Sample return - first class spaceflight
If there is one industry or scientific discipline in 2020 in which the infamous coronavirus pandemic has left relatively few traces, it is space exploration. Everything rises and falls; Newton and Kepler send their regards. There is simply no way around it. When expensive metal boxes are in orbit around Earth or out in the depths of the Solar System, with or without valuable human passengers, someone on the ground has to make sure that the mission continues. For obvious reasons, control of it must not simply be given up, as in most cases this would lead to the total loss of the very valuable spacecraft.##markend##
This is why the end of 2020 still offers two very special space technology highlights. In a way, they will crown (from the Latin - corona; that’s when it happened!) this messed-up year. Two robotic missions will return to Earth with extraterrestrial material. This is what the experts refer to as ‘sample return’, even though, of course, no samples are being returned to their own point of origin, but rather are being be brought back to us on Earth by the returning spacecraft and transported to laboratories after landing.
On Saturday evening in Europe, or Sunday morning before sunrise in Australia, a sample capsule will float down with a parachute into the desert of the Royal Australian Air Force's Woomera Range Complex. It contains valuable cosmic cargo - dust from the asteroid Ryugu, which is four-and-a-half billion years old. The Japanese spacecraft Hayabusa2 studied the asteroid for two-and-a-half years, releasing the Franco-German (CNES-DLR) MASCOT lander, which bounced across the kilometre-sized asteroid on 3 October 2018. Hayabusa2 acquired samples at two locations with the aim of then delivering them to Earth. ‘Deliver’ is deliberately chosen wording, because Hayabusa2, after the releasing the 16-kilogram landing capsule, will manoeuvre away from its collision course with Earth to an orbit around the Sun. From there, it will begin its journey to visit another asteroid, only 40 metres in size and rapidly rotating, in 2031.
A few days later, likely during the following week, a similar scenario will take place in Inner Mongolia. There, the sample capsule of the Chinese lunar mission Chang'e-5 will float down from the sky into the winter cold of the Central Asian steppe, filled with two kilograms of lunar samples. Two extraterrestrial samples in just one week - you can almost hear scientists across the world cheering. Cause enough to present here the robotic sample-return missions that have taken place actually very rarely over the 63-year history of space travel. We will also take a look at the largest extraterrestrial sample collection ever gathered, made possible by the fact that it was placed on the spacecraft, and thus on Earth, by human hands. This is, of course, the 382 kilograms of lunar samples collected by 12 Apollo astronauts across six missions between 1969 and 1972.
A 'bracket' of lunar geological chronology
In the afternoon of 2 December 2020 (CET), Chang'e-5, the fifth mission in the series named after the Chinese Moon goddess, landed in the northwest of the Moon's Earth-facing side. More precisely, it landed near Montes Rümker, on the dark plains of Oceanus Procellarum, the 'Sea of Storms'. The volcanic complex, which is named after the German astronomer Carl Rümker (1788-1862), has fascinated lunar geologists ever since it was discovered by the Lunar Orbiter spacecraft in 1966/67, and particularly since the exciting oblique view recorded from the window of Apollo 15 by Command Module Pilot Alfred Worden in July 1971. With samples from this region, lunar researchers can now do what they could not with the sample material collected by the Apollo missions, namely, transport volcanic rocks as young as possible to Earth. Young for the Moon, which has been only barely geologically active for three billion years, means between one and two billion years old.
This was actually planned in the 1970s for the Apollo 18-21 missions, for which the 'hardware' had already been produced. But US President Richard Nixon decided in 1970 that Apollo 17 would be the final mission - first and foremost for financial reasons, but also because Nixon was somewhat angry that he could no longer derive any political benefit for himself from further Moon landings. This led to a scientific deficit, as it was not possible to complete the lunar chronology. The beginning (samples of the oldest rocks) and the end (from the last volcanic activity on the Moon) were missing.
Everything in between was recorded quite well across the six missions that returned lunar samples and provided precise calibration points for the lunar 'impact chronology' - the age curve of geological units - by means of chemical isotope measurements. This is based on the number of impact craters of all sizes on these units. Many craters mean a unit is old, few craters indicate that it is young. Everything is initially relative, but with the age measurements conducted in a laboratory, it can also be determined absolutely. Chang'e-5 could now provide another 'anchor' in this chronology, as Mons Rümker is one of the youngest geological structures on the Moon. It is looking good; Bill Hartmann, one of the great American lunar geologists and chronologists, has examined the landing site using high-resolution orbital photographs and has made a statement to the community that the region could be dated to even just 1.7 billion years according to the methodology and models of the late DLR planetary researcher Gerhard Neukum (1944-2014). Parts of these lava plains could be even younger. This immediately set the pulse of the lunar research community racing.
