The diminutive US spacecraft New Horizons visited the distant Pluto-Charon system five years ago, on 14 July 2015. As it flew by, it performed unique scientific measurements and sent sensational images to Earth, revealing the binary system's turbulent past and unexpectedly dynamic development far from the Sun. The fifth anniversary of this Pluto flyby is a welcome opportunity to review this unique feat of aerospace engineering and the scientific findings of an outstanding mission to explore icy celestial bodies on the outer fringes of the Solar System. German planetary scientists are involved in New Horizons’ Radio Science Experiment (REX).
Clyde Tombaugh (1906–1997), an American junior astronomer at the Lowell Observatory in Flagstaff, Arizona, made the discovery of his life on 18 February 1930 when he identified a long-sought object, the suspected planet X, beyond the orbit of Neptune. This new, ninth 'planet' – a comparatively small celestial object with a diameter just shy of 2,400 kilometres – was soon named Pluto. It became the new planetary outpost of our solar system, a status that Neptune had previously held since 1846. Pluto held this role until 1992, when the Mauna Kea Observatory in Hawaii detected a trans-Neptunian object (TNO) even further away than Pluto, a small irregularly shaped body with a diameter of only 100 to 150 kilometres. Additional TNOs were discovered in quick succession, and their numbers have continued to grow, with over a thousand identified to date. For planetary researchers, these TNOs are highly informative natural history archives, although they also raise numerous new questions about the origins and development of the Solar System.
The long journey of NASA's New Horizons mission from inception to destination
It wasn't until around half a century after Pluto's discovery that NASA considered dispatching a small spacecraft to explore the dwarf planet on the outer fringes of the Solar System. The identification of Pluto’s moon Charon, first noticed by the American astronomer James W. Christy from the US Naval Observatory while studying high-resolution images in June 1978as a protrusion from Pluto that moved periodically around the image centre over 6.4 days, undoubtedly prompted initial tentative planning. With a diameter of 1,212 kilometres and one eighth of the mass of Pluto, Charon is a comparatively large and massive satellite of its 'planet', which is why they are often referred to as the Pluto-Charon binary system.
However, it was not until December 2000 that a proposal was brought to sufficient maturity to design a spacecraft mission not only conceptually but in terms of the engineering required. The name of the mission, 'New Horizons', came to Mission Director Alan Stern during a mountain hike when he let his gaze wander from a summit to the horizon. Another five years passed before New Horizons took off for Pluto from Cape Canaveral on 19 January 2006. The spacecraft finally reached its destination in July 2015; postponing the launch by only a month would have delayed arrival by five years!
Mass, acceleration, astronomy: A race against time
Navigating the labyrinth of discussions and decisions that are necessary to get the spacecraft to its perch at the tip of a rocket on the launch pad before the countdown begins can prove more difficult than the planning and technical implementation of the actual scientific instruments. In the end, it was the astronomy of the situation that accelerated the development of New Horizons on the ground. Pluto follows a highly eccentric solar orbit, with its perihelion at 4.5 billion kilometres. The celestial body passed this nearest point to our star in 1989 and has since moved further away. Its aphelion is at 7.4 billion kilometres and Pluto takes 248 Earth years to complete one solar orbit. Any additional delay would therefore have meant an increasingly long journey to Pluto. A delay would have also restricted the scientific capabilities of the mission because, as the planet drifts further away the Sun, Pluto's wafer-thin atmosphere condenses and falls as ice onto the surface, making it impossible to study.
In order to reach Pluto at all, the spacecraft needed to be designed with as little mass as possible. Including fuel, the spacecraft's final mass was less than 500 kilograms. In addition, the Atlas launch vehicle was equipped with an additional thrust stage that accelerated New Horizons to an escape velocity of 16.21 kilometres per second (58,356 km/h), the highest speed at which a spacecraft had ever departed Earth. What followed was a nine-and-a-half-year journey and a swing-by manoeuvre at the giant planet Jupiter to further accelerate the spacecraft. Then, five years ago at 13:50 CEST on 14 July 2015, New Horizons – now some 4.8 billion kilometres from home – flew barely one Earth's diameter over Pluto’s surface and past its moons.
