|Artist concept of the New Horizons spacecraft in the Pluto system. From bottom left to top right: New Horizons, Pluto, and Charon. Image credit: NASA.|
In their 2002-2003 Decadal Survey of future goals for planetary exploration, planetary scientists rated Kuiper Belt Object (KBO) exploration as their highest scientific priority. Nevertheless, the planetary science community, NASA, and the Congress have many competing priorities, so the NH II mission was judged to be a reach too far.
NASA had approved the NH mission proposal in November 2001. The compact 478-kilogram spacecraft was scheduled to launch atop a hefty Atlas V 551 rocket in January-February 2006. A Jupiter gravity-assist flyby in March 2007 would accelerate it toward Pluto with a flight time of only about eight years. If all went well, NH would bring to bear on Pluto and its satellites a suite of seven science instruments in July 2015. NH would then fly past one or more additional Kuiper Belt Objects (KBOs) in the 2016-2020 period.
For a time in 2004-2005, however, it appeared that it would leave Earth with a minimal supply of plutonium in its electricity-generating Radioisotope Thermal Generator (RTG). (In the image at the top of this post, the RTG is the black vaned cylinder at lower left.) The plutonium shortage stemmed from a security breakdown at the Department of Energy (DOE) laboratory that produced the plutonium New Horizons needed. Without an RTG fully loaded with plutonium, it was unlikely that NH could operate for long enough to reach any KBO beyond Pluto.
The shortage caused SwRI to propose a modified version of NH II. In its purest form, the new NH II would aim exclusively to explore one or more KBOs. It would leave Earth at least a year after NH, hopefully enabling it to 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 it had gained from NH, the NH II mission would cost only $472 million; that is, at least $200 million less than NH.
SwRI found that NH II could launch to one or more of the hundreds of KBOs known in 2004-2005 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 permit a Uranus flyby in the 2014-2017 period en route to the target KBO without dramatically increasing mission duration. This would make NH II only the second spacecraft to explore the Sun's seventh planet; the first was Voyager 2 in January 1986.
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 more than 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. Its 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. The Uranian 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 many intriguing features but no signs of present-day activity. In 1998, however, the Hubble Space Telescope revealed about 20 bright clouds in the Uranian atmosphere, and more bright clouds have since been observed. In addition, astronomers have spotted glowing aurorae at its magnetic poles, which do not match its rotational poles.
Small worlds similar to the Uranian moons in size and mass have turned out to be surprisingly active. Saturn's 500-kilometer-diameter moon Enceladus, to cite the best-known example, has squirting from warm areas at its south pole jets of water vapor laden with salt and organic compounds.
|Binary Kuiper Belt Object 1999 TC36, Image credit: NASA/Space Telescope Science Institute.|
NH II might instead be directed toward a flyby of 2002 UX25, a roughly 680-kilometer-diameter KBO with a 205-kilometer satellite. If launch took place in March 2008, the flyby could occur as early as July 2022. Earth departure in early May 2009 would yield a 2002 UX25 flyby in July 2023. 2002 UX25 currently orbits the Sun at about 41 times the Earth-Sun distance. With a fully fueled RTG, additional KBO flybys after the 1999 TC36 or 2002 UX25 flyby would be possible.
In late 2004, as the plutonium shortage became apparent, the NH team appealed to Congress for funds for an NH II mission study. NASA's Fiscal Year 2005 budget appropriation called for such a study, though Congress declined to fund it. Nevertheless, in early 2005 NASA Headquarters tasked NASA Goddard Space Flight Center in suburban Washington, DC, with an independent study of the NH II concept.
The DOE subsequently was able to resolve its security problems and provide a full load of plutonium for the NH RTG, so NASA dropped the NH II concept. NH left Earth on 19 January 2006, flew past Jupiter on 28 February 2007, and flew through the Pluto system in mid-July 2015.
Seven months after NH left Earth, Pluto's classification as a planet, long tenuous, was updated to take into account new knowledge of the outer Solar System. The NH flyby made Pluto both the first KBO to be discovered and the first to be visited. NH flew past a second KBO, provisionally designated 2014 MU69, in January 2019. In November 2019, the 35-kilometer-long KBO was named Arrokoth.
New Horizons 2: A Journey to New Frontiers, presentation materials, A. Stern, Southwest Research Institute, 10 June 2005.
New Horizons II Mission Design, presentation materials, Y. Guo, 16 June 2004.
“New Horizons II: Doubling UP in the Outer Solar System,” L. David, Space.com (no longer online), 17 June 2004.
“New Horizons Set to Launch with Minimum Amount of Plutonium,” B. Berger, Space News, 4 October 2004.
The Challenge of the Planets, Part Three: Gravity
The Seventh Planet: A Gravity-Assist Tour of the Uranian System (2003)