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From the James-Webb Space Telescope, which has just been launched by Ariane to reach its operational site 1.5 million km from Earth, to exploration missions of planets like Mars or Jupiter more than 600 million km away, the US and European space agencies are opening windows in the universe.
James-Webb: from pupa to butterfly
The James-Webb Telescope, the largest astronomical observatory in space, was launched on Christmas Day 2021 from the Kourou Space Center in Guyana. It will allow us to go back in time and witness the birth of the first stars and galaxies. The project, prepared for thirty years by the US (Nasa), Canadian (ASC) and European (ESA) space agencies, experienced setbacks and an additional cost of a factor greater than 3: the final budget reached $ 10 billion (including 700 million euro for Europe).
The telescope reached its orbit around Lagrange point L2, 1.5 million km from Earth, on January 24, 2022, after all its components were deployed in a spectacular way (20 m yarrow heat shield, primary and secondary mirrors). The primary mirror has a diameter of 6.5 m; it consists of 18 hexagonal segments.
For the launch, the telescope was folded up like a doll inside the hood of the Ariane rocket. Now that the butterfly is fully unfolded, it takes five months of adjustments and controls. The first astronomical observations begin in June 2022. In February, the telescope made an image of a star, which is actually divided into 18 images, which will allow to adjust the 18 segments with a precision well below a micron.
Unlike its predecessor, the Hubble Space Telescope (HST), which operates in the visible, the James-Webb Telescope (JWST) operates in the infrared, at wavelengths between 0.6 and 28 microns. Because the universe expands and the expansion of space also prolongs the wavelength of the light emitted by distant objects. The farther away the source is, the greater the redshift. In space, seeing is far to go back in time, because the signals emitted by these primordial galaxies cannot travel faster than light. JWST will see galaxies form shortly after the big bang, which goes back 13.8 billion years.
How galaxies are formed: in search of our origin
Although HST also observed very distant galaxies, it could detect only the brightest. With its diameter of 6.5 m (against 2.4 m for HST), JWST will capture seven times more light and will thus reveal the entire population of these young galaxies, which are the origin of the universe’s first glimpse, after the onset of darkness ages. This period, a few hundred million years after the big bang, is called the cosmic dawn. The universe is still mostly filled with neutral, atomic hydrogen gas. Little by little, the ultraviolet radiation from the first stars of the first galaxies will re-ionize the universe; this period of re-ionization will end approximately after one billion years. JWST will allow a breakthrough in this area of knowledge about our origins.
JWST carries four state-of-the-art instruments. Three of them work from 0.6 to 5 microns: Nircam, for making pictures; Nirspec, to observe the spectrum of 200 objects simultaneously; Niriss, a low spectral resolution spectrophotographer studying the temperature, mass, and chemical composition of stars. Only Miri will work from 5 to 28 microns and take pictures and spectra of cold, distant objects. Miri was built by Europeans – much of it in France. Europe thus has 15% of telescopic time.
An essential element in the evolution of galaxies is the presence of a supermassive black hole in their core. But this core is most often hidden by a dense cloud of dust. JWST will make it possible to cut through the dust and better understand what is happening near active nuclei, such as quasars, and how these nuclei regulate star formation in galaxies.
New stars, the atmosphere of planets outside the sun
The infrared JWST will be able to pass through the cocoons of gas and dust that give rise to new stars in our galaxy. Infrared is also the domain of thermal radiation from cold objects, colder than stars like the Sun, which radiate in the visible. This is the domain of dwarf stars, and also of exoplanets, the planets orbiting a star other than the Sun.
For more than twenty-five years now, nearly 6,000 exoplanets have been identified orbiting stars near the Sun. Astronomers are now trying to get to know them better and especially to study the composition of their atmosphere. An exoplanet is said to be “habitable” when the conditions are met for liquid water to exist on its surface, it must not be too close to its star because the water would already have evaporated, or too far, completely frozen.
But nothing beats direct measurement. Thanks to its high infrared sensitivity, JWST will be able to track these exoplanets and, during their transit in front of their star, measure the spectrum of their atmosphere in absorption on the star disk. A single transit may not be enough, but we can combine several transits if the period of rotation around the star is small. This period is equal to one year for the Earth around the Sun, but it is possible to find denser exoplanets, with much shorter periods, in the order of a few days.
This is the case with the Trappist-1 system, forty light-years from Earth, where seven rocky planets have been discovered. Some have properties close to ours, orbiting a star that is colder than our sun. Since their star is a cold red dwarf, the habitable planets are much closer, with revolutions of one to ten days. In their atmosphere, astronomers hope to find biosignatures, spectra of molecules, as we can see them in the infrared spectrum of the Earth’s atmosphere: such as water, oxygen, ozone, methane, carbon dioxide and ammonia.
Our galaxy, the Milky Way
The European Space Agency launched the astronomical Gaia satellite in 2013 to measure the distance and intrinsic motions of more than a billion objects, mostly stars, in our galaxy. The satellite is still operating at the Lagrange L2 point with great success. It sweeps the entire celestial vault to accumulate at the end of its mission at least 60 observations of all the objects that can be identified by its instruments. The mission is a huge challenge in gathering information and processing the hundreds of terabytes collected. The results, the last of which will be presented in 2022, are surprising. They show that our galaxy is far from in equilibrium: on the contrary, it is constantly disturbed by the interaction with small accompanying galaxies and by the star bar in its center. It could be shown that the Milky Way underwent a major fusion with another massive galaxy eight billion years ago, called Gaia-Enceladus, and this was the reason for an eruption of star formation.
Several missions have explored virtually every planet or satellite in the solar system, with Mars being the most visited. ESA even managed to put the Rosetta spacecraft into orbit around the comet Tchouri, which landed the Philae robot there in 2014. Comets have not been much transformed since the beginning of the solar system, which 4 billion years ago, and are precious witnesses to its formation . Today, several robots roam the surface of Mars, sending us images of the landscape, taking samples and sending their composition to Earth. Currently, NASA’s Perseverance rover is sublimating laser rock samples and examining the spectrum produced. The robot moves 3 meters forward a day and explores the ancient lake of Jezero. He collects stone cores, which will be deposited in several places and restored by another mission. The return of the samples to Earth, as for the Moon, should provide even more information.
Following the success of the Apollo missions and man’s trips to the Moon, some believe the next step would be to send humans to Mars. But above all, it is a huge challenge, a mission with great political prestige, certainly not a scientific necessity, since robots already allow us to explore the planet. Such a mission would, in addition to being extraordinarily expensive, be extremely dangerous to humans. During the trip, from six months to a year, it would be impossible to be protected from radiation, a source of cancer, whereas the magnetosphere on Earth protects us. Another concern, a 30,000 km / h encounter with dust and small debris can do a lot of damage to the pressure vessel. The planet Mars is not habitable. If there is water, it is trapped underground, in the state of ice. How much energy do we have for heating, to find oxygen? To date, these problems are far from solved, the huge credits for such an experiment have not been met, and meanwhile robot missions make it possible to explore the entire planet little by little.
Astrophysicist, professor at the Collège de France, holder of the chair “Galaxies and Cosmology”, Françoise Combes is a member of the Academy of Sciences. She works at the Paris Observatory with the formation and evolution of galaxies, their dynamics and their co-evolution with supermassive black holes, as well as on dark matter.