Currently sailing in the dark and icy regions of the Outer Solar System, the US space agency’s New Horizons probe crossed the 50 astronomical units (AU) mark on April 17, 2021. An AU corresponding to the distance between the Sun and the Earth, or 150 million kilometers, it is thus 7.5 billion kilometers from its place of origin! Difficult, for the human mind, to realize how far the artifact which spins at the speed of 58,000 km / h is today far from us. One of the ways of figuring it out, in addition to these distances which defy the imagination, is the time that the signals take to reach the vessel: almost 7 hours, therefore double (14 hours) for a round trip communication, the radio waves propagating at the speed of light, about 300,000 meters per second.
Artist’s impression of the American probe New Horizons. Credits: NASA / JHUAPL / SwRI
A symbolic and intangible border
Only four other vehicles – also under the American flag – have so far exceeded this very distant milestone. Pioneer 10, the first vessel to fly over the gas giant Jupiter, had reached it in 1990; and Pioneer 11, which passed the same pioneering studies for Saturn – the giant with the rings – the following year. The Voyager spacecraft also crossed this symbolic and immaterial border after having observed Jupiter and Saturn for Voyager 1 but also Uranus and Neptune for Voyager 2. The twin probes are today at 152 and 127 AU from Earth respectively, ie at – beyond the zone of material influence of the Sun, which we call the heliosphere, and in a territory which envelops and gives birth to all the stars: the interstellar medium.
Drawing representing the five probes – including New Horizons – which have crossed the 50 astronomical unit mark. Credit: NASA / Johns Hopkins APL / Southwest Research Institute
Unlike Pioneer ships, they are still functioning and transmitting invaluable data even though some instruments have failed for many years or been disabled to save power. “Four other missions we reach this distance [de 50 unités astronomiques]during the 20e century, but none of them were in perfect health, said on Twitter, astronomer Alan Stern, principal investigator of New Horizons. This is the tremendous legacy of the skills and meticulous care of the people who have designed New Horizons and the pilot for the past 15 years or so.. ”
The star in the shape of a snowman
Launched in 2006 from the Kennedy Space Center in Florida, the probe passed Jupiter the following year before embarking on a long transit to Pluto, its main target located nearly 5 billion kilometers from Earth. She reached it in July 2015, making the first flight over the dwarf planet and upsetting our knowledge of the geology and atmosphere of this icy star orbiting in a large ring-shaped area – the Kuiper belt. Continuing its course, New Horizons joined in 2019 a binary asteroid about thirty kilometers long resembling… a snowman! Located at 43.4 AU (6.6 billion kilometers) and named Arrokoth, it is the most distant object ever flown over. “Sending a ship through the Solar System to explore Pluto and the Kuiper Belt had never been done before New Horizons, said Alan Stern in a statement from NASA. Most of the team have been involved in this mission from the very beginning, when it was still just an idea, and all the while our children have grown up, we and our parents have grown old. …”
Composite image of Arrokoth reconstructed using data recorded by New Horizons. Credit: Nasa
500 scientists and engineers mobilized on this project
But while New Horizons, in perfect working order, has nuclear fuel for at least another fifteen years and is currently looking for a new target, NASA is already preparing the relief to reach even greater distances. She has been working for almost four years on a mission project that could reach 1000 astronomical units in fifty years! Led by Elena Provornikova of the Applied Physics Laboratory at Johns Hopkins University in Maryland, the project has been tentatively called Interstellar Probe and involves 500 international scientists and engineers. It has just entered its final year of preparatory studies and the most recent advances were presented at the annual congress of the European Union for the Geosciences (EGU), held from April 19 to 30, 2021.
A land that humanity has never reached
“Interstellar Probe would slip away towards the deep interstellar medium, unknown territory that humanity has never reached yet”Elena Provornikova told EGU. The probe, which would approach the Sun only 1.6 million kilometers away to use it as a “gravitational catapult”, would leave the heliosphere and reach the interstellar medium after about fifteen years, ie much more quickly than Voyager probes. It could be launched in the early 2030s and thus would not reach its final objective until 2080. “We would then take, for the first time, an image of the heliosphere from the outside and know what our solar system looks like.”, Notes Elena Provornikova.
Some characteristic distances between the Sun and Proxima Centauri, the closest star. Credit: Johns Hopkins University