Q&A with Heino Falcke, principal investigator of the Netherlands-China Low-Frequency Explorer
Heino Falcke, an accomplished radio astronomer at Radboud University in the Netherlands, leads a team of scientists and engineers who developed a scientific receiver flying on Chinaâs Queqiao spacecraft flying to a point in space beyond the far side of the moon.
The Dutch payload riding piggyback on the Queqiao mission was developed in less than two years, an unusually fast time for a space instrument, and launched May 20 aboard a Long March 4C rocket from the Xinhua space base in southwestern Chinaâs Sichuan province.
Queqiao is a communications relay satellite for Chinaâs Changâe 4 mission, a lunar lander and rover set for launch by the end of the year to attempt the first soft touchdown on the far side of the moon. China built the Queqiao spacecraft to link ground controllers with the rover after landing, enabling communications that are otherwise impossible between the lunar far side and Earth.
It turns out the far side of the moon, and the region of space around it, is prime real estate for astronomers who seek to study a time a few hundred million years after the Big Bang, when an ocean of hydrogen made up the universe before the first stars were born, a period known as the cosmic dark ages.
Astronomers need to detect low-frequency waves to probe this part of the universeâs ancient past, and Earthâs atmosphere blocks most of the signals from reaching ground-based radio observatories. And interference from Earthâs own radio emissions introduce noise to listening posts that could be launched into Earth orbit.
The far side of the moon is a âradio quietâ zone free of such interference, and the Netherlands-China Low-Frequency Explorer aboard the Queqiao spacecraft will demonstrate the potential of sending future radio astronomy missions to such an observation post.
Falcke spoke with Spaceflight Now shortly before the Queqiao missionâs launch May 20.
Q: How does it feel to be sending an instrument beyond the far side of the moon?
A: âIâve been dreaming about this since I was a kid. I saw the some of the last Apollo missions to the moon. I was like 5 years old sitting in front of the TV. I didnât see the first one, but I remember kids playing outside, and I was sitting inside watching the last man on the moon (in 1972). In 1969, I was just three. It was always something that was inspiring, and I think made me go into science in the first place. I got into low frequency radio astronomy, and the moon is the best place, in principle, to do that science. That connection with radio astronomy came up, and Iâve been working on this for the last 15 years perhaps. Iâve been involved in the Low Frequency Array (LOFAR) in the Netherlands. We have, I think, revolutionized low frequency radio astronomy with it because weâve increased, by an order of magnitude, resolution and imaging capability.
âThen the idea came up, whatâs next? The idea, of course, was LOFAR on the moon. We were part of ESA discussions. A European lunar lander was planned with a payload called the Lunar Radio Explorer to do the dark ages experiment. We were part of the strawman design. In principle, the lander had this radio experiment on-board, but then the entire lander was canceled. Space is always back and forth. The U.S. says weâre going back to the moon, th en no, weâre not allowed to touch to the moon, and now weâre going back to the moon. Itâs a back and forth, and it has reverberations around the world. That wasnât the only reason why it was canceled though.
âWe had these studies and this proposal ready, more or less, and this discussion with the Chinese, suddenly, out of nowhere, popped up in 2015. There was a trade mission by the Dutch king, which had a lot of industry people. The Dutch are good in radio astronomy, and they knew about that. The chinese said, âWeâre going to the moon,â and the link was made at the conference reception by people in the delegation. This opportunity was identified as part of this negotiation or discussion, and we were asked ot put the proposal in, which we did within two weeks.
âThat was accepted in April 2016 or so by the Chinese. This is now more than two years ago, and that meant then that we had to organize the funding. We had to build the experiment, and then we ha d the problem that the Americans are not allowed to cooperate with the Chinese. We have always, in this community, interacted very well with our American partners, and that wasnât possible anymore, so we had to build things from scratch, like the antennas. Typically, we would buy these antennas in Berkeley. Theyâre the main experts for low frequency antennas. But that wasnât possible, so we had to build them from scratch. We have a company in Delft, which is building smaller satellites. We build the digital electronics. ASTRON, the institute where I came from originally, which built the LOFAR array, provided the front end, and another company developed the control computer. That was a completely crazy timescale. We developed this essentially from scratch, so the boundary conditions changed. We couldnât use easily existing things.â
Q: Because they included U.S. parts?
