by Prasad Ganti
On the new year’s day, 2019, NASA’s New Horizons spacecraft passed the most distant object in our solar system, which any human made spacecraft came close to. This object is about four billion miles away from Earth, and a billion miles away from Pluto, the demoted planet and furthest one from Earth and thus the Sun. New Horizons is now so far that the radio signal takes about six hours to reach the Earth. At just twenty miles in size, shaped like a snowman with two spheres, it was a challenge to identify the new destination once New Horizons reached Pluto, and even greater challenge to navigate towards such a nondescript object at such a vast distance.
Pluto and Ultima Thule belong to the part of the solar system called Kuiper Belt (picture shown below), consisting of bits and pieces of rock and ice. They are supposed to be remnants of the formation of our solar system, much like the asteroid belt between Mars and Jupiter. New Horizons joins the club of spacecraft like Pioneer 10 and 11 and Voyager 1 and 2, which are at the edge of the solar system. Picture below shows the Kuiper belt and the different spacecraft and where they are relative to each other.
New Horizons is the fifth spacecraft to traverse the Kuiper Belt, but the first to conduct a scientific study of this mysterious region beyond Neptune. Credit: NASA/JHUAPL/SwRI/Magda Saina
The pictures we got of Ultima Thule are very rudimentary, with fewer pixels in them. That is because of the slow speeds of data transmission at thousand bits per second, all of it relayed by a radio transmitter sending out fifteen watts of energy. Such feeble energy is caught by NASA’s deep space network. The transmission can be done when Sun is not in the path to Earth. It will take roughly twenty months to send home all of the newly collected seven gigabits of data about Ultima Thule. The geology and composition, as well as the potential for rings or moons, will be beamed home first.
The data transmission speeds and the radio transmitter appear to be primitive. Considering that the New Horizons was launched in 2006 after years of design, it is a wonder that the technology works so well at such huge distances. The saga of conceiving, designing, launching and operating of New Horizons is well chronicled in the book “Chasing New Horizons” by Alan Stern who is the PI (Principal Investigator) for the project. The PI is the person who conceives the idea and sells it to NASA and carries it to fruition.
When Voyager 1 and 2 were launched in 1977, they went on a grand tour of all the outer planets. While Voyager 1 went towards Jupiter and Saturn, it was pulled off path to investigate Titan. It then veered off from the plane of the planets, with no chance of ever going to Pluto. Voyager 2 went to Uranus and Neptune, it was not destined to travel towards Pluto, as Pluto was at a different place in its orbit around the Sun at that time. Thus Pluto became the only planet not to have been visited by any spacecraft.
NASA has many competing proposals, all vying for the reducing budgets. Pluto mission, titled New Horizons, barely won the competition at the beginning of the century. Work started at a feverish pace. New Horizons was to be the fastest launched spacecraft, given the vast distance it had to traverse. Atlas V was the launch vehicle, with a custom built third stage solid rocket added atop to boost the fastest launched spacecraft towards Jupiter. Going so far away from the Sun meant that it had to use nuclear power (solar power is too feeble at such distances from the Sun). RTG (Radioisotope Thermoelectric Generator) uses plutonium packaged in small pellets made of plutonium dioxide. Clad in iridium and bottled up inside a black graphite casing. The radioactive decay produces heat which is used in a thermocouple producing 250 watts. 250 watts is equivalent of about four ordinary electric bulbs, During the long journey, all the systems in the spacecraft were put into hibernation. This also meant that the staffing at mission control was down to bare minimum. The systems on the spacecraft were very skeletal. There was no scan platform to turn the instruments during a flyby. It was simply too heavy. Cameras (capture light to take pictures) and spectrometers (which analyze the various frequencies present in a given sample of light) are the typical instruments used for such exploration.
On such a long journey, re-orienting the spacecraft is required. The initial aim may not be very accurate. ALMA (Atacama Large Millimeter Array), the ground based radio telescope in Chile, was used to correct the path of the New Horizons, as it sped towards Pluto after getting a gravity assist from Jupiter. With the proper maneuvering as it neared Jupiter, it got flung by Jupiter’s massive gravity towards Pluto. Like getting a free push.
As it neared Pluto in 2015, the mission control came back fully staffed to prepare for the flyby procedures. Days before the flyby, mission control lost communication with the spacecraft. During those nervous moments, the staff correctly guessed that the on-board computer became overloaded while compressing images. They restarted the computer and were able to recover most of the image files. And then loaded the computer with the sequenced commands for the flyby mission. Everything went well thereafter.
The space telescope Hubble was used to identify the distant object Ultima Thule, and when this small object moved in front of a star, its size was estimated as twenty miles (due to the dip in light from the star). The flyby of Ultima Thule was an accomplishment made beyond the initial goals of reaching Pluto. New Horizons is still alive and going. What is the next destination ? The excursion across the solar system covering tinier masses at the extreme edge is something amazing. These achievements are nothing short of a miracle.