Through the eye of the needle: Through the orbital plane of Pluto's moons
New Horizons had to fly almost vertically through the orbital plane of Pluto's moons Charon, Nix, Hydra, Kerberos and Styx. This caused great uncertainty during planning, as there might have been other moons or rings of ice and rock in this plane, concealed from telescopic imaging and hazardous for the spacecraft. But observations obtained just a few days before the flyby brought welcome relief: the photographs did not show any new obstacles. Every observation and measurement to be made during the flyby was planned to the second beforehand during years of arduous programming. Seven on-board scientific experiments were deployed within just a few hours. In addition to the three optical devices – the UV spectrometer Alice and the high-resolution camera systems LORRI and Ralph – the two plasma instruments PEPSSI and SWAP, the dust detector Venetia, and the radio experiment REX took detailed measurements of this distant, ice-cold world. REX is the only instrument on New Horizons involving German scientists, namely planetary researchers from the Rhenish Institute for Environmental Research at the University of Cologne (EURAD; see info box). Their participation was financed by Space Administration at the German Aerospace Center (DLR) using funds from the Federal Ministry for Economic Affairs and Energy (BMWi).
A heart of ice, muddy ground, mountains of crystalline ice, and a detergent
The on-board cameras observing Pluto and its lunar system brought to light a bizarre world with a turbulent past and perhaps even a dynamic present – a result whose magnitude no scientist had expected . Alan Stern’s excitement when the first close-ups appeared on the screen is legendary among planetary researchers. The scientists had waited decades for this moment. Pluto and Charon had remained largely mysterious until then; this distant world is too small even for the Hubble Space Telescope, which merely showed a dim variation in brightness between two miniscule discs of light. On average, only five ten-thousandths of the sunlight that falls on the Earth arrives at Pluto, and the daytime temperature is minus 234 degrees Celsius.
Most atmospheric gases freeze into their individual constituents at these temperatures. Pluto is therefore covered by a mixture of frozen water, nitrogen, methane, carbon monoxide, carbon dioxide and ammonia and – when still ‘close’ to the Sun – is enveloped by a wafer-thin atmosphere. It consists of nitrogen with some carbon monoxide and methane. As the new images taken towards the horizon showed, it is stratified and extends to an altitude of 1,600 kilometres. The average density of Pluto is 1,860 kilograms per cubic meter. From this, researchers deduced that Pluto consists of more than two-thirds rock and 30 percent ice in varying compositions. Once the dwarf planet had formed, the rock mass was therefore large enough to generate sufficient heat through the decay of radioactive elements on its inside to create a rock core at the centre, surrounded by a thick layer of ice. More than that, there might be enough available energy to drive dynamic processes in the ice layer, even today.
The scientists found themselves gazing at a dark surface strewn with impact craters, which probably froze four billion years ago, but also at vast swathes of bare ice, unblemished by craters, that have existed for less than 100 million years or in some cases had formed just recently. The numerous polygons carving a honeycomb-like pattern in the ice of the Sputnik Planitia are thought to be solidified convection cells in which liquid or solid material rose to the surface and separated during the freezing process. This expanse of ice, as if purposely designed by nature to attract the media, stretches over one thousand kilometres in total, with the outlines of a heart that was given the name Tombaugh Regio after the person who first identified Pluto.
