New Horizons: NASA’s space probe sent to Pluto

Pluto’s elliptical trajectory is particularly excentric, which means that the planet’s distance to the Sun varies from 7 to 4 billion kilometres. In 2015 Pluto was at its closest point to the Sun and, consequently, to the Earth. This only takes place once every 248 years. NASA’s astrophysicists wanted to take advantage of this window of opportunity to send a space probe to encounter Pluto and learn more about this distant and still little known heavenly body.

What did we know about Pluto before launching the project?

Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh. In 1978, James Christy, from the USA’s Naval Observatory, found out it possessed a moon with a relatively high mass. The two bodies turn around each other, the gravity centre being placed outside both of them—they are ‘binary bodies’. It is remarkable that we have later on noticed that such configuration is not rare in the Solar System. In fact, about 10 percent of the bodies within the Kuiper belt (where Pluto is located) are binary bodies.1

Picture of Pluto (bottom right) and Charon (top left) taken by the New Horizons probe on 14th July 2015 (source).

Pluto and its satellite, named Charon following its discovery, waltz so nicely together that they always see the same side of each other. That implies that each body permanently hides one of its faces to the other and that they are geostationary in relation to one another, being always overhead the same place. I can well imagine a Plutonian. At home, it never sees Charon. To see the magical spectacle of the enormous satellite in the sky, it must decide to trek a long way to the other side of the planet. As a comparison to our Earthling references, let us say that Charon is seven times bigger in Pluto’s sky than the Moon in ours. Besides, with such a massive object, Solar eclipses must take place much more often than on Earth, especially since the Sun is much smaller seen from a distance of 4 billion kilometres.

In 1985 a new breakthrough was made. Astronomers saw a star being hidden by Pluto.2 Telescopes pointing towards that direction showed how the star’s radiance progressively diminished before being completely hidden. This showed that Pluto was surrounded by a gaseous layer, an atmosphere. It is essentially formed by nitrogen, such as those of Triton, Titan and, by the way, the Earth.

Pluto is included in NASA’s agenda

Who could envision a journey to Pluto? NASA had the technological capacity. Yet, strategic will and funding were needed for such an endeavour. By the end of the 1960s, NASA had already conceived the project of a space probe that would visit many planets of the Solar System, among which, Pluto. It was the “Grand Tour” project. Sadly, the project was ended as Nixon cancelled it in 1969. NASA salvaged two “mini grand tour” missions from the Grand Tour project. First of all, Voyager 1, which passed near Jupiter in 1979 and near Saturn and its Satellite, Titan, in 1980. Then, Voyager 2, which flew over Neptune in 1986 and over Uranus in 1989. But there was no interest in reaching Pluto: too far away, too expensive.

Illustration by Klifton Kleinmann.

By the end of the 1990s, there were nonetheless those who still believed in the project, among whom Alan Stern, astrophysicist, then director of the Department of Space Studies at SwRI (Southwest Research Institute). Yet, NASA did not make up its mind… What could be done to persuade NASA’s decision makers? Alan Stern made politicians intervene. He pleaded the case for Pluto to members of the Congress, and this way he managed to persuade NASA. A Pluton project was launched. But, still, an inexpensive 700 million $ project. And the budget could not get out of control, as the project would then be mercilessly stopped. That is how NASA works: programme cutters are always lurking… Alan Stern, principal investigator, concedes that 80 percent of his activity was related to lobbying and fund seeking, whilst only 20 percent to scientific occupations.3 In 2002 the decision was made. NASA, as the general contractor, launches a call for tenders for the New Horizons project. Johns Hopkins University in Baltimore is chosen to build the space probe.4 It is a project that will involve a team of 50 people. It will have to conceive and build the probe in 4 years.

Planned trajectory and proposed agenda

New Horizons was launched on 19th January 2006 aboard an Atlas V rocket. That window of opportunity was interesting, as it took advantage of Jupiter’s gravitational assistance. The encounter with Pluto was expected in 2015.

To meet the expenditure, NASA did not allow any change in the programme: in between, Pioneer 10 and Pioneer 11 checked different trajectories which it would be interesting to study further in the future, but the decision was formal: the spacecraft would not be diverted.3

New Horizons is equipped with seven measurement instruments that have to analyse the surface and atmosphere of the bodies studied during its journey (Jupiter, contacted in 2007, and then Pluto and its satellite, Charon), as well as the composition of space on the edge of the Solar System. This could be considered as devoid of matter in a first approach, but that was an aspect the scientific community wanted to delve into. Cameras and spectrometers made it possible to take pictures, to measure the temperature, composition and ionisation of atmospheres, and to count dust particles.

Data collected by New Horizons instruments will allow us to better describe Pluto’s atmosphere, its past history and its future evolution. Solar winds, formed by particles emitted by the Sun and crashing into Pluto’s atmosphere, contribute to the ejection of a part of this atmosphere into the intersidereal space, and they do so easily, because Pluto’s gravitational force is small.13 On the other hand, Pluto’s strongly elliptical trajectory suggests periods in which solid matter sublimates in the surface, while Pluto is approaching the Sun, followed by periods when Pluto moves away from the Sun and the opposite phenomenon—solid matter solidifies and is deposited on the surface of the planet in the shape of ice dust—takes place. All these phenomena indicate a periodical modification of the atmosphere’s composition as well as a long-term evolution.

