ESA’s Solar Orbiter to swing past Earth this week • The Register


Interview ESA’s Solar Orbiter is to undertake a flyby of Earth, requiring a careful assessment of debris as it dips close to the orbit of the International Space Station (ISS) ahead of its main science mission.

The flyby is due to take place on the 26 and 27 of November.


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The amount of debris on orbit was helpfully increased last week by Russia’s anti-satellite missile demonstration, much to the consternation of NASA and other space agencies.

With the Solar Orbiter due to pass above North Africa and the Canary Islands at its closest approach on 27 November, it has to make it through two regions of potential space debris; geostationary orbit and low earth orbit.

“At 12km/s we’d be a really effective ASAT weapon,” Daniel Lakey, Solar Orbiter spacecraft operations engineer at ESA tells The Reg, in reference to the speed at which the spacecraft will be barrelling along.

The team has already assessed if the first Trajectory Correction Manoeuvre (TCM) – or “TCM-3d” in spacecraft operations speak – needed adjusting. The good news was, following Tuesday’s meeting, it didn’t.

“The ‘default’ guidance profile option executed as planned instead,” he says.

The first TCM window (aka TCM-6h) will occur just before midnight on 26 November, when the spacecraft will be just six hours from its closest approach to Earth.

“For the TCM-6h, FD [Flight Dynamics] will compute a series of alternative trajectories that will be checked by ESA’s Space Debris Office against known objects, who’ll report back on the likelihood of a collision,” says Lakey.

Hopefully all will go well, despite the increasing count of debris around Earth, and an adjustment will not be required. As for what would trigger an all-hands-to-the-pumps moment, “2*10-5 for TCM-6h,” he says. “If there’s a likelihood of a collision greater than that, a CAM [Collision Avoidance Maneouvre] trajectory will be selected that has a lower chance of a collision and the lowest delta-V requirement.”

If a change is needed to tweak the trajectory, the team will only have a matter of hours to put together the instructions and get them onboard the Solar Orbiter. It is, says Lakey, “about the quickest turnaround we can do.”

“Because we always have a complete ‘chain’ of spacecraft attitude guidance onboard, we need to juggle with the new commands to make sure they don’t clash with the old ones or end up with none at all,” he says. “Both equally bad.”

The trajectory was plotted years ago, and this flyby is required to decrease the energy of the spacecraft ahead of its next closest observation of the Sun.

In Deep Space, debris isn’t something the team is too concerned with, however, now it has jumped to the top of the agenda. ESA has plenty of experience with dodging debris, although Lakey points out: “Unlike our friends next door in the Earth Observation division, things are complicated for us because of the inherent uncertainties in our trajectory and the time needed to process the tracking data.”

“Although they are no doubt wondering why we’re making such a fuss about doing a CAM, seeing as they do them with some regularity these days,” he adds.

And the infamous missile shenanigans in orbit? “Whereas our colleagues in SDO [Space Debris Office] have not had their lives made any easier by the ASAT test, it doesn’t change our plan – we’re somewhat on rails at this point and will be hurtling past Earth at a height of about 450km at 04:30 on the 27th, come what may.”

ESA’s own figures put the altitude at 460km, to which Lakey says: “Flight Dynamics would be able to give an expected value to within some large number of decimal places but it would be relative to the centre of the Earth…”

As for the spacecraft itself, it remains in good health. The team is running well under fuel budget, although if it proves necessary to adjust the delta-V then that margin will start being eaten into. That said, the budget shouldn’t be exceeded, “but ultimately yes, the less we fire the thrusters the less fuel we use.”

The instruments are all collecting science, and the spacecraft is due to dip down to 0.32AU in March. The heat shield has also been performing well, “to the point we have developed a ‘de-icing’ manoeuvre to warm up the backside of the spacecraft,” says Lakey. “Mooning the Sun, if you will.”

Indeed, the mission is packed full of unknowns for ESA and Lakey tells us “there’s a lot to learn about how to fly a spacecraft there.”

The hope is that the mission will get extended. ESA’s Mars Express is, after all, 18 years into a two-year mission (launched in 2003, it was only meant to operate for two years. The mission was recently extended to the end of 2022). This is assuming the Solar Orbiter doesn’t hit anything as it passes Earth.

The team will be at the European Space Operations Centre, “watching the telemetry like hawks,” says Lakey. “We’re used to being hundreds of millions of kilometres away from Earth with long signal propagation delays, so it’s quite novel for us to see the data coming down in real-time.”

“If we had hatches we would be battening them down around now.”

And if the worst should happen? “I don’t think humanity has created any structure that would withstand a debris strike at the speeds we’re going.”

“I prefer not to think about it too much.” ®



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