When Clyde Tombaugh (1906–1997) discovered a tiny object on a pair of photographic plates, one has to wonder if he could have foreseen that it would take 62 more years to find another object in the distant solar system or that it would lead to a yet to be discovered region of space. In 1930, rocketry was still in its infancy and deep space travel was the work of popular science fiction. Therefore, it would have been a far off dream for Tombaugh to think about visiting his discovery, which we now know as Pluto. However, on July 14, 2015, that is exactly what he will do, when the New Horizons spacecraft makes a Flyby of the Pluto-Charon system. New Horizons is carrying a sample of Tombaugh’s ashes donated to the mission by his wife, Patricia Tombaugh (1912–2012), to commemorate his discovery of Pluto.
For people that grew up in the educational system of the United States prior to 1992, Pluto was always the Ninth planet from the Sun and an anomaly when compared to the rest of the planets in our solar system. The Inner planets are characterized by being similar rocky bodies that are relatively close to the sun. Next there were the Outer planets, consisting of large gaseous planets with tumultuous atmospheres. Then there was Pluto; a small planet rotating around the sun in an elliptical orbit that was out of plane with the rest of the planets. The questions of why Pluto was different were not able to be answered until technology allowed for better views of deep space. In 1992, The first trans-Neptunian object since Pluto and Charon was discovered in 1992 and since then more than 1,500 objects have been identified. This region is characterized by small ice worlds that orbit the sun in vast number of Astronomical Units beyond Pluto and has been called the Kuiper Belt.
The idea to send a probe to visit the region started to form in the early-1990’s. And although there were many proposals for missions that would visit the small planet, it wasn’t until NASA established as part of the New Frontiers program that a stable stream of funding was made available to fund such a mission. It was in this climate that the New Horizons mission was born. Led by Alan Stern as Principle Investigator, New Horizons is a joint effort between the South West Research Institute and the Johns Hopkins University Applied Physics Laboratory (APL). APL provides for the mission management of the spacecraft.
New Horizons was launched from Kennedy Space Center on January 19, 2006 aboard an Atlas V rocket with second and third stages to provide it the necessary velocity to be the first spacecraft launched directly into a solar escape trajectory. The primary goals of the mission are to map the surface composition and to characterize the global geology and atmosphere of Pluto. This data will help provide context for the formulation of the Pluto system and establish some understanding of its role in the formation of the early solar system. Its extended mission is to encounter one or more objects in the Kuiper Belt beyond Pluto and conduct similar data collection exercises.
In order to accomplish these goals, the spacecraft has a suite of seven science instruments.
- Alice is an ultraviolet spectrometer used for measuring gas composition
- Ralph combines an infrared spectrometer (LEISA) for mapping surface composition with a color optical imager (MVIC) for mapping surface structure and composition
- REX is a radio experiment for measuring atmospheric composition and temperature
- LORRI is an optical telescope that provides the highest resolution imaging of the surface
- PEPSSI is a plasma-sensing instrument for measuring particles escaping from Pluto’s atmosphere
- SWAP is a plasma-sensing instrument for measuring the properties of the solar wind at Pluto, Pluto’s atmospheric escape rate, and for searching for a magnetosphere around Pluto. The “solar wind” is a stream of charged particles streaming away from the Sun at high speed.
- SDC, an instrument used to measure dust impacts at the New Horizons spacecraft during its entire trajectory, was built by students at the University of Colorado!
To get a sense of the size of the spacecraft, it is possible to see a scale model of it hanging in the Udvar-Hazy Center, which is the National Air & Space Museum Annex at Dulles International Airport. The spacecraft has been compared to the size of a baby grand piano.
Subsequent to the launch of New Horizons, the International Astronomical Union reclassified Pluto from planet to dwarf planet status. However, this does not diminish the historical nature of the mission. The science data collected will greatly enhance the science communities understanding of Pluto, be able to validate assumptions and speculations about its surface features and compositions, and hopefully inspire the next generation of deep space scientists.
My Recollections of New Horizons IV&V
The IV&V planning and scoping efforts for the New Horizons mission began in late 2002. The CARA process was used in establishing the scope of the analyses performed. It was determined that Command and Data Handling (C&DH), Guidance, Navigation & Control (GN&C) and the Ralph instrument were to receive full life-cycle IV&V. There were two instruments which were determined to be of sufficiently low risk and not significant contributors to the primary science goals and therefore were not provided any IV&V coverage. Those were the REX and SDC instruments. The remainder of the instruments suite were addressed via IV&V requirements and test analysis activities. There was also some initial work performed on the Ground Software, but it was high reuse and it was determined that further work in that area would not be productive use of IV&V efforts.
One of the tasks performed during the New Horizons test campaign was an analysis of the Comprehensive Performance tests. This required additional IV&V analysis resources to be added to support the timely analysis of that large set of test artifacts. This type of analysis was needed due to the way that APL had structured their acceptance testing, for which IV&V had generated a risk. They had placed requirements verification into the system test world where it was exercised in a more day in the life kind of way.
Due to the tight constraints that were placed on the launch window, APL decided to slip functionality to a post-launch upload. They had one period that extended in January 2006 and allowed for the Jupiter Gravitational Assist and one that was in February 2006 that was a direct to Pluto launch and would have added four years to the time line (missing those two would have been a significant launch delay). They ended up making the window for the gravitational assist. Therefore, we performed C&DH and GN&C analysis post-launch, primarily this was code analysis and final test analysis. I believe there were two in-flight issues with the GN&C processor, which we supported. Ultimately, one was determined to be in the Detection and Correction Code (EDAC) hardware and the other was a problem with the autocoder.
Over the 9 year history of the mission, there were only two safe mode entries, that I am aware of. One occurred back in 2007 and was similar to the GN&C reset related to EDAC hardware. The other happened July 4th and appeared to come from trying to use the software differently than originally intended. The original operations philosophy was that they would start N number of weeks prior to the fly-by and start taking data. They would keep taking data until about the same N number of weeks past the fly-by (part of that was the occultation data collection and some radio experiment), then compress, then downlink. Downlink was to take on the order of nine months. Over the years, it seems they have revamped their operational plans based on the lessons learned from their Jupiter fly-by and from yearly encounter planning meetings. So when the safe mode entry happened, they were uploading a command sequence, while taking data and compressing data, so the sequencing got overwhelmed.
I find this mission fascinating. In the time that New Horizons has been cruising to Pluto, I was married, my son was born, I watched him learn to crawl, learn to talk, learn to walk, lost my wife, and have seen my son complete nearly a quarter of his schooling. I hope this event inspires kids of his age to aspire to be the next generation of discovery leaders.