Jul 192019
 

The world is celebrating the 50th anniversary of one of the most momentous events in human history: the first time a human being set foot on another celestial body.

It is also a triumph of American ingenuity. Just as Jules Verne predicted a century earlier, it was America’s can-do spirit that made the Moon landing, Armstrong’s “one small step” possible.

And today, just like 50 years ago, their success was celebrated around the world, by people of all nationality, religion, gender or ethnicity.

But that’s not good enough for some New York Times columnists.

Instead of celebrating the Moon landing, Mary Robinette Kowal complains about the gender bias that still exists in the space program. Because, as we learn from her article, this evil male chauvinistic space program was “designed by men, for men”. Because, you know, men sweat in different areas of their body and all. Even in the office, temperatures are set for men, which leaves women carrying sweaters.

Sophie Pinkham goes further. Instead of celebrating America’s success on July 20, 1969, Pinkham goes on to praise the Soviet space program in a tone that might have been rejected even by the editors of Pravda in 1969 as too over-the-top. Because unlike America, the Soviets put the first woman in space! Their commitment to equality did not stop there: They also sent the first Asian man and the first black man into orbit. Because, we are told, “under socialism, a person of even the humblest origins could make it all the way up.”

Just to be clear, I am not blind to gender bias. We may have come a long way since the 1960s, but full gender equality has not yet been achieved anywhere: not in the US, not in Canada, not even in places like Iceland. And racism in America remains a palpable, everyday reality. Back in 1969, things were a lot worse.

But to pick the 50th anniversary of an event that, even back in the turbulent 1960s, had the power to unify humanity, to launch such petty rants? That is simply disgraceful. Or, as the New York Post described it, obscene.

The New York Post also makes mention of one of the female pioneers of the US space program, Margaret Hamilton, whose work was instrumental in making the Apollo landings possible. Yet somehow, neither Pinkham nor Kowal found it in their hearts to mention her name.

I have to wonder: Are columnists like Pinkham or Kowal secretly rooting for Donald Trump? Because they certainly seem to be doing their darnedest best to alienate as many voters as possible, from what appears to be an increasingly bitter, intolerant, ideological agenda on the American political left.

 Posted by at 6:49 pm
Jul 162019
 

Fifty years ago today, fifty years ago this very hour in fact, at 9:32 AM EDT on July 16, 2019, Apollo 11 was launched.

Moonbound Apollo 11 clears the launch tower. NASA photo

And thus began a journey that, arguably, remains the greatest adventure in human history to date.

I was six years old in 1969, hooked on the novels of Jules Verne. With Apollo 11, Verne’s bold imagination became the reality of the day.

 Posted by at 9:25 am
Jul 152019
 

Galileo is the world’s third global satellite navigation system, built by the European Union, operating in parallel with the American GPS system and Russia’s GLONASS. It has been partially operational since 2016, with a full constellation if satellites expected to enter service this year.

But as of early Monday, July 15, Galileo has been down for nearly four days, completely inoperative in fact:

As of the time of this writing, no explanation is being offered, other than one article mentioning an unspecified issue with Galileo’s ground-based infrastructure.

It really is difficult to comprehend how such a failure can occur.

It is even more difficult to comprehend the silence, the lack of updates, explanations, or any information about the expected recovery.

 Posted by at 12:59 am
Jan 062019
 

I almost forgot: a couple of months ago, I was interviewed over the telephone by a journalist who wanted to know my thoughts about one of my favorite moments in manned space exploration: The Apollo 8 “Genesis” moment, the reading of the opening verses of the Old Testament, on Christmas Day, 1968, by the astronauts of Apollo 8 as their spacecraft emerged from behind the Moon.

Today, something reminded me of this interview and I did a quick search. Lo and behold, there it is: My words, printed in The Boston Globe on December 23, 2018:

“It was a beautiful moment, and Genesis is part of our Western cultural heritage,” said Viktor Toth, an atheist and a senior research fellow at Carleton University in Ottawa, Canada, who played the lead role in the investigation of the Pioneer Anomaly, the mysterious acceleration of the Pioneer 10 and 11 spacecrafts in deep space. “This was an awe-inspiring thing: Human beings for the first time cut off from the Earth, and then they reemerged and saw the Earth again. The message was entirely appropriate.”

Though shortened, this pretty accurately reflects what I actually said during that roughly 10-minute conversation with the journalist.

 Posted by at 10:12 pm
Jan 042019
 

Even as China was celebrating the first successful landing of a spacecraft on the far side of the Moon, NASA’s New Horizons continued to radio back data from its New Year’s Day encounter with Ultima Thule: a strange, “contact binary” asteroid in the Kuiper belt, far beyond Pluto.

Ultima Thule will remain, for the foreseeable future, the most distant celestial object visited by spacecraft. While there is the odd chance that New Horizons may find another target within range (as determined by the on-board fuel available, which limits trajectory corrections, and the aging of its nuclear power source that provides electricity on board), chances are it won’t happen, and it won’t be until another deep space probe is launched, quite possibly decades from now, before we get a chance to see a world as distant as Ultima Thule.

Another piece of news from the New Horizons project is that so far, the probe found no moon orbiting Ultima Thule. No Moon At All.

 Posted by at 8:46 pm
Jan 032019
 

OK, the far side of the Moon is not really dark, but it is kind of hard to see. But now, from the department of unqualified good news: China successfully landed its spacecraft, Chang’e 4 (named after the Chinese Moon goddess), on the dark side of the Moon, and it has already sent us back some pictures.

This is big. Really big. To make it happen, China first had to launch a lunar orbiter, Queqiao (“Magpie bridge”), in order to maintain communication with the lander. And being on the far side of the Moon, the lander is completely shielded from radio signals from the Earth, which means an unprecedented opportunity to study radio signals of extrasolar origin.