Another surprise was the discovery of the gigantic Norgay and Hillary Montes, high-mountain ranges with summits of up to 3,500 metres. However, they are not made of stone and rock, but consist instead of an extremely hard, low-temperature form of water ice that prevents the base of these frozen giants from melting under their own weight. The dark, reddish-brown material in the north of Pluto, which is also found on Charon, probably consists of a complex mixture of organic molecules such as carbon, nitrogen and hydrogen, which are formed in the atmosphere of gas planets, moons or comets when the surface material interacts with ultraviolet radiation and the particles of the solar wind. They are called 'tholins' (Greek for 'muddy'). On Charon, the scientists also identified water ice crystals containing additional traces of ammonium hydroxide (NH4OH) – a substance that is widely known as ammonia and used as a household detergent.
The next trick: Flying past 30-kilometre Arrokoth on New Year's Day 2019
After its successful and – from a scientific perspective – immensely productive flyby of Pluto, New Horizons continued its voyage through the Kuiper Belt. This tubular region surrounding the eight planetary orbits, sometimes referred to as the Edgeworth-Kuiper Belt, is the cosmic home to icy, sometimes extremely ancient bodies ranging in size from a few kilometres to several thousand kilometres in diameter. Dwarf planets such as Pluto, Eris, Makemake and Haumea are among the known objects. The Kuiper Belt directly adjoins the outer planet Neptune and extends to a distance of approximately 18 billion kilometres from the Sun. It is also the reservoir for most short-period comets. Even taken together, all objects found in the Kuiper Belt have a combined mass equivalent to only a fraction of Earth's.
Even before New Horizons arrived at Pluto, the Hubble Space Telescope had already spotted another trans-Neptunian object that appeared suitable for a relatively close flyby after its rendezvous with Pluto. The new object was initially given the label 1110113Y but was officially designated 2014 MU69 in May 2015 once its orbit had been determined with sufficient accuracy. Sifting through the submitted proposals, the New Horizons team selected 'Ultima Thule' as a temporary name in March 2018, based on the Earth's northernmost land point at the upper tip of Greenland, a place that throughout human history has given birth to as many legends as Atlantis. It has since been renamed 'Arrokoth', which means 'sky' in the Algonquin language. Arrokoth orbits the Sun at a distance of between 6.4 and almost 7.0 billion kilometres and is currently located around 44 Astronomical Units (1 AU = 150 million kilometres) from the Sun.
In 2017 and 2018, observations of Arrokoth's stellar occultations provided the first indications that the object may consist of two bodies orbiting each other. SOFIA, the airborne observatory operated by DLR and NASA, also played a key role in these measurements. The promising preliminary investigations were confirmed when, on 1 January 2019, New Horizons passed Arrokoth at a distance of just 3,000 kilometres and at a speed of 14.3 kilometres per second (51,500 km/h). The spacecraft sent back images reminiscent of the shape of the dual lobes of comet 67P/Churyumov-Gerasimenko, on which the European Rosetta lander, Philae, had landed five years earlier.
Only a detailed analysis revealed the difference: while the comet's nucleus as a whole has a certain spatial depth, the two parts of Arrokoth are rather flat but, like 67P, do not have many craters. The latter aspect is a strong indicator that the surface could yield insights into the early days of our solar system, as there have likely been few changes due to chemical reactions caused by sunlight or the energy released during impacts since the object formed. Indeed, as planetary scientists had suspected, a thorough geological analysis of the images revealed structures considered to be the very first ‘building blocks’ in the formation of two separate bodies.
The images obtained during the flyby show that Arrokoth initially consisted of two individual bodies, which have since grown together along their longitudinal axes to form a 31-kilometre, peanut-shaped 'contact binary': a body resulting from the connection of two initial bodies through contact. This is probably a common process in the outer Solar System, as analyses suggest that comet 67P/Churyumov-Gerasimenko, which was visited by the European Space Agency's (ESA) Rosetta probe, also formed due to gentle contact between two smaller, original bodies. The two rust-brown bodies do not reveal any colour variance, with only the barely discernible 'neck' in the contact zone appearing far brighter.
