The two spacecraft of the Proba-3 mission will fly in precise synchronization to create an artificial eclipse for six hours on each orbit, allowing them to study the Sun’s elusive corona. Credit: ESA – P. Carril
UPDATE: Proba-3 successfully launched Thursday morning at 5:34 EST (10:34 GMT). You can watch a replay of the launch here.
In a fabulous feat of formation flight, the European Space Agency’s (ESA) Proba-3 mission, scheduled to launch Thursday morning, will fly two spacecraft to 490 feet (150 meters, or a football field and a half ) away, to an accuracy of just 0.04 inches (1 millimeter)—the width of a human fingernail.
The launch is currently scheduled for Thursday, December 5 at 5:38 a.m. EST (10:38 GMT), pending a successful software fix to resolve an anomaly in the redundant propulsion system of one of its two spacecraft.
Live coverage will begin half an hour before the scheduled launch time. You can watch the event on ISRO’s YouTube channel or on ESA’s Web TV.
See the sun
While proof of the technology is its own justification, the spacecraft also has a scientific mission: to observe the delicate corona of the Sun, usually visible only during a solar eclipse. The corona, the Sun’s outermost layer, is a million times fainter than its face, making it impossible to see most of the time. It is only when the Sun’s brightest disk is blocked that light from the corona can be observed. During an eclipse, the Moon naturally blocks that light for observers on Earth. Proba-3 will attempt to recreate this effect by flying in careful formation, so that one spacecraft will eclipse the other for six hours in each orbit.
“By aligning with the Sun, one spacecraft will cast a precisely controlled shadow over another, to entirely cover the Sun’s bright disk, so that the million-fold dimmer solar corona becomes visible for prolonged observation. Will it work or not: that is the challenge we set ourselves,” Damien Galano, Proba-3 mission manager, said in a statement.
If successful, the mission will reveal valuable details about the elusive solar corona and pave the way for future cloaking missions in space, such as those envisioned in the direct search for exoplanets around bright stars.
Related: Bringing the sun to light
Delete it
If astronomers want to observe something right 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, be it the solar corona, an exoplanet, or even a much smaller star, will appear a thousand or a million times dimmer. You can’t photograph such a dim object if there’s something so outrageously bright next to it. Starlight overwhelms the image, spreading onto nearby pixels and making it impossible to image nearby objects. To see these fainter, closer targets, astronomers must block the light from the star so it doesn’t overwhelm their instruments.
Such a device is called a coronagraph and works in exactly the same way as someone shielding their eyes from the Sun by raising their hand in front of their face. But scientists, of course, need more precision. While it might seem simple to place a tiny disk (an occulter) directly in front of the camera or imager, such an arrangement causes severe diffraction, with light leakage in spikes around the sides of the coronagraph. This is because light is both a particle and a wave, so some amount of light from the star will always creep around the occulter and find its way into the camera. The further apart the camera and occulter are, the less diffraction is observed.
This is why Proba-3 will separate its Occultator and Coronagraph aircraft by 150 yards (137 meters). Even then, enough light passes through that the Coronagraph spacecraft has its own small internal occulter, just 3.5 mm (0.14 in) wide, to get the clearest image possible.
And for this concept to work, Proba-3 must function as a massive 150 meter long instrument. This is where flight precision comes into play. The two satellites must remain in perfect alignment, the Occultor exactly between the Coronagraph and the Sun, for the entire duration of the observations, approximately six hours each.
They will achieve their locked orbit with a series of targeting tests, including LED lights on the Occultor for the Coronagraph to aim at, a laser and retroreflector system, and a shadow sensor, which sends an alert if the Occultor’s shadow becomes moves at all on the Coronagraph imager.
Smooth the path
You might think that space would be the perfect environment for such precision formation flight. Unlike a Thunderbird airshow, Proba-3 in space has no drafts, drafts or wind to contend with.
But space is not 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 the spacecraft over time. For Proba-3, all these disturbances would require extra maneuvers to stay in place, which means extra fuel – always a costly decision in space.
So engineers opted for a highly elliptical orbit, which brings the spacecraft closer to Earth for a short time, before sending it farther into space for a long, slow orbital spin. (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.)
As spacecraft move quickly and swoop close to Earth, plunging into its upper atmosphere, they will fly in a safer, less constrained formation. But as they approach apogee, the furthest point from Earth, they will move into formation and stay there for six hours until they fall back home, to repeat the process again.
Engineers expect the spacecraft to have enough propellant to continue this cycle for about two years.
Take flight
The Proba (PRoject for OnBoard Autonomy) mission series is ESA’s way of launching relatively cheap projects that test new technologies using standardized components. The name also comes from the Latin “let’s try”, reflecting the experimental nature of the series.
The first Proba mission lasted more than 20 years. It was built to test now-common space technologies, including lithium-ion batteries and gallium arsenide solar panels. The craft eventually transitioned to basic Earth observation, using its two onboard imagers. It was followed by Proba-2, which had a similar array of new technologies but nominally observed the Sun, and Proba-V (short for Vegetation), which imaged vegetation and land cover on Earth and completed a more Wide Call Spot.
The latest Proba mission costs about $200 million, more expensive than previous versions, largely due to the difficulties of flying two aircraft at the same time. The satellite will be launched from India’s Satish Dhawan space center 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 later. The Indian space agency ISRO also launched Proba-1, but not the two intermediate Proba missions.
For approximately 18 weeks, Proba-3 will remain in the commissioning phase, where operators ensure that subsystems and individual instruments function properly. It is only then that the spacecraft will separate into its two parts and begin testing maneuverability, then begin the tight formation flight and eclipse observations that will define the mission.
