As part of our partnership with the Academy of Sciences, academicians analyze and shed light on the major challenges of the contemporary world through scientific questions that make the news.
Francoise Combes
astrophysicienne
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 for planets such as Mars or Jupiter more than 600 million km away, the American and European space agencies are opening windows on the Universe.
The James-Webb: from chrysalis 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 American (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 dollars ( including 700 million euros for Europe).
The telescope reached its orbit around the Lagrange point L2, 1.5 million km from Earth, on January 24, 2022, after all its components deployed in a spectacular fashion (20 m yarrow heat shield , primary and secondary mirrors). The primary mirror has a diameter of 6.5 m; it is made up of 18 hexagonal segments.
For the launch, the telescope was folded up like a chrysalis inside the fairing of the Ariane rocket. Now that the butterfly is fully deployed, it takes five months of adjustments and checks. The first astronomical observations will begin in June 2022. In February, the telescope made an image of a star, which has in fact broken down into 18 images which will make it possible to align 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 is expanding, and the expansion of space also lengthens the wavelength of the light emitted by distant objects. The further away the source, the greater the redshift. In space, to look far is to go back in time, because the signals emitted by these primordial galaxies cannot travel faster than light. The JWST will see galaxies forming, shortly after the big bang that dates back 13.8 billion years.
How galaxies form: in search of our origins
Although the HST also observed very distant galaxies, it could only detect the brightest ones. With its diameter of 6.5 m (against 2.4 m for the HST), the JWST will capture seven times more light and will thus reveal the entire population of these young galaxies which are at the origin of the first glimmers of the Universe, after the dark 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 in the first galaxies will re-ionize the Universe; this period of re-ionization will end approximately after a billion years. The JWST will allow a breakthrough in this area of knowledge of our origins.
The JWST carries four state-of-the-art instruments. Three of them work from 0.6 to 5 microns: Nircam, to make images; Nirspec, to observe the spectrum of 200 objects simultaneously; Niriss, a low spectral resolution spectro-imager, studying the temperature, mass and chemical composition of stars. Only Miri will work from 5 to 28 microns, taking images and spectra of cold, distant objects. Miri was built by Europeans – much of it in France. Europe thus has 15% of the telescope 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. The 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 extra-solar planets
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, those planets that orbit around a star other than the Sun.
For more than twenty-five years now, nearly 6,000 exoplanets have been identified, orbiting stars in the vicinity of the Sun. Astronomers are now trying to get to know them better, and in particular 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 have already been evaporated, or too far , completely frozen.
But nothing beats direct measurement. Thanks to its high infrared sensitivity, the 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 stellar disk. A single transit may not be enough, but we can combine several transits, if the period of revolution around the star is small. This period is equal to one year for the Earth around the Sun, but it is possible to find closer exoplanets, with much shorter periods, of the order of a few days.
This is the case of the Trappist-1 system, forty light years from Earth, in which seven rocky planets have been discovered. Some have characteristics close to ours, revolving around a star colder than our Sun. As 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 bio-signatures, spectra of molecules as we might see them in the infrared spectrum of Earth’s atmosphere: such as water, oxygen, ozone, methane, carbon dioxide and ammonia.
Our Galaxy, the Milky Way
The European Space Agency launched the Gaia astrometric satellite in 2013 to measure the distance and proper motions of more than a billion objects, mainly stars, in our Galaxy. The satellite still works, at the Lagrange L2 point, with great success. It sweeps the entire celestial vault so as to accumulate, at the end of its mission, at least 60 observations of all the objects identifiable by its instruments. The mission is an immense challenge of collecting 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 being in equilibrium: on the contrary, it is constantly disturbed by the interaction with small companion galaxies and by the stellar bar in its center. It could be shown that the Milky Way underwent a major merger with another massive galaxy, eight billion years ago, called Gaia-Enceladus, and this was the occasion for an outbreak of star formation.
space exploration
Multiple missions have explored virtually every planet or satellite in the Solar System, with Mars being the most visited. ESA even managed to put the space probe Rosetta into orbit around comet Tchouri, which landed the robot Philae there in 2014. Comets have not been transformed much since the beginning of the Solar System, there 4 billion years ago, and are precious witnesses of its formation. Today, multiple robots roam the surface of Mars, send us photos of the landscape, take samples, and send their composition to Earth. Currently, NASA’s Perseverance rover sublimates rock samples by laser and examines the spectrum produced. The robot advances 3 meters per day and explores the ancient Great Lake Jezero. He collects rock cores, which will be deposited at several sites and recovered by another mission. The return of the samples to Earth, as for the Moon, should provide even more information.
After the success of the Apollo missions and the trips of man to the Moon, some believe that the next step would be to send humans to Mars. But it is above all a huge challenge, a mission of great political prestige, certainly not a scientific necessity, since robots already allow us to explore the planet. Such a mission, besides being extraordinarily expensive, would be extremely perilous for humans. During the time of the trip, from six months to a year, it would be impossible to be sheltered from radiation, a source of cancer, whereas, on Earth, the magnetosphere protects us. Another concern, an encounter at 30,000 km/h with dust and small debris can do a lot of damage to the pressurized vessel. The planet Mars is not habitable. If there is water, it is confined underground, in the state of ice. How much energy do we have for heating, for finding oxygen? To date, these problems are far from being solved, the huge credits for such an experiment are not met, and, meanwhile, robotic missions make it possible to explore the whole planet little by little.
Profile
Astrophysicist, professor at the Collège de France, holder of the “Galaxies and cosmology” chair, Françoise Combes is a member of the Academy of Sciences. She works at the Paris Observatory on the formation and evolution of galaxies, their dynamics and their coevolution with supermassive black holes, as well as on dark matter.
–
–
