horizons into safe mode, an event that was much hyped in many news media outlets but was in reality a rather minor inconvenience to the mission that resulted in no loss or impact to the primary scientific objectives), New Horizons officially entered proximity operations and flyby mode on 8 July 2015.
With the close approach flyby at 11:48 UTC on 14 July 2015 (07:48 EDT in the United States), New Horizons passed within 7,800 miles (12,500 km) of the surface of Pluto and 17,900 miles (28,800 km) from Charon.
For the flyby campaign, the New Horizons mission carries multiple scientific objectives divided into three categories.
The primary mission objectives of the flyby campaign include: mapping of the chemical compositions of Pluto’s and Charon’ surfaces, the characterization of global geologies and morphologies of both Pluto and Charon, and the characterization of the neutral atmosphere of Pluto and its escape rate.
Secondary mission objectives include: imaging of Pluto and Charon in stereo with LORRI and Ralph; characterization of the time variability of Pluto’s surface and atmosphere and of Pluto’s ionosphere and its interaction with the solar wind; mapping of the chemical compositions of select areas of Pluto and Charon (determined en route to Pluto), the terminators (day/night border) of Pluto and Charon, of surface temperatures on Pluto and Charon, and of additional surfaces on Nix, Hydra, Kerberos, and Styx.
Other objectives include: determination of bolometric (measurement of the power of incident of electromagnetic radiation by heating a material with a temperature-dependent electrical resistance) Bond albedos (used to determine how much energy a body absorbs) for Pluto and Charon; searching for and determining the composition of any atmosphere on Charon; and searching for neutral hydrogen, hydrocarbons, hydrogen cyanide, and other nitriles in the atmosphere of Pluto (and possibly Charon).
Finally, New Horizons also carries a series of tertiary objectives, including: refining of the radii, masses, and orbits of Pluto and Charon, searching for additional moons and rings, and characterizing the energetic particle environment at Pluto and Charon.
To accomplish part of these objectives, on 11 July, New Horizons’ instruments began mapping the surfaces of Pluto and Charon to within 40 km (25 miles) resolution.
Mapping operations began on 11 July to ensure that all features of Pluto’s and Charon’s surfaces were mapped as the two objects complete one full revolution about their axes during the flyby campaign.
Moreover, this three-day mapping campaign will allow New Horizons’ LORRI camera to obtain four complete color dayside maps of Pluto at a maximum of 1.6km resolution.
To accomplish all of the mission’s objectives, New Horizons carries a suite of instruments tailor made for the Pluto encounter.
Included in the probe’s science package is LORRI, an 8.2-inch aperture camera capable of resolving to approximately 1 asec (1.6km resolution) that will provide high-resolution images within the visual range.
Joining LORRI is the PERSI platform, consisting of the Ralph telescope (which will provide broadband and color channels in the visible light spectrum as well as near infrared imaging spectrometry) and the Alice instrument (which will provide ultraviolet imaging spectrometer capability to resolve wavelength bands in the far and extreme ultraviolet range).
Both Ralph and Alice will help determine the composition of Pluto’s atmosphere.
New Horizons also carries the PAM experiment and its two instruments that will measure the particles of the solar wind and the concentration of those particles at Pluto’s distance from the Sun.
Meanwhile, the REX instruments will allow for radio scientific observations of the dwarf planet system while VBSDC – a student-built experiment from the University of Colorado at Boulder – will measure dust particle concentration throughout the entirety of New Horizons journey from the distance of Uranus’ orbit out into the Kuiper Belt.
No dust collector instrument has ever operated successfully beyond the orbital bounds of Uranus, and models for dust accumulation and concentration in the outer solar system remain speculative.
Thus, VBSDC will help validate those speculative models and help refine our understanding of dust concentrations in the outer solar system.
With all these instruments concentrating their focus on Pluto and Charon during approach and close flyby, the investigation of Pluto will not end once New Horizons swings beyond the dwarf planet.
Excitingly enough, since Charon is more than half the diameter of Pluto, its large, relative size and position “behind” Pluto – as viewed from the Sun – at the time of flyby will allow enough reflected light from Charon’s surface onto Pluto’s nightside to allow for some nightside imaging and observations of Pluto.
New Horizons will also perform backlight, post-flyby observations of Pluto to once again search for any rings around the dwarf planet.
At the same time, New Horizons’ REX instrument will perform radiometry on the nightside of Pluto.
Beyond Pluto: To other Kuiper Belt objects
Once the Pluto encounter is complete, New Horizons will not enter orbit of the dwarf planet but instead continue on into the deep recesses of the Kuiper Belt.
Pre-mission planning hoped that New Horizons would be able to fly by at least one and possibly two additional Kuiper Belt Objects (KBOs) after the Pluto encounter.
However, since the entirety of the mission was based on the successful encounter with Pluto, any KBO visited afterword would have to fall within 1 degree of New Horizons’ trajectory at the time of the Pluto encounter and fall within an orbital boundary of 55 AU.
The restriction of 1 degree from New Horizons’ trajectory is because of the minimal amount of hydrazine fuel that will remain within the probe following the Pluto campaign.
The restriction to 55 AU has to do with the probe’s communications and power abilities.
Beyond 55 AU, New Horizons’ communications link will become too weak to support a flyby.
Likewise, New Horizons’ power source (its RTG wattage) will have decayed too much to allow for scientific observations of objects beyond 55 AU.
Moreover, the New Horizons team hoped that any KBO visited after Pluto would be more than 50 km (31 miles) in diameter, neutral in color, and, if possible, have a moon.
By 15 October 2014, the Hubble Space Telescope had revealed three potential KBO targets for New Horizons post-Pluto.
All three objects fell within an estimated diameter range of 30-55 km and were observed at distances from the Sun between 43 and 44 AU.
Of the three objects identified as PT1, PT2, and PT3, estimates for fuel probability of reaching these objects were found to be 100%, 7%, and 97%, respectively.
Moreover, all three KBOs are low-inclination and low-eccentricity classical KBOs that are quite different from Pluto.
If PT1 is chosen for flyby (the object with a 100% probability of enough fuel for flyby), New Horizons will reach it in January 2019.
However, PT3 (the object with a 97% probability of enough fuel for flyby) might be more preferable since it is brighter and therefore probably larger than PT1.
As of writing, PT2 is no longer in consideration for flyby, and PT1, with a diameter now estimated at 40–70 km, is the preferred flyby target.
A final decision on which object to take New Horizons to after Pluto will be made in August 2015.
After this proposed flyby of a KBO, New Horizons will join the Voyager probes in their exploration of the outer realm of the solar system, specifically in the mapping of the heliosphere.
It is currently estimated that New Horizons will end its mission based on RTG plutonium decay in 2026, thus resulting in intermittent heliosphere data collection if instrument power-sharing is required – as is currently done on the Voyager probes.
If, like the Voyager probes, New Horizons is still functioning when it reaches the outer heliosphere, it is expected that the probe will encounter the heliopause in 2047 and join Voyager 1 (and by that point, Voyager 2) in the interstellar medium between the stars.
(Images via NASA)