The New Horizons II (NH II) mission was conceived in mid-2002 as an economical backup for the New Horizons mission to Pluto, its moons, and the Kuiper Belt. The brainchild of Southwest Research Institute (SwRI) scientist Alan Stern, NH II was meant to ensure that NASA could satisfy the desires of the planetary science community. In their 2003 Decadal Survey of future directions for planetary exploration, work on which commenced in 2002, planetary scientists rated Kuiper Belt exploration as their highest priority.
NASA had selected SwRI's New Horizons mission proposal in November 2001. The compact 478-kilogram spacecraft (image at top of post) was scheduled to launch on an Atlas V 551 rocket in January-February 2006. A Jupiter gravity-assist flyby in March 2007 would accelerate it toward Pluto in only eight years. The spacecraft would bring to bear on Pluto and its satellites a suite of seven science instruments in July 2015.
New Horizons was meant to fly past one or more Kuiper Belt Objects (KBOs) in the 2016-2020 period, after it finished exploring the Pluto system. For a time in 2004-2005, however, it appeared that it would have to leave Earth with a minimal supply of plutonium in its electricity-generating Radioisotope Thermal Generator (RTG). (The RTG is the black vaned cylinder at lower left in the image above.) The shortage was the result of a security-related interruption in activities at the Department of Energy laboratory that processed plutonium. The New Horizons team could not be certain that the interruption would be resolved before their spacecraft's launch window opened. Without a fully loaded RTG, it was unlikely that New Horizons could operate for long enough to reach a KBO. This bolstered support for NH II, a dedicated KBO mission, which would, presumably, be able to avoid security-related problems and launch with a topped-off RTG.
To cut costs, NH II would be a "clone" of New Horizons. SwRI estimated that, by avoiding new development and by drawing on the experience gained from New Horizons, the NH II mission would cost only $472 million; that is, at least $200 million less than New Horizons.
SwRI found that NH II could launch to one or more of the hundreds of known KBOs any time that a launch window for Jupiter opened (that is, every 13 months). The March 2008 and April 2009 launch opportunities were especially attractive, however, because they would enable a Uranus flyby in the 2014-2017 timeframe. This would make NH II only the second spacecraft to explore the Sun's seventh planet; the first was Voyager 2 in January 1986.
All of the planets except Uranus rotate on an axis more or less perpendicular to the plane of their orbit around the Sun. Earth, for example, is tilted at 23.44° relative to the plane of its orbit about the Sun. Uranus is tipped on its side relative to the other planets, meaning that its axis is nearly parallel to the plane of its orbit. Uranus has at least 27 moons, of which five (Miranda, Ariel, Umbriel, Titania, and Oberon) range from 450 to 1600 kilometers in diameter. It also has a system of at least 11 rings. The rings and moons revolve around Uranus in the plane of its equator, which means that the entire Uranus system appears to pivot around the Sun on its side. Uranus needs a little over 84 years to circle the Sun once.
When Voyager 2 flew past Uranus, the planet's south pole was pointed toward the Sun; that is, its southern hemisphere was near the middle of a 21-year summer. Uranus's northern hemisphere was pointed away from the Sun, so was locked in dark winter. The same applied to its moons; their southern hemispheres were fully lit and their northern hemispheres were cloaked in cold darkness. This meant that Voyager 2 could not image their northern hemispheres. Uranus's equator would be turned more toward the Sun when NH II flew past, so the spacecraft would be able to observe the Uranus system in its entirety.
Uranus appeared bland to Voyager 2, and the visible parts of its largest moons showed intriguing features but no signs of present-day activity. In 1998, however, the Hubble Space Telescope revealed about 20 bright clouds in Uranus's atmosphere, and more bright clouds have since been observed, along with glowing aurorae. In addition, small worlds not too different from the Uranian moons have turned out to be surprisingly active; Saturn's 500-kilometer moon Enceladus, for example, has squirting from its south pole jets of water vapor laden with salt and organic material.
After flying past Uranus, NH II would be on course for its primary destination. If launched from Earth on March 19, 2008, April 29, 2009, or April 30, 2009, the spacecraft would zip past the binary KBO 1999 TC36 on September 15, 2020, September 15, 2021, or April 8, 2023, respectively. 1999 TC36, currently orbiting the Sun at about 31 times the Earth-Sun distance, consists of two close KBOs, one about 285 kilometers across and the other about 265 kilometers in diameter; circling the close pair is a moon about 140 kilometers across. Launch on March 20, 2008, March 21, 2008, or May 1, 2009 would yield a flyby of 2002 UX25, a roughly 680-kilometer-diameter KBO with a 205-kilometer satellite, on July 15, 2022, September 15, 2020, or July 16, 2023. 2002 UX25 currently orbits the Sun at about 41 times the Earth-Sun distance. Additional KBO flybys after the 1999 TC36 or 2002 UX25 flyby would be possible.
In late 2004, after the plutonium shortage became apparent, the New Horizons team appealed to Congress for funding for a New Horizons II mission study. NASA's Fiscal Year 2005 appropriation called for such a study, though it failed to fund it. In early 2005, NASA Headquarters tasked Goddard Space Flight Center with studying the NH II concept. Oddly enough, Goddard did not invite the New Horizons team to participate in its study. The Department of Energy was eventually able to resolve its security problems and provide a full load of plutonium for the New Horizons RTG, and the NH II backup concept was dropped. New Horizons left Earth on January 19, 2006, and flew past Jupiter on February 28, 2007. If all goes well, the 480-kilogram spacecraft will fly past Pluto on July 14, 2015, then will explore at least one small (40-to-90-kilometer-diameter) KBO by 2020.
References:
New Horizons 2: A Journey to New Frontiers, presentation materials, A. Stern, Southwest Research Institute, June 10, 2005.
New Horizons II Mission Design, presentation materials, Y. Guo, June 16, 2004.
"New Horizons II: Doubling UP in the Outer Solar System," L. David, Space.com, June 17, 2004.
"New Horizons Set to Launch with Minimum Amount of Plutonium," B. Berger, Space News, October 4, 2004.