A: âYes, our designs used those parts. W e had to find other parts. We were still in good discussions with our U.S. partners. They are quite happy that something is happening now in space in this arena. This will all give us some experience.
Q: Will you share data from this experiment with U.S. scientists?
A: âThe data will be shared between us and the Chinese. The goal is to share our results with the world. The results could be general. I want share the data, as we always do, with our colleagues. Weâll have the first look at it, but we want to use others to collaborate. So the data first goes to China and the Netherlands, but we want to share the results with the world.â
Q: Can you give me an idea of the size and mass of this instrument?
A: âItâs about 10 kilograms total. The most impressive part, when unfolded, are these three orthogonal monopole antennas, which are 5 meters long. They are sort of tape measure anten nas. Itâs carbon fiber, and theyâre rolled out by a motor. It takes half an hour per antenna to unroll. What you do is you roll out one antenna a half-meter, then do the next one a half-meter, and the next one. So it takes two hours or so, in total, to deploy all three antennas.â
Q: And those were all built in the Netherlands?
Q: I undersrtand that your primary objective is to use this as a technology demonstration to detect these very low frequencies in space. So whatâs so interesting about astronomy in these frequency bands?
A: âYou had the Big Bang. The universe was cooling, and particle s were coming into existence, and at some point it became cool enough that hydrogen was formed, and thatâs when the universe became optically thin. So you had this hot plasma, which goes into the neutral hydrogen phase. Thatâs 390,000 years after the Big Bang, and thatâs when you get the cosmic microwave background radiation. Then, for a few hundred million years, thereâs essentially nothing but this atomic hydrogen, and theres nothing to be seen. Itâs just an ocean of hydrogen, more or less, with some dark matter in the background. No stars, no planets, no heat or light, except the cosmic microwave background radiation, which is still there. The hydrogen has this 21-centimeter line, a spin flip of the electron, which is emitted. That has to do whether itâs emitted in absorption or transmission. It has to do with the state of the plasma of the gas at that time.
âThe only thing youâd see is this 1.4 gigahertz emission, which is essentially a cell phone frequenc y. Itâs going to be absorbed in this phase, but then this light is red-shifted, or stretched, due to the expansion of the universe when it reaches us. So the emission is shifted. Itâs not seen as a 21-centimeter wave, but is actually seen as a 10-meter wave or longer. So from 21-centimeter to more than 10-meter wave. This wave is stretched by the expanding universe because it goes from the early universe to here. So what you expect to see, if you look at all the sky, there should be a faint deficit of light at a few tens of meters wavelength, which comes from the very early universe, which is the early gas absorbing this emission. So itâs this 21-centimeter emission redshifted by a factor of 70 or so.â
Q: This canât be observed from Earth?
A: âBelow 30 megahertz, the ionosphere becomes very, very difficult. Below 10 megahertz, it shuts off completely and you have all the shortwave emissions in the 10 to 20 megahertz range a s well, as interference. You have reflection. You have emissions going partially through the atmosphere, and partially not. So itâs almost impossible to make this measurement from the ground because of the changing environment. Youâd need to integrate for quite some time to see it.
âWe have the EDGES (Experiment to Detect the Global Epoch of reionization Signature) signal, and it was very hard work to get it going in Australia, a radio quiet region of the Earth. But at below 10 megahertz, you canât see anything at all. You really have to do this in space.â
Q: And the far side of the moon is a good place?
A: âItâs the best place for radio astronomy because, in the long run, you need multiple of these antennas, and then you can just place on the ground there on the far side of the moon. Most importantly, behind the moon, you are always shielded from emissions from the Earth, which is the strongest emitter in these fr equencies. Thatâs why the far side of the moon is protected by ITU radio regulations as a radio quiet zone to make sure it stays that way.â
Q: Youâre setting out to demonstrate this concept, and this capability, but do you expect to learn anything about the cosmic dawn from this particular experiment?
