RocketSTEM Issue #13 - September 2016 | Page 87

Pluto encounter planning Dr. Young, who is based at the Southwest Research Institute in Boulder, Colorado, describes some key considerations when planning for New Horizons’ Pluto encounter and beyond. Describing herself as a ‘Pluto person, rather than a spacecraft person’ and ‘pretty well-rounded when it comes to all aspects of Pluto science’, she was also heavily involved in planning the mission as the Pluto Encounter Planning Team Leader. Invited by Principal Investigator Dr. Alan Stern at the mission proposal stage to plot the quality of the data needed, which required her to plan for how the data was to be collected and how well the different instruments might work, she says she ‘was in a good position to be his right hand.’ She says that her early work on stellar occultation on Professor James Elliot’s team at MIT, a field involving meticulous advance planning, and which led to her passion for Pluto, also stood her in good stead. ‘It was the discovery that Pluto had an atmosphere while I was working with Jim Elliot in 1988 that changed my life!’ Obviously, Pluto’s atmosphere was a key target for New Horizons, as was its interaction with the solar wind and the complex relationship with Pluto’s surface, especially regarding the behaviour of volatiles like nitrogen and methane. And then, of course, there was Pluto’s large moon, Charon, discovered Figure 2: Size of Pluto and Charon compared with America. Credit: NASA by United States Naval Observatory astronomer James Christy in 1978. At just over half Pluto’s size and about one-eighth its mass (see Fig. 3 for statistics on Pluto and its moons), Charon is large enough to be considered as part of a binary system with Pluto, especially as the two orbit a barycentre (common centre of gravity) outside Pluto itself. Figure 2 shows the sizes of Pluto in Charon compared to that of the USA. Charon’s contrasting brightness with that of Pluto and Earth-based observations of its surface chemistry had indicated that it could be an equally intriguing world. Diameter (km) Mean distance from the system’s centre± of mass (km) Orbital period (days) Mass (1019kg) (Earth = 5.97x1024kg) Pluto 2,374 2,035 6.38723 1,305 (0.002 Earth’s) Charon (P1) 1,212 17,536 6.38723 158.7 Styx (P5) 16x9x8 42,410 20.16 ? Nix (P2) 50x35x33 48,690 24.85 0.005 ± 0.004 Kerberos (P4) 19x10x9 57,750 32.17 ? Hydra (P3) 65x45x25 64,720 38.20 0.005 ± 0.004 Figure 3: Pluto and its family of moons – Selected statistics. Credit: NASA Earth-based observations of Pluto’s mysterious, mottled surface, barely resolved by the Hubble Space Telescope, also informed the choice of hemisphere for the encounter. Figure 4 shows (a) the best low-resolution mapping of Pluto from Earth compared to (b) the global map of Pluto based on imaging by the New Horizons spacecraft. Says Leslie Young, ‘We knew from early low resolution mapping from the Earth, before we even considered the mission, that Pluto is one of the most contrasty bodies in the solar system. We knew that where you have rapid transitions from very dark to very bright, there’s going to be 85 www.RocketSTEM .org 85