As skywatchers and scientists converge on a transcontinental band of totality for Monday’s solar eclipse in the United States, engineers in Europe are building a unique pair of satellites to create artificial eclipses lasting for hours — a feat that that could be a boon for solar physicists but will escape the view of Earth-bound spectators.
The European Space Agency’s Proba-3 mission, scheduled for launch in late 2020, is made possible by two satellites, one about the size of a refrigerator, and another slightly smaller spacecraft with the rough dimensions of a coffee table.
The basic idea is to fly the smaller satellite directly between the sun and the field-of-view of cameras and instruments mounted on the bigger spacecraft, blocking the sunlight and revealing the glow of the sun’s corona, or super-hot atmosphere, and filament-like eruptions called solar flares.
The light coming from the surface of the sun is a million times brighter than the corona, requiring special measures to see the solar atmosphere.
The concept of obstructing the brightest light emanating from the sun to study activity around it is not new. Scientists have made observations of the corona for centuries during solar eclipses, and there are other space missions that carry coronagraphs, light-blocking discs buried inside telescopes used to make the relatively dim solar atmosphere visible.
But coronagraphs mounted inside telescopes are prone to stray light, a common problem in optics. Light escaping around the coronagraph disc can distort or mask views of the corona.
One simple way to think of the stray light problem is to compare an image of a total solar eclipse, a spectacular phenomenon where the faint corona suddenly springs into view. Holding your thumb over the sun at arm’s length does not produce the same result because sunlight has already been scattered by particles in Earth’s atmosphere.
“One of the science goals of Proba-3 is to reproduce the conditions of a total solar eclipse as much as possible,” said Andrei Zhukov, principal investigator for Proba-3’s coronagraph at the Royal Observatory of Belgium, in response to questions from Spaceflight Now.
In general, the longer the distance between an observer or a camera and the object obscuring the sun, the better the result. Scientists also do not have to worry about atmospheric distortions in space.
“This problem can be minimized by extending the coronagraph length, the distance between the camera and the disc, as far as possible – but there are practical limits to coronagraph size,” Zhukov said in an ESA press release.
“Instead, Proba-3’s coronagraph uses two craft: a camera satellite and a disc satellite,” Zhukov said. “They fly together so precisely that they operate like a single coronagraph, 150 meters (492 feet) long.”
The duo will launch together into an highly elliptical, oval-shaped orbit around Earth taking the satellites as high as 37,611 miles (60,530 kilometers) and as low as 372 miles (600 kilometers).
In that orbit, the satellites will complete one lap around the planet every 19.6 hours. For six of those hours, cameras on Proba-3’s larger satellite will have an artificial eclipse.
Proba-3 will see the features down to 34,500 miles (55,600 kilometers) from the sun — about 8 percent of the solar radius — resolving activity closer to the solar limb than any current space mission. Zhukov said ground-based observers looking at a total solar eclipse can still see more of the corona than Proba-3, but the advantage of a space mission is the eclipse’s longevity.
“During two years of its nominal mission, Proba-3 will provide around 1,000 hours of coronal observations,” Zhukov wrote in an email to Spaceflight Now. “This has to be compared with several minutes of duration of ‘natural’ eclipses during the same time.
“Proba-3 will also be free from disturbances produced by the Earth’s atmosphere in all astronomical observations,” Zhukov wrote.
ESA is developing the Proba-3 mission as an experimental demonstration, with scientific observations of the sun a secondary goal.
Engineers want to test out technologies for autonomous formation flying on Proba-3, which will use ranging measurements with the help of GPS navigation signals and optical sensors.
The two spacecraft will be connected with an inter-satellite radio link, and the so-called occulter satellite — the smaller of the pair — will carry low-power micro-thrusters for fine maneuvers, keeping the two vehicles positioned with millimeter precision.
Proba-3 will create an eclipse when the satellites are farthest from Earth. The satellites will passively drift apart during the rest of each orbit, a fuel-saving measure to minimize consumption of the mission’s limited supply of propellant.
The capabilities to be proved out on Proba-3 could be used on future missions to repair satellites in orbit or return samples from Mars, according to ESA.
Already approved for development as a tech demo mission, Proba-3 won the backing of ESA’s science program committee earlier this year. The agency’s scientific division will pay for Proba-3’s science operations center to ensure astronomers get the most out of the project.
Proba-3 was scheduled for launch in 2019, but officials recently pushed back the mission’s liftoff to the fourth quarter of 2020.
“The complexity in the development of the formation flying technology does not allow the launch in late 2019 as was planned earlier,” Zhukov said. “The project schedule is now consolidated, and the launch in the fourth quarter of 2020 is the new baseline. That does look feasible.”
Solar Orbiter on track to launch before next total solar eclipse
Stevenage, 21/08/2017 - Today’s total Solar Eclipse across the United States of America will provide a spectacular view of the mysterious Solar Corona, the one million degree “halo” around the sun, which can only be seen from Earth when the Moon passes in front of the bright solar disk, which otherwise completely drowns out the faint light of the corona.
Scientists lucky enough to be able to see the Solar Eclipse from Earth will be studying this rare glimpse of the corona during the one hundred and sixty seconds or less that the eclipse lasts, to try to answer the many unanswered questions about this mysterious corona. Noone yet knows for instance, why the corona is more than 100 times hotter than the surface of the Sun.
Meanwhile the European Space Agency spacecraft Solar Orbiter is in the final stages of spacecraft integration at the Airbus spacecraft assembly hall in Stevenage, UK.
Solar Orbiter will be launched in February 2019 into a close orbit around the Sun and will allow scientists to study the solar corona in much more detail, for much longer periods, and at a much closer distance that can ever be reached here on the ground, or for that matter, by any spacecraft circling the Earth. 10 instruments will be flown that will study not only the corona but the Sun’s disk in great detail, the solar wind and the solar magnetic fields which will give us unprecedented insight into how our star works, and how we can better predict periods of stormy “space-weather” that the Sun throws our way from time to time.
The last of the ten instruments is being installed this month and the next step is system testing before the heatshield, antennas and boom are added towards the end of the year. In addition, the first instrument end-to-end electrical test has been performed successfully showing that the system works completely as expected.
By the time of the next global total eclipse, across the Pacific Ocean and South America on 2nd July 2019, Solar Orbiter will have begun its three and a half year journey to the inner solar system to get close to our Sun.