Spectral measurements indicate that the reddish colouring is caused by the compounds methanol (CH3OH), hydrocyanic acid (HCN), water ice, and hydrocarbons. The immense distance from Earth at which New Horizon is travelling means that the transmission of data from the flyby of Arrokoth is not yet complete, even 18 months later, and will continue until the end of 2020. Only then will the final results be available.
First fixed star parallax measurement from a spacecraft
New Horizons set another record just recently. Beyond Arrokoth's orbit, at a distance of around 47 AU, the spacecraft observed two nearby stars, enabling the first interplanetary parallax measurement. The experiment’s results were announced on 11 June 2020.
On 22 and 23 April of this year, the spacecraft – travelling far from Earth and the Sun – imaged of two stars that are relatively close to our solar system, Proxima Centauri and Wolf 359 in the constellation of Leo. A comparison of the images taken by the spacecraft and those obtained simultaneously on Earth clearly reveals how the two stars occupy slightly different positions relative to the background stars, which are over 100 times further away, in each set of images. In astronomy, this is called the parallax effect.
"The trigonometric parallax method has played, and continues to play, a key role in astronomy because it can be used to determine the distance to nearby stars within a radius of approximately 100 light years," explains Manfred Gaida, astronomer and researcher at DLR. "This method of determining interstellar distances is the first step in the process of cosmic range-finding."
The principle underlying this measuring method is extremely simple and is comparable to the way that the thumb of an outstretched hand will appear to jump back and forth if viewed alternately through the left or the right eye. This phenomenon is caused by the distance between the centres of the pupils and the difference in viewing angle that results. For astronomers, the equivalent distance is twice the radius of Earth's orbit around the Sun, approximately 300 million kilometres. This is equivalent to double the semi-major axis of the Earth's orbital ellipse, or two AU. If the position of a relatively close star is measured from two positions far apart on the Earth’s orbit, for example in spring and autumn, the distance to the star can be determined by means of trigonometry.
Stellar parallax with a baseline of seven billion kilometres
New Horizons enabled researchers to increase the baseline from 1 AU to over 47 AU. This means that Proxima Centauri and Wolf 359 'jump' noticeably back and forth in front of the more stationary background stars when the Earth and spacecraft images are viewed alternately – a hitherto unseen and unique visualisation of the stellar parallax effect that was not possible before. Alan Stern, the New Horizons project director, and his team have therefore achieved an impressive feat of experimental astronomy.
Parallax angles are very small – less than one arcsecond relative to the Earth’s radius. Moreover, the parallactic effect is overlaid by many other physical phenomena such as the star's aberration and proper motion. As a result, it took a long time to complete the first successful determination of a star's parallax, but this measurement paved the way to an appreciation of the true scale of the universe. Over the centuries, many attempts were made to fathom the distance between stars, but the real breakthrough did not come until 1835. Working independently, the three astronomers Friedrich Georg Wilhelm von Struve (1819–1905), Friedrich Wilhelm Bessel (1784–1846), and Thomas Henderson (1791–1844) carried out parallax measurements almost simultaneously. From his base at Dorpat Observatory, Struve picked the bright star Vega in the constellation Lyre; in Königsberg, Bessel focused on star 61 Cygni in the constellation Swan; and Henderson used Alpha Centauri at the Cape Observatory. In 61 Cygni, Bessel chose a strong candidate, as its high tangential proper motion of 5.2 arcseconds per year suggested that it might be relatively close to the Sun. The first reliable determination of a stellar parallax was later ascribed to Bessel due to the extensive data he produced, and without dispute from Struve or Hendersen.
Bessel began his investigations in Königsberg, hometown of German philosopher Immanuel Kant, in August 1837. He conducted his observations using a heliometer made by the Bavarian technician Joseph von Utzschneider (1763–1840) and the celebrated Munich-based manufacturer of optical lenses, Joseph von Fraunhofer (1787–1826). This instrument enabled very accurate measurements of the small angular distances between 61 Cyg and two neighbouring stars. By October 1838, Bessel had made a total of 183 measurements of the positions of the two adjacent stars and was therefore able to determine a parallax of 0.3136 ± 0.0202 arcseconds for 61 Cyg, a value that has since been corrected to 0.2859 ± 0.0001 arcseconds. By taking the reciprocal of the arcsecond value, this corresponds to a distance from the Sun of 3.5 parsecs, or just under 11 light years, making 61 Cygni one of the 20 stars closest to our Sun.
