The two spacecraft of the Proba-3 mission will fly in careful synchronization to create an artificial eclipse for six hours out of every orbit, allowing them to study the Sun’s elusive corona. Credit: ESA – P. Carril
In a fabulous feat of formation flying, the European Space Agency’s (ESA) Proba-3 mission, now set to launch Thursday morning, will fly two spacecraft 490 feet (150 meters, or one and a half football fields) apart, at a precision of just 0.04 inch (1 millimeter) — the width of a human fingernail.
The launch is currently scheduled for Thursday, Dec. 5, at 5:38 AM EST (10:38 GMT), pending a successful software solution to address an anomaly in the redundant propulsion system of one of its two spacecraft.
Live coverage will begin a half-hour before the scheduled launch time. You can watch the event on ISRO’s YouTube channel or ESA’s Web TV.
Seeing the Sun
While the test of technology is its own justification, the spacecraft also have a scientific mission: observing the Sun’s delicate corona, usually visible only during a solar eclipse. The corona, the outermost layer of the Sun, is a million times fainter than its face, making it impossible to see most of the time. It’s only when the Sun’s brighter disk is blocked that the light from the corona can be observed. In an eclipse, the Moon blocks that light naturally for observers on Earth. Proba-3 will attempt to recreate this effect by flying in careful formation, so that one of the spacecraft eclipses the other for six hours each orbit.
“By lining up with the Sun,one spacecraft will cast a precisely controlled shadow onto another, to cover the Sun’s brilliant disk entirely, so that the million-times-fainter solar corona will become visible for sustained observation. This will either work or it won’t: that is the challenge we have set ourselves,” said Damien Galano, Proba-3 mission manager, in a statement.
If successful, the mission will reveal valuable details about the elusive solar corona, and pave the way for future spaceborne occulting missions, like those envisioned in the direct search for exoplanets around bright stars.
Related: Bringing the Sun to light
Blot it out
If astronomers want to look at anything just next to the Sun — or any other star, for that matter — they face a basic problem: The Sun is very, very bright. Compared to a star, anything else — whether it’s the solar corona, an extrasolar planet, or even a much smaller star — will appear a thousand or a million times less bright. It’s not feasible to take a picture of such a dim object if there’s something so outrageously bright next to it. The starlight overwhelms the picture, bleeding across nearby pixels and making imaging nearby objects impossible. To see these dimmer, nearby targets, astronomers need to block the light from the star so it doesn’t overwhelm their instruments.
Such a device is called a coronagraph, and it works by exactly the same method as someone shielding their eyes from the Sun by raising their hand in front of their face. But scientists, of course, need more precision than that. While it might seem straightforward to put a tiny disk (an occulter) directly in front of the camera or imager, that arrangement results in severe diffraction, with light leaking in spikes around the sides of the coronagraph. That’s because light is both a particle and a wave, so some amount of light from the star will always sneak around the occulter and find its way into the camera. The farther apart the camera and the occulter are, the less diffraction is observed.
This is why Proba-3 will separate its Occulter and Coronagraph craft by 150 yards (137 meters). Even then, enough light sneaks through that the Coronagraph spacecraft has its own tiny internal occulter, just 0.14 inch (3.5 mm) across, in order to get the clearest image possible.
And for this concept to work, Proba-3 must operate as one enormous, 150-yard-long instrument. That is where the precision flying comes in. The two satellites must stay in perfect alignment, the Occulter exactly between the Coronagraph and the Sun, for the duration of the observations, some six hours each.
They’ll achieve their lockstep orbit with a series of targeting tests, including LED lights on the Occulter for the Coronagraph to target, a laser and retro-reflector system, and a shadow sensor, which sends an alert if the shadow of the Occulter drifts at all on the Coronagraph’s imager.
Smooth the path
One might think that space is the perfect environment for such precision formation flying. Unlike an aerial Thunderbird exhibition, Proba-3 in space has no air currents, drafts, or wind to contend with.
But space isn’t quite that simple. Satellites in low Earth orbit still experience a slight but measurable amount of drag as they pass through the extremely thin upper vestiges of Earth’s atmosphere. It’s also close enough for Earth’s slight gravitational variations to disturb spacecraft over time. For Proba-3, all those disturbances would require extra maneuvering to stay in place, which means extra fuel — always a costly decision in space.
So engineers decided instead on a highly elliptical orbit, which brings the spacecraft close to Earth for a brief period, before sending them farther out to space for a long, slow orbital loop. (Newton’s laws tell us that planets — and satellites — move fastest when they are closest to their stars or planets, and slowest near their farthest point.)
When the spacecraft are moving quickly and swooping close to Earth, dipping into its uppermost atmosphere, they will fly in a safer, less constrained formation. But as they approach apogee, the farthest point from Earth, they will move into formation and remain there for six hours until they fall back toward home, to repeat the process over again.
Engineers expect the spacecraft have enough propellant to continue this cycle for about two years.
Take to the skies
The Proba (PRoject for OnBoard Autonomy) series of missions are ESA’s way to launch relatively cheap projects that test new technologies using off-the-shelf components. The name is also taken from the Latin for “let’s try,” reflecting the experimental nature of the series.
The first Proba mission lasted more than 20 years. It was made to test now common space technologies including lithium ion batteries and gallium arsenide solar panels. The craft eventually shifted to basic Earth observation, using its two onboard imagers. It was followed by Proba-2, which had a similar wide range of new technologies but nominally observed the Sun, and Proba-V (short for Vegetation), which imaged greenery and ground cover on Earth and complemented a larger mission called Spot.
The latest Proba mission costs about $200 million — more expensive than the previous iterations, in large part due to the difficulties of flying two craft at once. The satellite will be launched from India’s Satish Dhawan Space Centre aboard a PSLV-XL rocket. Separation from the rocket will occur 18 minutes after liftoff, and mission control expects to hear the first signal from the spacecraft 15 minutes after that. The Indian space agency ISRO also launched Proba-1, but not the two intervening Proba missions.
For some 18 weeks, Proba-3 will remain in Commissioning Phase, where operators ensure the subsystems and individual instruments are all performing correctly. It is only then that the spacecraft will separate into their two parts and begin testing maneuverability, eventually beginning the tight formation flying and eclipse observations that will define the mission.
