We are counting down to 14 July 2015 when the New Horizons spacecraft will fly by the binary dwarf planet Pluto-Charon. A lot of people, especially the New Horizons team at the Johns Hopkins Applied Physics Laboratory in Maryland, will keep their fingers and toes crossed, hoping that the painstaking preparations and the navigation of this unique mission will render the results as planned.
New Horizons will approach the Pluto system face-on almost perpendicular to the Pluto-Charon orbit which itself is almost perpendicular to the ecliptic. This view, as we also see it at the moment from Earth, happens during two periods of Pluto’s long orbit around the Sun. Early images from New Horizons have already shown the interesting “dance” of this binary system.
The animation here, which was made about at the time that New Horizons was launched, shows the mutual Pluto-Charon orbit to scale. The red dot indicates the joint centre of mass of the system, which lies more than 1000 km above Pluto’s surface (more than Pluto’s radius). Because of their relative size and distance, Pluto and Charon are in mutual tidal locking, which means that they face each other with the same side all the time.
New Horizons will fly close past this system on 14 July at 11:50 UT, at a distance 12,500 km from Pluto's surface. This is well within the orbits of the four moons Styx, Nix, Kerberos and Hydra, the furthest of which (Hydra) has an orbital radius of almost 65,000 km. During the flyby all systems aboard the spacecraft will work overtime, but the observations of the Pluto system have already begun long before that.
New Horizons has been doing science from the moment it came out of hibernation in December 2014. The science team has defined several approach phases) and the third approach phase has begun 23 June and continues until 12 July. After that it is really getting full on during the four days around the actual closest approach (the Near Encounter Phase) while the spacecraft speeds at 13.8 km per second relative to Pluto. The science team has prepared hundreds of tightly orchestrated observation sequences during that phase. The most significant events (see image) during the approach phase are the actual close approach to Pluto at 11:50 UT, the Pluto-Sun occultation at 12:48 UT and the Charon-Sun occultation at 14:15 UT.
Key targets are the surface and atmosphere of both Pluto and Charon. High resolution images of the surface will detail down to 70 m per pixel and we will measure the mass of the two dwarf planets ten times better than we know it at present. Atmospheric composition, temperature and pressure gradients will be measured and the instruments will search for possible aurorae, smaller satellites than the four irregular moons we have already discovered, rings, etc. After the closest approach, New Horizons will observe the back-lit atmospheres of Pluto and Charon which gives further opportunities to study the atmospheres, in particular with stellar occultations. Very crucial are the moments that New Horizons will fly through the Pluto and Charon shadows from the Sun, as it will observe a “Solar Eclipse” from both dwarf planets. During these phases a lot of detail of the atmospheric compositions will be revealed with the help of the light from the Sun.
Communications with the spacecraft take more than 4.5 hours at the distance of some 5 billion km. Moreover the spacecraft will collect far too much data during the Close Encounter phase to broadcast in real time and the complete data set will only be available after more than a year. So we will have to be patient to see all the results.
We could well be in for some big surprises! This encounter is truly historic; we have never observed a system in close-up that far away from the Sun. In comparison with the many surprise discoveries at the closer planets and moons during the last half century, it is to be expected that this binary dwarf planet system and its smaller moons will reveal long held secrets. Whatever we find out, it will be spectacular.
Read this preview from Planetary scientist S. Alan Stern, the Principal Investigator of NASA's New Horizons mission.
Animation: Ngā Whetū Resources
Approach: NASA / John Hopkins – APL
New Horizons instruments: NASA