A distance of 11 light years corresponds to an unimaginable 104 trillion kilometres. The distance to the star closest to the Sun, Proxima Centauri, is somewhat smaller at 'only' 4.2 light years – or 40 trillion kilometres. Wolf 359 is just under eight light years from the Sun. Compared to the Milky Way’s diameter of a hundred thousand light years, these distances are tiny, but still vast in comparison to those encountered in our solar system.
Located in the southern celestial hemisphere, Proxima Centauri cannot be observed from Europe, but only from locations South of the 27th parallel. The star has an apparent magnitude of approximately 11 and was discovered in 1915 by Robert Innes (1861–1933) in Johannesburg using an astrograph. Innes found that the proper motion of the newly discovered star was nearly the same as that of Alpha Centauri and concluded that the two stars were gravitationally related. He also suspected that Proxima Centauri was a little closer to the Sun than Alpha Centauri, which itself is a binary star system. However, this was not proven until later by the American astronomer Harold Lee Alden (1890–1964), for whom a crater on the dark side of the moon is named. It has also been known since 2016 that an Earth-sized satellite orbits Proxima Centauri every eleven days.
Also known as CN Leonis or Gliese 406, Wolf 359 is located in the northern celestial hemisphere and is a red dwarf star with an apparent magnitude of 13.5. It is named after Max Wolf (1863-1932), an astronomer from Heidelberg who released a ‘Catalogue of 1053 fixed stars with more pronounced proper motion’ in the publications of the Baden Observatory in Heidelberg in June 1919, the same year in which Albert Einstein became famous for his proof of light deflection at the sun's limb. The 359th star listed in this catalogue is the one which – after acquiring fame through the adventures of the Starship Enterprise – has been brought back into the public eye by the New Horizons mission.
In an Earth-bound parallax measurement, a large number of successive position measurements are made for a particular star over the course of six months or a whole year in order to calculate its distance based on the 'reflection' of the Earth's orbital ellipse on the celestial sphere. In contrast, the current interplanetary parallax measurement involved only two simultaneously obtained measurement points, but they are 47 Astronomical Units apart. Since a parsec corresponds to the distance at which the radius of the Earth's orbit (1 AU) subtends an angle of one arcsecond, this angle – transferred to the Sun/New Horizons baseline – is now, in a first approximation, 47 times greater. Based on 1 AU, the parallax for Wolf 359 is 0.415 arcseconds and 0.769 for Proxima Centauri. Compared to the Earth’s orbital value, 16 arcseconds were now measured for Wolf 359 and 32 arcseconds for Proxima Centauri.
Although New Horizon's parallax measurements do not improve on the already known distances to Proxima Centauri, and are primarily intended to illustrate how the perspective on our familiar view of the starry sky changes at a greater distance from Earth, they do, in the long term, pave the way for new application opportunities such as for interstellar navigation. A spacecraft carrying a star catalogue containing the Earth-based locations of stars as a reference could make their own parallax measurements to navigate safely through interstellar expanses, just as sailors once used the stars to traverse uncharted seas. But the New Horizons measurements have also heralded a renaissance for the idea originally proposed by the legendary American physicist Freeman Dyson (1923-2020). Dyson suggested systematically deploying spacecraft for stellar parallax measurements with a larger baseline than the Astronomical Unit. Clyde Tombaugh would undoubtedly be delighted at the pursuit of these plans. The small spacecraft, carrying some of Tombaugh's ashes to the dwarf planet he discovered and beyond, has certainly lived up to its name and truly opened up new horizons for us all.