In 2004-2005, a scarcity of plutonium posed a threat to the New Horizons Pluto/Kuiper Belt spacecraft, potentially limiting its supply of nuclear fuel and putting its scheduled Kuiper Belt Object flyby at risk. To address this challenge, a suggestion emerged for an alternative mission named New Horizons II. This proposed mission aimed to navigate past Uranus and explore a minimum of two Kuiper Belt Objects. The details of New Horizons II and its designated targets are outlined by space historian David S. F. Portree.
The concept of the New Horizons II (NH II) mission was initiated in the mid-2002 as a cost-effective contingency plan for the New Horizons mission, which aimed to explore Pluto, its moons, and the Kuiper Belt. Conceived by Southwest Research Institute (SwRI) scientist Alan Stern, NH II aimed to ensure NASA could meet the priorities of the planetary science community. Kuiper Belt exploration, rated as the highest priority by planetary scientists in the 2003 Decadal Survey, influenced the need for NH II.
NASA had approved SwRI's New Horizons mission proposal in November 2001. The compact 478-kilogram spacecraft, set to launch on an Atlas V 551 rocket in January-February 2006, would use a Jupiter gravity-assist flyby in March 2007 to reach Pluto within eight years. Equipped with seven science instruments, it was scheduled to study Pluto and its satellites in July 2015.
While New Horizons planned to explore Kuiper Belt Objects (KBOs) after studying the Pluto system, a plutonium shortage in 2004-2005 threatened its nuclear fuel supply. Due to a security-related issue at the Department of Energy, the New Horizons team considered launching with a minimal plutonium supply, jeopardizing the ability to reach a KBO. This circumstance led to increased support for NH II, a dedicated KBO mission expected to launch with a fully loaded Radioisotope Thermal Generator (RTG), bypassing potential security-related problems.
To minimize costs, NH II was designed as a "clone" of New Horizons, leveraging existing developments and experience gained from the original mission. SwRI estimated that NH II could be executed at a cost of $472 million, at least $200 million less than New Horizons.
NH II was designed to launch to known KBOs during any Jupiter launch window (every 13 months). Launch opportunities in March 2008 and April 2009 were particularly appealing as they allowed for a Uranus flyby in the 2014-2017 timeframe. This would make NH II the second spacecraft to explore Uranus after Voyager 2 in January 1986.
Uranus, unique among planets, is tilted on its side with an axis nearly parallel to its orbit plane. NH II aimed to observe the entire Uranus system, unlike Voyager 2, which could only image the southern hemispheres of Uranus and its moons during its flyby. NH II's launch window would enable a more comprehensive study of Uranus.
Following the Uranus flyby, NH II would proceed to its primary destination. Depending on the launch date (March 19, 2008, April 29, 2009, or April 30, 2009), the spacecraft would pass the binary KBO 1999 TC36 on September 15, 2020, September 15, 2021, or April 8, 2023, respectively. Additional KBO flybys were also considered after the initial encounters.
In response to the plutonium shortage, the New Horizons team sought Congress funding for a New Horizons II mission study in late 2004. Although NASA's Fiscal Year 2005 appropriation called for the study, it was not funded. Goddard Space Flight Center was tasked with studying NH II in early 2005, excluding the New Horizons team. Ultimately, the Department of Energy resolved its security issues, providing a full plutonium load for New Horizons, leading to the abandonment of the NH II backup concept. New Horizons launched on January 19, 2006, flew past Jupiter on February 28, 2007, and aimed to reach Pluto on July 14, 2015, followed by potential KBO exploration by 2020.
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