New Horizons and Pluto
The objective of the NASA mission, New Horizons, was to study Pluto, its moons, and other objects in the Kuiper Belt. New Horizons was the first mission in the New Frontiers program, a competitively selected, medium-class, and principal investigator-led series of missions. New Horizons was the first spacecraft to reach Pluto, a relic from the solar system's formation. By the time it reached Pluto, the spacecraft had traveled for a longer period and farther away than any previous deep-space spacecraft ever launched.
New Horizons launched from Cape Canaveral on January 19, 2006, becoming the fastest human-made object ever launched from Earth. It was put directly into an Earth-and-solar escape trajectory with a speed of about 16.26 km/s (10.10 mi/s). Controllers implemented course corrections through January and March of that year, and on April 7, 2006, New Horizons passed the orbit of Mars. An opportunity arose to test the spacecraft's scientific instruments, especially Ralph, the visible and infrared imager and spectrometer, on June 13, when New Horizons passed by the tiny asteroid 132524 APL at a range of about 101,867 km (63,300 mi).
After this brief encounter, New Horizons proceeded to Jupiter. The closest approach was made at a distance of 2.3 million km (1.4 million mi) just over 13 months after launching, on February 28, 2007. This Jupiter flyby was instrumental to the overall mission. It provided an opportunity to do a general test of the spacecraft's scientific capabilities, such as its ability to return data about the planet's moons, magnetosphere, and atmosphere. It also provided a critical gravity assist that increased New Horizons' speed by about 14,000 km/hr (9,000 mi/hr), shortening its trip to Pluto by three years. New Horizons did detailed observations during the four-month flyby. Data was gathered on Jupiter's ring system, atmosphere, and moons. Although it was observing Jupiter's moons from distances much farther than Galileo was able to, New Horizons returned impressive pictures of Io, Ganymede, and Europa. After the Jupiter encounter, the spacecraft sped toward the Kuiper Belt and was put into hibernation mode. Most of New Horizons' significant systems were deactivated and revived every two months each year during its voyage to Pluto. New Horizons was brought out of hibernation on December 6, 2014, for the Pluto encounter. Just under 8 years after launch, on January 15, 2015, the spacecraft began its approach phase to Pluto. At that time, it took over four hours for a signal to reach Earth from the spacecraft.
On March 12, 2015, with nearly four months remaining until its close encounter, the spacecraft finally got closer to Pluto than Earth is to the Sun. Pictures of Pluto taken in April 2015 revealed distinct features, and each picture that followed had increased detail as the distance between New Horizons and Pluto closed. On July 10, 2015, data collected was used to conclusively answer the mystery of Pluto's size! Scientists concluded that Pluto is about 2,370 km (1,470 mi) in diameter, making it slightly larger than previous estimations. Its moon, Charon, was subsequently confirmed to be about 1,208 km (750 mi) in diameter.
New Horizons finally passed above Pluto's surface at about 7,800 km (4,800 mi) on July 14, 2015, and became the first spacecraft to explore the dwarf planet. On July 15, a 15-minute series of status messages were received at mission operations, confirming that the flyby had been entirely successful.
New Horizons was not only tasked with collecting data on Pluto and Charon, but on Pluto's other satellites, Hydra, Nix, Styx, and Kerberos. It took NASA over 15 months to download the entire data set collected during the Pluto, and Charon encounters. Despite how long this may seem, it was necessary considering the distance between Earth and New Horizons, and the spacecraft could only transmit 1-2 kilobits per second.
The data collected from New Horizons informed us that Pluto and its satellites are far more complicated we initially thought, and scientists were particularly surprised by the level of activity on Pluto's surface. The atmospheric haze and lower-than-predicted atmospheric escape rate forced scientists to revise earlier models of the system fundamentally. There was also evidence of intense shifts in atmospheric pressure, which lead to the possibility Pluto had standing or running liquid material on its surface in the past, or even now!
Stunning photographs showed a giant heart-shaped glacier made of nitrogen, named Sputnik Planitia, on Pluto's surface. The glacier is about 1,000-km wide (600 mi), making it undoubtedly the largest glacier in the solar system that we know of. Images of Charon showed a giant tectonic belt, further suggesting liquid on the surface of Pluto as a long-gone water/ice ocean.
In late 2015, after its Pluto encounter, mission planners began to redirect New Horizons toward Ultima Thule, a Kuiper Belt Object that is approximately 6.4 billion km (4 billion mi) from Earth for a future 2019 flyby. This encounter aimed to measure surface temperature, study the surface geology of the object, search for signs of activity, map the surface, detect any satellites or rings, and measure its mass.
The spacecraft was halfway to its new target from Pluto on April 3, 2017, and soon after entered hibernation mode for "a long summer's nap" that lasted until September 11, 2017. On July 14, 2017, the anniversary of its Pluto-Charon flyby, the New Horizons team unveiled new detailed maps of both planetary bodies.
Nearly 13 years after its launch, New Horizons flew past Ultima Thule, the most distant target in history, on January 1, 2019. Primary images showed a reddish, snowman-like shape. However, further analysis of images taken near 3,500 km (2,200 mi) revealed its unusual shape. Ultima Thule has a large, flat lobe connected to a smaller, rounder lobe that is almost 35 kilometers (22 miles) long in total. The odd shape was the most surprising part of the flyby, as no moons or rings were found. New Horizon's findings from Ultima Thule have genuinely changed the planetary science community, as nothing similar had ever been seen in the solar system. New Horizon's findings there have changed scientists' very understanding of how planetesimals, the building blocks of the planets, form.
In terms of other scientific discoveries made possible by New Horizons, in August 2018, NASA confirmed the existence of a "hydrogen wall" with evidence from New Horizons at the outer edges of the Solar System. This "hydrogen wall" theory was first detected and proposed in 1992 by the Voyager spacecrafts. New Horizons was about 6.6 billion km (4.1 billion mi) from Earth, as of March 2019, and was operating normally and speeding deeper into the Kuiper Belt at nearly 53,000 km/h (33,000 mi/h). The New Horizons mission has been extended through 2021 to explore additional Kuiper Belt objects.