The New Horizons Spacecraft
By: S. Alan Stern & Andrew Cheng
Edited by: Ted A. Nichols II

The first exploration of the Pluto-Charon system and the Kuiper Belt promises to be a scientific watershed. It will provide valuable insights into the origin of the outer solar system and the ancient outer solar nebula, the origin and evolution of planet-satellite systems presumably formed by giant impacts, and the comparative geology, geochemistry, tidal evolution, atmospheres, and volatile transport mechanics of icy worlds.

The New Horizons mission begins with the launch of a Discovery-class interplanetary spacecraft in January 2006 onto a trajectory that reaches Pluto-Charon via a March, 2007 Jupiter Gravity Assist (JGA). Pluto-Charon will be reached in the summer of 2016 or 2017, depending on the launch vehicle selected by NASA. Multiple KBOs will be encountered in the five succeeding years after the Pluto-Charon encounter.

The New Horizons spacecraft mass is 416 kg, including propellant for a 290 m/s propulsion budget. The spacecraft subsystems are based on APL's Discovery/CONTOUR spacecraft. CONTOUR is scheduled for launch this July. CONTOUR itself, a multiple comet flyby spacecraft, is based in part on APL's TIMED earth orbiter mission (launched in 2001). Use of CONTOUR design heritage reduced schedule and cost risk, allowing a substantial, 22% dry mass margin, a healthy 20% power margin at Pluto encounter, and a significant, multi-year margin against the NASA AO's 2020 Pluto-Charon arrival date limit.

This spacecraft will carry four complementary reconnaissance instruments. The payload consists of the PERSI Vis/IR/UV remote sensing package, the REX radio/radiometry experiment, the PAM plasma suite and the LORRI long-focal-length imager. Notably, New Horizons accommodates an infrared imaging spectrometer, which Voyager did not have, and which is essential to characterize the composition and the physical state (including temperature) of the ices on the surface. In addition, New Horizons will achieve a best imaging resolution at Pluto that is several times superior to the best achieved by Voyager at Triton, allowing, for example, better discrimination among possible geologic processes. The disk-average surface temperatures of the daysides and the nightsides of Pluto and Charon will be determined by measurement of the microwave brightness temperatures by REX; surface temperature mapping across each body will be achieved by measurement of temperature-sensitive spectral features of ices by LEISA. Table 2 provides additional detail regarding the payload and its sensor suite.

As the next mission to Jupiter, New Horizons will conduct an intensive, 4-month campaign of Jupiter system observations in early 2007. Closest approach will occur in March 2007 at a distance of 45±5 Rj (as set by the Pluto aim point); this is over three times closer than Cassini's Jupiter flyby in 2000-2001. This encounter affords irresistible opportunities for studies such as long time base imaging studies of atmospheric and auroral dynamics, new observations of the Galilean and irregular satellites of Jupiter, and in situ exploration of the jovian magnetosphere.

During the cruise from Jupiter to Pluto, New Horizons may be able to reach a Kuiper belt escapee (a so-called Centaur object), but this depends upon groundbased searches finding a suitable target along the mission trajectory.

The Pluto-Charon encounter begins 6 months prior to closest approach. For a period of 75 days on either side of closest approach, New Horizons images will exceed the best the Hubble Space telescope can achieve at Pluto-Charon. This allows advance planning to optimize the close approach sequence, and a substantial timebase of disk-resolved images to study time-variable phenomena such as volatile transport and meteorology.

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New Horizons Payload Overview:
Instrument Type Sensor Characteristics Builders

-PERSI Remote sensing suite MVIC (panchromatic and four-color CCD imager, 0.4-1.0 microns, 20 microradians/pixel)
-LEISA (near infrared imaging spectrometer, wedged filter, 1.25-2.5 l/Dl = 600 for 2.1-2.25 microns and 300 otherwise, 62 microradians/pixel), and ALICE (UV imaging spectrometer, 500-1850 Å, spectral resolution 3 Å, 5 milliradians/pixel) Ball, SwRI, NASA/GSFC
-REX Uplink radio science, passive radiometry Signal/noise power spectral density 55 db-Hz; ultrastable oscillator stability 1x10-13 in 1 second samples. Disk-averaged radiometry to ±0.1 K. Stanford U., JHU/APL
-PAM Plasma and high energy particle spectrometers SWAP (solar wind plasmas up to 6.5 keV, toroidal electrostatic analyzer and retarding potential analyzer), and PEPSSI (ions 1-5000 keV and electrons 20-700 keV, time-of-flight by energy to separate pickup ions) SwRI, JHU/APL
-LORRI High resolution imager Panchromatic, narrow angle CCD imager, 0.30-0.95 microns, 5 microradians/pixel JHU/APL

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Long focal length approach imagery will include 40 km-class mapping of the so-called farside hemispheres of Pluto and Charon 3.2 days out (one half the rotation period of Pluto-Charon). This obviates the well-known farside mapping dilemma imposed by Pluto's slow (6.4 d) rotation for a single-spacecraft flyby mission.

The spacecraft-planet relative flyby speed of the Pluto-Charon encounter will be 11 km/sec. Near closest approach, New Horizons will obtain maps of both Pluto and Charon with km-scale resolution; at closest approach, images at scales as high as 25 m/pixel may be achieved (depending on the final flyby distance selected). In addition, the Group 1 objectives call for mapping the surface composition and distributions of major volatile species, for which New Horizons will obtain: (i) four-color global (dayside) maps at 1.6 km resolution, (ii) diagnostic, hyper-spectral near-infrared maps at 7 km/pixel resolution globally (dayside) and at 0.6 km/pixel for selected areas. Characterization of the neutral atmosphere and its escape rate will be accomplished by a battery of investigations including: (i) diagnostic ultraviolet airglow and solar occultation spectra to determine the mole fractions of N2, CH4, CO and Ar to 1% in total mixing ratio and to determine the temperature structure in the upper atmosphere, (ii) radio occultations at both Pluto and Charon, measuring the density/temperature structure of Pluto's neutral atmosphere to the surface, (iii) in situ determination of the atmospheric escape rate by measuring Pluto pickup ions, and (iv) H Lya mapping of the Pluto-Charon system in order to determine the rate of Roche-lobe flow of atmosphere from Pluto to Charon.

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Check out the Pluto Portal's Page of Links for the Pluto Mission