The Week in Space and Physics
On the James Webb commissioning, the SLS, Sagittarius A* and an ancient impact on Ganymede
Half a millennium ago, as they traversed the great southern oceans, Portuguese sailors wrote of two clouds lighting up the night skies. These clouds, long known to the people of the Southern Hemisphere, are nearby galaxies: now named the Large and Small Magellanic Clouds. Astronomers believe, although there is some doubt about the matter, that the pair orbit the Milky Way.
Over the centuries these two clouds have often drawn the attention of astronomers. Over ten thousand research papers speak of their mysteries, from the great Tarantula Nebula to the site of SN1987, the only supernova to shake our galactic environs in decades. Now they are demonstrating the awesome power of the James Webb Space Telescope.
NASA directed the telescope to image a portion of the Large Magellanic Cloud. They then compared the result to images made by the earlier Spitzer Telescope, then the world’s strongest infrared telescope. The difference in the two images – shown clearly at the top of this page – is staggering.
With its larger mirror and enhanced detectors, the Webb shows the infrared sky – that light sitting just beyond visible red – in unprecedented clarity. Indeed, even NASA seem surprised by the quality of the telescope: calling the alignment of the telescope “perfect”. As a result, even sharper measurements of the cosmos than expected now seem possible.
Astronomers still need to wait a few months for the real scientific observations to begin. According to NASA, all science instruments on the telescope are now powered on and cooled to their operating temperatures. Each instrument has successfully photographed individual stars, thus demonstrating their functionality. But mission controllers still need to calibrate those instruments and find the distortions of each and their responses to light.
Here the Large Magellanic Cloud comes into play again. Back in 2006, anticipating the future need of the James Webb, astronomers directed Hubble to observe the nearby galaxy. The telescope plotted the positions of two hundred thousand stars, creating the “Webb astrometric field”. The James Webb will now observe that same patch of the Magellanic Cloud. Astronomers can then precisely measure the distortions and performance of the telescope.
A second step will target closer objects. Unlike distant stars, things in the Solar System - planets, asteroids and comets - appear to move across the sky. To capture them the telescope must swing around its axis, tracking their motion. Mission controllers plan to test this ability before declaring the James Webb ready.
Once that is done – hopefully in mid-July – the team will release the Webb’s first package of scientific data. This, a set of colour images and light spectra, will mark the start of a decade or more of activity by the telescope. Astronomers are coy about what will be in that package – saying only that it will highlight the key themes of the telescope, from exoplanets to the early universe.
When will the SLS Fly?
What to make of the delays to the SLS? NASA has persistently delayed the testing and launch of their mammoth new rocket, pushing the first launch date from the end of last year to no earlier than August this year. Even that goal looks unlikely – and that, in turn, makes NASA’s attempt to reach the Moon by 2025 look ever more impossible.
In April NASA rolled the SLS out to the launch pad for a wet dress rehearsal – a set of tests that should have brought the rocket to within ten seconds of a launch. A series of faults stopped that happening, and NASA was left scrambling for fixes. By mid-April, however, the space agency decided to return the rocket to the hanger for further work.
Some of the issues appear to have been fixed, and NASA now plans to try the rehearsal again. Recent reports suggest the SLS will return to the pad at the end of May, with tests taking place in early June. If all goes to plan – a big if – the rocket could be ready for the first test launch in August. That launch would send the Orion capsule towards the Moon, the first such flight since the days of the Apollo programme half a century ago.
NASA has come under some criticism for the frequent delays and high cost of the SLS. Unlike SpaceX – who seem perfectly happy to blow up an enormous rocket once a month or so – NASA operate under tight political constraints. They cannot afford to see the SLS explode on the launch pad, neither in terms of cost (at least four billion dollars per launch) or in terms of the political scrutiny such a disaster would bring.
In the end, however, these constraints may well spell doom for the SLS before it even gets off the ground. If SpaceX can get their Starship rocket to work and, crucially, to land, the rationale for NASA’s enormously expensive rocket will be hard to justify. Political needs mean it is unlikely the rocket will ever be cancelled. But high costs also mean it is unlikely to fly as regularly as a real return to the Moon would demand.
Astronomers Finally See the Milky Way’s Black Hole
Astronomers this week announced the first image of the Milky Way’s central black hole. Lying around twenty-seven thousand light years from Earth, the black hole – Sagittarius A* - has never been seen directly before. Indeed, even the Nobel Prize committee careful danced around the topic in 2020. Awarded for the "discovery of a supermassive compact object", the prize wording carefully avoided calling it a black hole.
That’s because black holes, despite strong evidence of their existence, have long been regarded with suspicion. Even Einstein - whose theories predicted their presence - once called them abhorrent. The laws of relativity, he believed, must prevent them from forming. But this image, and those of two other black holes in neighbouring galaxies, has mostly laid such doubts to rest.
Somewhat ironically, indeed, the images prove black holes behave exactly as Einstein’s theory predicts. Still, some surprises did emerge: astronomers spotted odd fluctuations in the brightness of the ring of debris orbiting the black hole.
Models had indeed predicted some fluctuations - but not to the scale seen in the new observations. Something unknown, then, is happening to the gas swirling around the edge of the black hole. Future measurements will hone in on those fluctuations, seeking to explore their properties. Scientists are already talking about making “movies” of the black hole, showing how its brightness varies day-by-day.
Does a Giant Crater Scar Ganymede?
When Voyagers 1 and 2 passed by Ganymede in 1979, planetary scientists noticed a curious property of its surface. Much of the moon is covered by grooves and furrows, for which no clear origin has ever been found.
More images from the Galileo probe, visiting Jupiter in the 1990s, found these grooves stretch across the whole moon. Now a new analysis, from researchers at Kobe University in Japan, shows that all these grooves, even those on opposite sides of the moon, are aligned in concentric circles.
This pattern, they say, could have formed in a single enormous collision. Their calculations suggest Ganymede was hit by a giant asteroid – or a small moon – one hundred and fifty kilometres across. Such a violent collision should have left clear traces in the geology and structure of Ganymede. Those traces, if they exist, should be visible to JUICE, a probe due to visit Jupiter in the 2030s.
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