Mission accomplished

He calculates the positions of satellites to the centimetre. Professor Pieter Visser, aka ‘Satellite Pete’, has been appointed as professor of astrodynamics and space missions.

Snow, and nothing else - that is all Pieter Visser saw on his computer screen in the autumn of 2009. He had spent twenty years working on the GOCE mission. GOCE – the Gravity field and steady-state Ocean Circulation Explorer – was designed to map the earth‘s gravity field in unprecedented detail. Visser’s dissertation, which he defended in 1992, examined methods for interpreting satellite measurements and converting them into gravity field models. He later went on to become a member of the GOCE mission advisory group of the European Space Agency, ESA, continuing to closely monitor the construction and launch activities, and everything related to them.

As he explained during his inaugural address on 18 March, the astrodynamics expert almost came to view the mission as a person. "And sometimes as a rebellious adolescent." Understandably so, because several months after the launch, he received all this clutter on his screen. Where were the nice and tidy curves - the gravity signals - that he had been anticipating?

Did that make you anxious?

"You are indeed nervous beforehand. Satellites come with a hefty price tag. But fortunately, everything turned out fine. We just needed to make one correction. The satellite was rotating: the centrifugal forces had to be filtered out more effectively. Then, the gravity signal appeared. I took a photograph, which was passed all around ESA. Everyone was pleased. It gave an enormous kick."

What exactly gave the kick?

"The fact that we had demonstrably achieved a ridiculously high level of accuracy. GOCE calculated gravitational speeds to an accuracy of ten to the power of minus twelve metres per second squared. That is ten thousand billion times less than the speed of gravity on earth. The level of precision involved would be like measuring the force exerted by a snow flake pushing on an oil tanker. And, of course, it is also gratifying that my work can be directly applied elsewhere. We calculate down to the centimetre how high satellites fly, and that enables us to carry out measurements for oceanographers, climate researchers, and magnetic field researchers. We are able to measure rises in sea levels and the degree to which ice is melting in Greenland, to give a couple of examples. But my fascination lies in the technology."

In 2013, the satellite was almost completely burnt in the atmosphere.

"Yes. Some fragments fell back to earth, landing near the Falkland Islands. I spent twenty-five years working on it. Nothing has dominated my career as much as this."

You are currently in the Swarm steering group - three satellites that are measuring the earth's magnetic field, and which were launched in 2013, just as GOCE came to an end.

"In this field, you count yourself lucky if you experience just one successful mission in your entire career. So we in our group have been very fortunate."

But you have also known things to go badly wrong.

"CryoSat exploded in 2005. This was an earth observation satellite that was supposed to measure the changes to the thickness of the most important layers of ice on earth. I was also on the steering group. In October 2005, we all met at the Space Expo in Noordwijk. The satellite was launched from Russia. The Russians kept us informed second by second. The first rocket stage went fine. But just after the second stage was ignited, we heard nothing. After ten minutes, we felt very uneasy. We learned a few hours later that the satellite had crashed into the Arctic Ocean as a result of a software error. It was quickly decided to build the satellite again. CryoSat-2 was launched in 2010."

You originally came to Delft in order to learn how to build aircraft. Something completely different, in other words. What brought about the change?

He laughs. "Indeed, I came to Delft on account of my interest in aircraft. I hardly dare mention that now. Thanks to a lecture by Professor Karel Wakker, who would later be my PhD supervisor, I became fascinated by the field of astrodynamics. Wakker talked about the International Sun/Earth Explorer 3, a satellite that had previously been mothballed and which was being used to achieve a first - a meeting with the comet Giacobinni-Zinner. The satellite even came close to the better known Halley‘s comet. It made a remarkable journey using so-called gravitational slingshots past the Moon and Earth - all for the purpose of arriving at the 'target’ comet at exactly the right time. I thought that was such a fantastic achievement, I decided the field of astrodynamics was for me."

Last year, ESA created a furore with the spectacular journey of the European Rosetta space probe to the comet 67P/Churyumov-Gerasimenko. Are you going to focus on interplanetary missions as well?

"We are involved with Juice, Jupiter Icy moons Explorer, a satellite aimed at Jupiter and its moons. The calculation methods are largely the same as for the satellites that we put into orbit around the earth. However, this is uncharted territory. With interplanetary missions, it is easier to make bigger discoveries. For example, we hope to find underground oceans on Jupiter's moons. To work out the position of Juice, we will be using Doppler measurements, among other things. For that, we can use the large Westerbork radio telescopes. Doppler measurements are also used by the police to carry out speed checks on roads. According to the current - optimistic - timetable, Juice is due to be launched in 2022. This is very much a long-haul project. The first measurements will not arrive until 2030. I will be nearing retirement by then."

But you'll be sticking around a bit longer?

"Here, almost everyone carries on working even after they have reached retirement age. You work passionately on a mission, and then you are suddenly supposed to stop because you have reached a certain age? No. I meet people at conferences who are in their eighties, for example Professor Byron Tapley, who is still going strong. In 1993, I spent a year working under his leadership at the Center for Space Research of the University of Texas at Austin. It was one of the highlights of my career. Tapley is still engaged with new missions. Wakker is still working too, for that matter. He is in his seventies and is still giving lectures."

Apart from the increase in the number of interplanetary missions, what are the most important developments in your field.?

"There are more and more satellites, nanosatellites, which fly in swarms. The question is how you keep them at the correct distance from each other. These small probes are much more sensitive to disruptive forces, such as the pressure from solar radiation and the low level of air pressure in space. Meanwhile, the problem of space debris is getting worse. Satellites often have to change course to avoid colliding with debris. If we are better able to calculate the orbits of space debris, we can predict when exactly a satellite has to change course and when not. Changing course is an expensive matter, because satellites only have a limited amount of fuel on board."

In your inaugural address, you referred to the increasing commercialisation of space travel.

"Space travel is changing very fast. The activities of a company like SpaceX are nothing short of spectacular. SpaceX appears to be able develop less expensive rockets with private investors than was previously the case when space travel organisations were involved. This means that launches in the future may be achievable at considerably less cost. In addition, a number of other companies are operating on the interface of aviation and space travel, such as Virgin Galactic and XCOR (companies that seek to take tourists on brief flights just outside the earth's atmosphere, Ed.). For me, space travel begins when you are in an orbit around the earth. These companies are not offering that. They are, as it were, just scratching the surface. With XCOR, you can also carry out measurements during the long freefall back to earth. For example, you can test equipment that you want to use in a satellite at a later stage. I think companies of this kind can bring about a revolution in travel. You will soon be able to fly from Amsterdam to Sydney in an hour, outside the atmosphere. The question is whether this can be achieved in an environmentally friendly manner. Travel of this kind requires better models of the atmosphere. And who knows, maybe we can make a contribution in that regard."

Are people going to colonise space?

"There is no longer a space race between countries, as was the case with the Moon missions in the 1960s and 1970s. But the Chinese want to walk on the Moon and the Americans on an asteroid. Colonising Mars is a much more complicated issue. Safety is a major challenge. If there are solar eruptions, you could be threatened by radioactive particles. It may be that you have to wear a suit of lead, or build a huge shield around the satellite. The journey time is another problem. At the moment, it would take several years to fly to Mars and back. We have to cut that to a few months. It may be that the journey time will be much reduced with the help of electrical propulsion. And perhaps we can one day tackle nuclear fusion and be able to travel to Mars in a week."