North Rhine-Westphalia has the Netherlands, Schleswig-Holstein has Denmark and Bavaria has Austria. What we are referring to is the most popular destination for a quick get-away. After only a brief journey the street signs look different and the people speak a different language or - in the case of Austria - at least a different dialect. Seen in terms of space, this equates to the inhabitants of planet Earth spending their holidays on the Moon. Astronauts frequently describe Earths's satellite as a “dream destination”, even though it has neither palm trees nor unspoiled beaches. After all, sand is present on the Moon in large quantities: in a particularly fine-grained, electrostatically charged, sharp-edged form. Like the holiday regions close to the border, compared to the other celestial bodies in our solar system the Moon is overrun by astronauts in spite of its inhospitable environment and covered with the remnants of human visits. Which means, that there have been people on its surface before. However, all they spoke was English and and all they set up were various scientific instruments, but no strange street signs. Still, there are seven other planets in our solar system with about 200 other moons, as well as countless asteroids and other small bodies that no human has ever visited offering destinations “off the beaten track”.
What other planets are out there?
The question as to what other planets are out there and what conditions are like on them is something that humankind has been pondering for many millennia. Answers were first sought through observation with the naked eye, then with increasingly powerful terrestrial telescopes and finally, since the 1960s, with space telescopes, probes and astronautical missions. Although it should be admitted that astronauts have so far not ventured beyond the Moon. But why? Is it not possible for us to journey to other planets as well? Or even to the Sun?
The sun: “I definitely want to spend my holidays in the sun!”
All those who still adhere to the geocentric view of the world are in for a great disappointment: the Sun - and not the Earth - is the center of our solar system, around which all other planets, moons and small bodies orbit. And: the Sun is an average-sized star that is located in a disc-shaped galaxy known as the Milky Way along with billions of other stars.
But in our solar system the Sun is easily the dominant object. Indeed, it accounts for 99.86% of the mass of the solar system. And it’s incredibly hot. In its interior, where the nuclear fusion of hydrogen into helium takes place, temperatures exceed 15 million degrees Celsius. On the surface, temperatures are significantly lower but still in excess of 5,500 degrees Celsius. So much for a vacation in the sun.
Despite the extreme heat and intense radiation in its vicinity, several probes have already been sent to the Sun. The first one, Helios 1, was launched on December 10, 1974. Helios 1 and its twin, Helios 2, which was launched in January 1976, have come the closest to the Sun to date, coming within a distance of around 45 million kilometers.
Missions to the Sun pose great challenges for the thermal systems fitted to the probes. Strategies for preventing overheating include finding a shape that reduces exposure to radiation, the installation of reflectors and shields, the inclusion of powerful radiators and strong insulation. Having the probe rotate quickly around its own axis while it is in a solar orbit can also prevent overheating.
On August 12, 2018 Parker Solar Probe, a solar probe developed by NASA, was launched. The probe will observe the Sun at close range, coming within a distance of around six million kilometers for the first time in December 2024. At this distance, the temperatures exceed 1,400 degrees. To withstand this extreme heat, the probe is fitted with a thick hexagonal, thermal protection shield behind which the solar modules and antennas can be folded back. The purpose of the mission is to explore the Sun’s outer atmosphere, which is known as the corona.ESA is also currently working on a solar orbiter, which is to be launched in 2020 and positioned in its target orbit around the Sun in 2023. The probe will investigate the space weather emanating from the Sun from an increasingly polar orbit.
Mercury: “But at night it has to cool down, otherwise I can’t sleep!”
Mercury is the smallest planet in our solar system. It is also the one closest to the Sun, orbiting it more quickly than any of the other planets. It takes Mercury only 88 Earth days to complete one full orbit. However, a day on Mercury does not last 24 hours as on Earth but about 1407.5 hours, which is equivalent to a good 58 Earth days.
Like Earth, Mercury is a terrestrial planet, which means that it has a solid surface, has been shaped by its own gravity into a sphere, and consists mainly of rock. Since it has no atmosphere to speak of, Mercury’s exterior appearance resembles that of the Moon, littered as it is with impact craters. As far as we are aware, however, its internal structure is similar to that of geologically active Earth and, although it is significantly smaller, its gravitational acceleration of 3.70 meters per second squared roughly corresponds to that of Mars due to its higher density. By comparison, Earth has a gravitational acceleration of 9.81 meters per second squared. Accordingly, a man weighing 80 kilograms on Earth would have a weight of only 29.6 kilograms on Mercury.