A: âIn principle, we could. If we integrate long enough, we could see it. The reason why Iâm careful to not claim that we can, because we have not demonstrated that we can do this, and the reason is itâs a new receiver, of course, but we are on this satellite on which we donât really know its properties, its electromagnetic properties. It has not been particularly designed to be very radio quiet, and has this shape, and the shape influences your beam pattern of the radio antennas, so you would need to know this shape very precisely, and that shape is sort of uneven. It has a big dish in front, and that would change the beam pattern as a function of frequency. You canât model this very well. In fact, we have not even had the design drawings from the Chinese. We donât get to know all the details about the satellite that we are flying on, and that will make it very difficult to calibrate the antenna.â
Q: That strikes me as rather unusual, to not have detailed information about the characteristics of the spacecraft your payload is flying on?
A: âYes. This is as much a technological experiment as it is a sociological experiment in collaborating with our Chinese partners, and communicating. This is, to my understanding, the first Western experiment theyâve had in their exploration program. I think they have more (Western) experiments on the (Changâe 4) lander. This is not an easy or standard way for the Chinese to communicate, so we donât get all the information about their space program, obviously.
Q: In your mind, whatâ s the importance of collaborating with the Chinese like this?
A: âI think collaborating with Chinese scientists has become quite standard these days in all kinds of fields. Chinese colleagues work in the U.S. and in Europe and go back to China. Thereâs a good and very friendly collaboration in the academic world with our Chinese partners, so there we see very little problems when it comes to pure science programs. Many of them are very well known colleagues in us to the west. The space progam is somewhat different. It is still part of the military branch in China. Itâs much more a part of national prestige, so I think it is much less common to collaborate in the space program. I know that people will look to us with a bit of curiosity to see how does it work. We also see from the Chinese side that sometimes they have to get used to the fact that thereâs this foreign experiment, because they used to do things internally. Sometimes, we donât hear abou t things. Weâll make an agreement with our Chinese colleagues, but then someone in the hierarchy doesnât know about it, or has different priorities, and then we get a different signal. Itâs not that thereâs a very clear line of information always, and maybe we donât always pick up the right signals.â
Q: How much did this instrument cost?
A: âAbout 3 million euros.â
Q: Through the Netherlands Space Office?
Q: When will the observations with your instrument begin
A: âThat was another issue. We had a certain deadline when we had to deliver things. The Chinese actually moved it forward, which made our timeline very challenging. We sensed some nervosity among the Chinese partners because our antenna was new and so forth, and I can partly understand that. Initially, the idea was that after the launch weâd get to t he moon, theyâll go around 100 kilometers from the moon during the flyby, and then they shoot over the moon and brake at the L2 point. But we donât get all the details and things change, so we sometimes donât have the latest information.
âIt takes a while to get there, but itâs hanging around, so to speak, behind the moon and waiting for the lander to come. The idea was to use that time to do our measurements, but then they said they want to do the deployment of the antenna only after the main mission is over, and I think thatâs quite sensible, so that will be likely in March of 2019.â
Q: Is there a follow-up to this mission, to do low frequency astronomy in space?
A: âYes. There are U.S. efforts. Jack Burns from the University of Colorado is doing something on this. We certainly have plans to have a follow-up mission based on this. Initially, scientists wanted to gonna have a Lissajous orbit that would have pass ed through the moonâs shadow more frequently. Thatâs something you can still do with a smaller satellite more like a CubeSat, and something more symmetric with three dipole antennas. That would be ideally suited to do these kinds of measurements. I think if we can design our own spacecraft, I suspect we could do a relatively low-cost mission to do these measurements, with an improved receiver and a more radio quiet spacecraft. With all the expertise we gain about this location on this mission, then thatâs an interesting option. Of course, if thereâs a lunar lander, you could take along an antenna and place it ideally on the far side, or in the shadow of a crater or mountain on the south pole, then most of the time you have the Earth blocked. That, I think, would be the obvious next step to do, and we hope this will be the first step toward that.âSource: Google News Netherlands | Netizen 24 Netherlands