Telecommunication issues

Good communication quality between the space probe and the Earth is essential for the success of such a project. At the speed of light, it takes 4h 30 to transmit information through space from Pluto. But that is not the only factor to take into account. New Horizons sends signals to receiving antennas which are part of the DSN (Deep Space Network) device.5 The antennas are positioned in three DSN centres: in Canberra (Australia), in the Mojave Desert in California (USA) and to the west of Madrid (Spain).

DSN must keep in touch with more than 40 missions6 launched by NASA and other space agencies from around the world.

New Horizons is only one among them, which limits the availability of the receiving antennae. Transfer speed is, all in all, limited to 2 kb/sec.

On 14th July 2015, New Horizons was at its closest to Pluto. The probe passed by the planet at a height of 11,000 km. It was necessary to take full advantage of that passage to aim the measurement instruments to the main objects of interest. The priority was for the probe to collect as much data as possible. Once the most optimal observation phase is over, the transmission of the yields will be done in a second stage. Besides, in order to transmit to the Earth, the probe must have a specific orientation. It is thus necessary to separate the time for data collection and the time for transmitting the data to the Earth. So, during certain time intervals, New Horizons cuts its communication to the Earth, focusing completely on measuring, collecting data and taking pictures. By the end of October 2016, i.e. after 15 months, New Horizons finally sends the last bytes of data collected during its flight over Pluto.

Specialists also hypothesised there is an atmosphere exchange system between the two bodies. Data collected by New Horizons should let us know.7,8,9

Astrophysicists usually analyse another phenomenon: the refraction of light passing from the void to an environment containing matter.

Like in the measurements of the 80s, astronomers would like to see again Pluto occulting a star. This time, the goal would be to measure Pluto’s atmosphere refraction index: instead of traveling in a straight line, light slightly changes its direction when it penetrates the atmosphere, as it would when passing through the surface of water. The lag allows us to infer the atmosphere refraction index.10

Sun eclipsed by Pluto: the atmosphere is blue (source).

But why do it from the Earth in relation to a star whatsoever? New Horizons has all the necessary instruments to do it and, once it passed Pluto, we could for the very first time take this measure using a very particular star: our own Sun.

This is a vision that we, Earthlings, had never been able to have, as one must be further away than Pluto in the Solar System to see such an eclipse. And New Horizons has sent a magnificent picture of Pluto with back lighting as it passes in front of the Sun. As with the Earth, the atmosphere gives Pluto a magnificent blue halo.

Continuation of the mission

New Horizon’s encounter with Pluto was a success8,11,3,12 and this favours the probe. The team decides that the mission will continue by sending its device to an object in the Kuiper belt that had not even been discovered when the New Horizons probe was sent: 2014 MU 69, discovered in 2014. This is from now on New Horizons’ new objective. The next encounter is expected on 1st January 2019. But in the meantime new events allowed us to know the object a bit better before New Horizon flies over it. Indeed, during summer 2017, the occultation of a star by “2014 M69” has been observed. From this observations, we try to figure out 2014 M69’s shape, to know whether it is surrounded by an asteroids belt and whether it has an atmosphere. Data analysis is in process.


  1. On Kuiper belt’s binary systems, see: and Fraser, Wesley C., et al. "All planetesimals born near the Kuiper belt formed as binaries." Nature Astronomy 1.4 (2017): 0088. 

  2. Pluto’s lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations Lellouch, Sicardy, de Bergh, HU Käufl, S Kassi, A Campargue, Astronomy & Astrophysics, jan 2009 

  3. Conference quoted from Sciences: 

  4. John Hopkins University site: 

  5. About Deep Space Network: 

  6. Missions monitored by the Deep Space Network: 

  7. On gaseous exchanges: Hale, J. P. M., and C. S. Paty. "Pluto–Charon solar wind interaction dynamics." Icarus 287 (2017): 131-139. 

  8. Icarus Science Journal/ Volume 287, Pages 1-334 (1 May 2017) Special Issue: The Pluto System Edited by Richard P. Binzel, Catherine B. Olkin, Leslie A. Young and Philip D. Nicholson 

  9. Rarefied gas dynamic simulation of transfer and escape in the Pluto-Charon system: William A.HoeyaSeng KeatYeohaLaurence M.TraftonbDavid B.GoldsteinaPhilip L.Varghesea, Icarus Volume 287, 1 May 2017, Pages 87-102 


  11. NASA’s website: 

  12. Starn,A. "Pluto's Secrets Revealed: NASA's New Horizons changed everything we thought we knew about this distant planet" Scientific American (2017). 

  13. Brain, D. A., et al. "Atmospheric escape from unmagnetized bodies." Journal of Geophysical Research: Planets 121.12 (2016): 2364-2385.