The pioneering observatory will use two spacecraft to create artificial solar eclipses from Earth orbit in order to study the solar corona.

An artist's conception of the Proba 3 mission in space. ESA

The rare spectacle of a total solar eclipse offers us a chance to see the elusive solar corona in person. But this moment is fleeting, making this region near the dazzling Sun poorly understood. Now, the European Space Agency (ESA) has launched Proba 3, an innovative mission to create on-demand solar eclipses in space.

An Indian Space Research Organization (ISRO) Polar Satellite Launch Vehicle launched with the mission from Satish Dhawan Space Center in India on December 5th at 5:34 a.m. EST (10:34 UT).

Spacecraft separation occurred 18 minutes after liftoff, and ESA controllers in Reda, Belgium, picked up signals from the mission in orbit 15 minutes later. Controllers now report that the spacecraft has successfully deployed its solar panels, and is drawing power.

Next comes the critical commissioning phase for the spacecraft. If all goes according to plan, the two pieces of Proba 3 should separate in early 2025. We should see the first science observations in about four months time.

"Today's liftoff has been something all of us in ESA's Proba 3 team and our industrial and scientific partners have been looking forward to for a long time," Damien Galeno (ESA Proba 3 mission manager) says in a recent press release. "I'm grateful to ISRO for this picture-perfect ascent to orbit.

The Proba 3 team ESA

This is the first ISRO/ESA collaboration since the launch of Proba 1 in 2001, where Proba stands for Project for Onboard Autonomy. The Proba missions are used to test new technologies while also carrying useful scientific instruments. Proba also means "let's try," an appropriate motto for these pathfinder missions. Proba 2, also a solar mission, launched in 2009 aboard a Russian rocket. Both Proba 1 and 2 are still operational.

Unlocking the Secrets of the Sun

Proba 3 will observe the solar corona region in multiple wavelengths using two spacecraft flying in tight formation. A 1.4-meter disk on the occulter spacecraft will block the Sun's light. The shadow it casts will fall on the coronagraph spacecraft's instrument package. To be effective, the two will have to fly 150 meters (500 feet) apart — about 1.5 times the length of a soccer field — and maintain that separation to a precision of less than a millimeter, about the thickness of a U.S. dime.

In that formation, Proba 3 will image the corona as close as 1.1 solar radii from the Sun. That provides the closest view we have right now, as the Solar Heliospheric Observatory's LASCO C2 camera can only image as close as 1.5 solar radii to the Sun.

Proba 3's view won't be constant, though. The mission has an elliptical, 19 hour 30 minute orbit and will conduct operations over a six-hour span near its apogee, 37,300 miles (60,000 kilometers) from Earth. Constraining observations to this window means that the spacecrafts will use minimal fuel because the spacecraft is under minimal atmospheric drag. The spacecrafts will also experience less wobble due to gravitational variations when it’s farthest from Earth.

The elliptical orbit for the Proba 3 mission is shown in an artist's illustration. ESA

Science from Proba 3

The concept of coronagraphs dates back to 1931, when astronomer Bernard Lyot demonstrated the first such instrument from an observatory high in the French Pyrenees. However, although Earth-based coronagraphs are feasible to build, atmospheric scattering limits their use. Space offers an ideal alternative. (An early attempt to eclipse the Sun using spacecraft — the Soyuz spacecraft attempted to observe the Sun using the Apollo spacecraft in 1975 — was disappointing, as thruster gases scattered light just as Earth's atmosphere does.)

The occulter and coronagraph segments of the mission, separated in the lab prior to launch.   ESA

A chief science question the Proba 3 team hopes to address is the coronal heating problem: Why is the corona so much hotter than the Sun's visible surface? The Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona (ASPIICS) instrument will address this issue by probing the corona close to the Sun.

Proba 3 will work to address the coronal heating problem. ESA

ASPIICS will also investigate what accelerates the solar wind. Researchers are hoping to see slow-moving "blobs" of solar wind at their point of origin deep in the solar corona. They also hope to probe the origin of coronal mass ejections at their starting point. Observations should also give insight into how coronal holes can give rise to the fast solar wind, which reaches speeds of more than 800 kilometers per second (2 million mph).

Separately, Proba 3's long elliptical path gives scientists the chance to probe the high-energy particles trapped within Earth's Van Allen radiation belts. The 3DEES Energetic Electron Spectrometer will measure the particles' flux and direction of the motion across a 180° field of view.

Finally, the Digital Absolute Radiometer (DARA) instrument will observe total solar irradiance, a measure of the Sun's total energy output. Tracking solar irradiance is crucial to understanding climate, as it accounts for more than 99.9% of Earth’s total energy input.

Precision Flying

Precision flying in space presents several challenges. Previous ESA missions, such as ESA’s Cluster constellation of space weather satellites, have accomplished the feat on a scale of tens to thousands of kilometers, but never at so close a range as 150 meters. Proba 3 will carry out this fine celestial ballet using a number of instruments.

First, the Vision-Based Sensor on the occulter spacecraft will follow a flashing constellation of LED lights on the coronagraph. This tracking works in concert with the Fine Lateral and Longitudinal Sensor to assure submillimeter accuracy.

LEDs active on on Proba 3. ESA/J. Versluys

The Shadow Positioning Sensor system will keep the 8-millimeter shadow of the occulter centered on the coronagraph instrument package. If any misalignment occurs, it will alert either spacecraft to gently fire its thrusters, nudging them back into place. The occulter uses cold gas nitrogen thrusters, while the coronagraph uses hydrazine thrusters.

Proba 3's nominal mission is two years long. While Proba 1 and 2 lasted far longer than their nominal missions, atmospheric drag at perigee, which takes the spacecraft just 600 km (400 miles) above Earth's surface, will likely cause Proba 3 to reenter our atmosphere in about five years.

We look forward to seeing Proba 3’s on-demand solar eclipses as it probes the mysteries of the Sun.

Source: skyandtelescope.org