Mercury does not have any moons of its own. In fact, one theory posits that Mercury is itself a moon that once orbited around Venus.
The absence of any atmosphere means that solar radiation hits the surface virtually unfiltered, but also that the heat absorbed during the day is immediately radiated back into space at night. This results in higher temperature differences between the day and night sides than on any other planet in the solar system: whereas daytime temperatures sometimes exceed 430 degrees Celsius, the surface cools to below -170 degrees Celsius at night.
These temperature differences, together with the intense radiation, are the reason why Mercury is the least explored planet in our solar system to date. Even observation from Earth is difficult due to the planet’s proximity to the Sun. It is almost impossible to observe Mercury from an Earth orbit as objects that are in the immediate vicinity of the Sun cannot be viewed with space telescopes. Anyone who has ever tried to light a fire with a magnifying glass on a sunny day will know why.
Similarly, space probes can only reach Mercury with great effort – even though on average the planet is closer to Earth than Venus. Since the probe must fly towards the Sun but takes off from Earth, which orbits the sun at a speed of 30 kilometers per second, the speed must be altered radically in order for the probe to enter an elliptical transfer orbit leading it close to Mercury. In addition, the potential energy of the probe is successively converted into kinetic energy during the flight (or, to be more precise, the “fall”) towards the Sun, which leads to a constant increase in flight speed. If this is not corrected by braking, the probe overshoots and literally flies “into the sun”.
Another obstacle is the lack of atmosphere. In a vacuum, a feather falls as fast as a bowling ball and a probe with a parachute as fast as a probe without a parachute.
Due to these difficulties, only two space probes have visited Mercury so far, while another one has been launched and is expected to pass by Mercury for the first time on October 2, 2020:
NASA’s Mariner 10 probe was launched on November 3, 1973 and passed Mercury three times between March 29, 1974 and March 16, 1975. It photographed a total of 45 % of Mercury’s surface in 9000 individual images.
The second NASA probe to reach Mercury was MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging); it was also the first and, to date, the final Mercury orbiter. After being launched on August 3, 2004 the probe swung into an orbit around Mercury on March 18, 2011 with an extended braking maneuver during its fourth fly-by. For the first time, it was possible to fully map Mercury from orbit. In addition, frozen water and organic molecules were found in eternally dark craters on the planet’s surface.
The third Mercury probe, named BepiColombo, was launched on October 20, 2018 from the European space center in Kourou, French Guiana, in a partnership between ESA and the Japanese space agency JAXA. After completing several swing-by maneuvers on Earth, Venus and Mercury, the probe will enter into an orbit around Mercury by the end of 2025. Various scientific investigations will be carried out during the flyby maneuvers and from orbit. Thus, Mercury’s surface is to be mapped in different spectral ranges and examined to determine its height relief. As well as this, information about the inner structure of the planet and its gravitational and magnetic fields is to be collected. Among other things, it is hoped that this will clarify whether Mercury has a liquid or a solid nucleus.
Two OHB instruments are flying to Mercury as part of the BepiColombo mission:
Working in cooperation with the Italian space agency ASI and the Instituto di Astrofisica e Planetologia Spaziali (IAPS), OHB Italia has played a key role in the development and assembly of SERENA. SERENA is a combination instrument consisting of four different particle detectors which will analyze the interaction of Mercury’s surface with its exo- and magnetosphere by tracking particle movements.
OHB System AG assembled another combination instrument known as MERTIS for the German Aerospace Center (DLR). An imaging spectrometer for determining the mineralogical composition of Mercury’s surface, MERTIS will be collecting high-resolution data on thermal infrared radiation. At the same time, the temperature and thermal conductivity of the planet’s surface are to be measured using a micro-radiometer in order to produce a temperature map.
These efforts will undoubtedly confirm what has already been suspected for quite some time: Namely, that the temperatures close to the sun are too extreme for a visit by human astronauts. So, there’ll be no vacation on Mercury, not even in the distant future. But what about Venus and Mars? After all, these are Earth’s neighbors.
More about this in the next part.