China conducts successful activities on the Moon one step at a time
It is not yet known exactly when Chang'e-5 will return to Earth. The design of the mission is not unlike the Apollo programme - flying to the Moon, uncoupling the lander module and descending to the lunar surface, ascending to the orbiter again after the task is complete, docking, returning to Earth, separating the lander capsule again and descending through the atmosphere. This is certainly not a coincidence, as the Apollo design made it possible to fly to the Moon nine times, six of them including a landing. And in 2030, as the Chinese space agency CNSA has already announced, the nation also wants to fly to conduct a crewed spaceflight to the Moon. So far, it seems to be on the right track. After all, just the name of Chang'e-5 implies its four predecessor missions. Two successful orbits, a somewhat less robust first lunar landing with a rover on the Earth-facing side of the Moon, and above all a landing with a very robust rover on the far side of the Moon, including a communications satellite to maintain radio contact. Certainly, a strong performance. With Tianwen-1, Mars is now also on the agenda, first via robotic exploration, with arrival scheduled for February 2021.
Space exploration is always complex and demanding, but anyone who can land, drill, dig, stow samples remotely and then bring them back to Earth has placed themselves firmly in the first class of space travel. If we increase the number of people participating in the whole programme, we will be able to go one step further - let us call this the 'imperial class'. Japan already has its second horse in the running in first class with Hayabusa2, the predecessor of which was Hayabusa (without the '1'). Launched in 2003, this first mission collected samples on the 500-metre-long asteroid Itokawa in September 2005, before returning to Earth. Along the way, some things went wrong, but in the end, there was a happy ending and Japanese spaceflight succeeded in saving the mission in a way that was almost unimaginable. After an odyssey with an almost completely 'wingless' craft (two of the three reaction wheels for controlling the orientation had failed), the sample container was successfully ejected over Australia on 13 June 2010. There, also in Woomera, the valuable cargo was recovered. However, the (not entirely unexpected) disappointment was great, as it turned out that the sampling mechanism on Itokawa had not worked as planned. Unfortunately, 'only' 1600 dust grains from the asteroid could be sent to the laboratories - including the DLR Institute of Planetary Research, where they were examined under Ute Böttger’s microscope and analysed using Raman spectroscopy.
50 years of robotic sample retrieval
When did the first samples from another celestial body arrive on Earth? Actually, it happens all the time, and has for 4.5 billion years. It happens when an asteroid or meteoroid collides with Earth and something arrives on the surface that did not burn up due to friction in the atmosphere. Arrival on the surface can be very destructive – dinosaurs would tell us a thing or two about this, if they still existed. 65 million years ago they were wiped out by the impact of an asteroid 15–20 kilometres in size. However, not much is left of the asteroid itself. Most of it evaporated (the energy converted during this process was 10–100 billion times greater than that of the Hiroshima nuclear bomb). But a millimetre-thin, globally traceable layer in Earth’s crust that is enriched with the element iridium (whose isotope ratios reveal that it is not from Earth) reveals the catastrophic event. Meteorites are much more harmless. For scientific research, they also represent important sample material from the early days of the Solar System.
However, spaceflight has made it possible to bring samples to Earth from locations on other celestial bodies that can be precisely chosen, and which are geologically quite well understood. Across all continents there are laboratories, institutions and researchers who, with all the analytical equipment at their disposal, would study this material again and again for as long as they possibly could - with increasingly insightful results. That is why it is so it is so regretful that we can only return with 'hand baggage' when we fly a spacecraft to a celestial body and back to Earth.
The Apollo lunar samples, the 'Holy Grail'
This was achieved for the first time with the Apollo missions - in other words, with humans, not robots. The 12 astronauts across the six missions that landed on the Moon between July 1969 and December 1972, brought back 382 kilograms of rock, dust and (unfortunately few) core samples from the Moon. It was the 'Holy Grail' of planetary exploration. The investigations conducted on these samples revolutionised our view of the formation and development of the inner Solar System – that is, the five spheres with a rocky crust and mantle, and a metallic core.
Little is known here, however, about one spectacular facet of the race to the Moon at that time. Apollo 11 took off on 16 July 1969, followed four days later by the first crewed Moon landing on 20 July (or 21 July in Europe) and Neil Armstrong's famous 'giant leap for mankind'. Three days earlier, however, on 13 July, a proton rocket took off from Baikonur, Kazakhstan, with no people on board. The Soviet Luna 15 spacecraft was to land in the Mare Crisium, approximately 500 kilometres northeast of Tranquillity Base, to take samples and return them to Earth as quickly as possible. The mission failed. Neil Armstrong and Buzz Aldrin were already waiting in the Eagle lander for their return trip up to Michael Collins in lunar orbit (with 21.6 kilograms of lunar rocks and dust on board) when Luna 15 hit Mare Crisium on 21 July at 16:51 CET (perhaps this impact was even recorded by the Apollo 11 Passive Seismic Experiment, PSEP?).
But even if Luna 15 had been successful, the USSR would not have won the sample return race. Apollo 11 would have arrived back at Earth half a day earlier. As already mentioned, five further Apollo missions have contributed to the most extensive stock of extraterrestrial material brought to Earth. What the Soviet Union did not succeed in doing with Luna 15, however, it did succeed at almost exactly 50 years ago in September 1970 with the follow-up mission Luna 16 - and again with Luna 20 (February 1972) and Luna 24 (August 1976). Three robotic Moon landers that successfully collected samples and returned 321.1 grams of lunar rock - NASA does not have this in its trophy collection (although it was never in the programme either, as their focus on the Moon lay entirely with crewed spaceflight). Chang'e-5 will therefore be the first mission in 44 years to transport samples from the Moon back to Earth.
Stardust, genesis - and world peace
What else has been delivered to Earth via spacecraft? Not much! Besides Apollo, Russian cosmonauts have also brought particles back to Earth from space. These were obtained using the legendary Mir (meaning 'world', or 'peace') space station, on whose outer wall the Orbital Debris Collection Experiment (ODC) was mounted for 18 months. In 1996 and 1997, interplanetary dust particles, but also human-made dust located in Earth’s orbit, were trapped within an aerogel.
Capturing non-anthropogenic particles was then the goal of the NASA Genesis mission, launched in 2001, which collected approximately 15,000 solar wind particles from a complex orbit beyond that of Earth. The returned capsule entered Earth’s atmosphere on 8 September 2004, but unfortunately the parachutes did not open, and the capsule slammed into the Utah desert at 300 kilometres per hour and burst. Actually, a helicopter was originally intended to capture the sample container as it hung from the capsule in the air - an already audacious manoeuvre! Due to the unsuccessful landing, the samples were partly contaminated, but a portion could be saved and examined, so the mission was not a complete failure. After all, these were the first samples ever collected from beyond the Moon’s orbit.
Two years later, however, the NASA probe Stardust, launched in 1999, brought a capsule of interplanetary dust to Earth. It collected this across two phases lasting several weeks in 2000 and 2002. It was also able to collect dust from the coma of comet 81P/Wild (Wild 2) as it flew past its nucleus on 2 January 2004. Samples of comets are at the top of the scientific wish list. There are currently discussions about sending a second mission to comet 67P/Churyumov-Gerasimenko - this time with the intent to return samples to Earth. The comet was explored from 2014 to 2016 by the European comet mission, Rosetta, and its lander, Philae. Such a second mission would deepen, solidify - or in some cases perhaps even refute - the countless insights gained thanks to Rosetta.
The next samples are already underway
In this exciting week for the exploration of Solar System bodies, it should be remembered that after Hayabusa2 and Chang'e-5, another spacecraft is on its way to back Earth, its sample container already filled to capacity. On 20 October 2020, the NASA spacecraft OSIRIS-REx placed its sampling tube on the 500-metre asteroid Bennu (which is very similar in appearance to the asteroid visited by Hayabusa2, Ryugu, but belongs to a different class of small bodies) for a few seconds. The photos showed an abundance of material, but the flaps of the sample container could not be closed properly at first, so a manoeuvre to rotate the spacecraft using its 'outstretched arm' to determine the mass of the samples had to be abandoned. Everything is fine with the sample return capsule itself. It is sealed, and NASA expects a yield of between 60 and 2000 grams. In March 2021, the window for the return to Earth will open. As of today, the OSIRIS-REx sample capsule is scheduled to land on Earth on 24 September 2023, again at the US Air Force testing and training facility in Utah.
Fascinating and exciting - but not for the faint of heart
These are exciting times for Solar System exploration with spacecraft, which, as discussed, can occasionally return to Earth, but which, of course, is no easy task. For those responsible, the finale is a moment of enormous tension and, if after so many years of waiting it fails, it can take on the characteristics of a tragedy. A spacecraft like this arrives at Earth travelling at over 10 kilometres per second and slams into the atmosphere ballistically at high speed - that is, without using engines to decelerate. Although it is slowed down considerably, it is also heated up to 1500 degrees Celsius, a temperature that the casing must be designed to withstand, and for which precious metal alloys are used in order to protect the valuable contents. The parachute must unfold, and the capsule must also be found after landing. This is no light entertainment, and certainly not for the faint of heart. But it is wonderful for all, if all goes to plan.
The most exciting project of this kind is already on the horizon. On 18 February 2021, the Perseverance rover of NASA's Mars 2020 mission will land on the Red Planet. It will drill into the rocks as it travels, leaving behind three dozen small sample tubes. These will be collected by a robotic mission provided by ESA in the second half of the decade, and then transported to Earth by a joint NASA-ESA mission. Here too, at the end of the mission, there will be a few minutes of anxiety as the samples are subjected to the hot ride through Earth’s atmosphere. But even before this, and here for the first time ever, there will also be anxious minutes during the ascent of the return craft through the gas envelope of Mars. We must always go one step further.