Chang’e 4 also carried a rover, Yutu-2, which has since been deployed.

By any reasonable measure, this is a huge success for China’s space program, and for humanity overall. Hopefully, both lander and rover will remain operational and able to fulfill their scientific objectives.

 Posted by at 9:40 pm
Dec 242018
 

A quote from 50 years ago is the most appropriate one tonight, considering that our world is just as troubled as the world of 1968:

And from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.

 Posted by at 5:25 pm
Aug 212018
 

Yesterday, I received a nice surprise via e-mail: A link to a new article in Astronomy magazine (also republished by Discover magazine) about our efforts to solve the Pioneer Anomaly.

I spent several years working with Slava Turyshev and others on this. It was a lot of very hard, difficult work.

As part of my (both published and unpublished) contributions, I learned how to do precision modeling of satellite orbits in the solar system. I built a precision navigation application that was sufficiently accurate to reconstruct the Pioneer trajectories and observe the anomaly. I built a semi-analytical and later, a numerical (ray-tracing) model to estimate the directional thermal emissions of the two spacecraft.

But before all that, I built software to extract telemetry from the old raw data files, recorded as received by the Deep Space Network. These were the files that lay forgotten on magnetic tape for many years, eventually to be transferred to a now obsolete optical disc format and then, thanks to the efforts of Larry Kellogg, to modern media. My own efforts, to make sense of these telemetry files, is what got me involved with the Pioneer Anomaly project in the first place.

These were fun days. And I’d be lying if I said that I have no tinge of regret that in the end, we found no anomalous acceleration. After all, confirmation that the trajectories of these two Pioneers are affected by an unmodeled force, likely indicating the need for new physics… that would have been tremendous. Instead, we found something mundane, relegated (at best) to the footnotes of science history.

Which is why I felt a sense of gratitude reading this article. It told me that our efforts have not been completely forgotten.

 Posted by at 8:05 pm
Mar 102018
 

There is a very interesting concept in the works at NASA, to which I had a chance to contribute a bit: the Solar Gravitational Telescope.

The idea, explained in this brand new NASA video, is to use the bending of light by the Sun to form an image of distant objects.

The resolving power of such a telescope would be phenomenal. In principle, it is possible to use it to form a megapixel-resolution image of an exoplanet as far as 100 light years from the Earth.

The technical difficulties are, however, challenging. For starters, a probe would need to be placed at least 550 astronomical units (about four times the distance to Voyager 1) from the Sun, precisely located to be on the opposite side of the Sun relative to the exoplanet. The probe would then have to mimic the combined motion of our Sun (dragged about by the gravitational pull of planets in the solar system) and the exoplanet (orbiting its own sun). Light from the Sun will need to be carefully blocked to ensure that we capture light from the exoplanet with as little noise as possible. And each time the probe takes a picture of the ring of light (the Einstein ring) around the Sun, it will be the combined light of many adjacent pixels on the exoplanet. The probe will have traverse a region that is roughly a kilometer across, taking pictures one pixel at a time, which will need to be deconvoluted. The fact that the exoplanet itself is not constant in appearance (it will go through phases of illumination, it may have changing cloud cover, perhaps even changes in vegetation) further complicates matters. Still… it can be done, and it can be accomplished using technology we already have.

By its very nature, it would be a very long duration mission. If such a probe was launched today, it would take 25-30 years for it to reach the place where light rays passing on both sides of the Sun first meet and thus the focal line begins. It will probably take another few years to collect enough data for successful deconvolution and image reconstruction. Where will I be 30-35 years from now? An old man (or a dead man). And of course no probe will be launched today; even under optimal circumstances, I’d say we’re at least a decade away from launch. In other words, I have no chance of seeing that high-resolution exoplanet image unless I live to see (at least) my 100th birthday.

Still, it is fun to dream, and fun to participate in such things. Though now I better pay attention to other things as well, including things that, well, help my bank account, because this sure as heck doesn’t.

 Posted by at 12:59 pm
Dec 242017
 

It’s the same, each and every Christmas. As Christmas Eve approaches, I remember that famous moment from 49 years ago. The astronauts of Apollo 8 just orbited the Moon. It was Christmastime. These three men were a thousand times farther from the Earth than any human being in history. It was an awe-inspiring moment. Once radio contact with the distant Earth was re-established, the three astronauts took turns reading the first ten verses of Genesis. Frank Borman then closed the broadcast with words that, in my mind, remain the most appropriate words for this evening: “good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.

 Posted by at 7:49 pm
Sep 152017
 

NASA’s Cassini spacecraft is no more.

Launched 20 years ago, Cassini arrived at Saturn in 2004 and has been studying the ringed giant ever since. Cassini also carried the Huygens probe, which executed a successful descent into the dense atmosphere of Saturn’s moon Titan, and even transmitted data from its surface.

Its fuel nearly exhausted, Cassini was steered into a trajectory that led to its intentional demise: a fiery plunge into Saturn’s atmosphere earlier this morning. As planned, the spacecraft was able to transmit observations until the very end, when its thrusters were no longer able to maintain its attitude during the descent.

Program manager Earl Maize and operations team manager Julie Webster embrace after signal loss.

I feel sad that Cassini is gone, but I should also feel elated because it has been an incredibly successful mission. I just hope I live long enough to see another probe visiting Saturn, perhaps a probe or set of probes that are designed to land on Titan, maybe even sail its hydrocarbon seas, in search of possible life on that icy world.

 Posted by at 10:11 am
Jul 272017
 

There is a brand new video on YouTube today, explaining the concept of the Solar Gravitational Telescope concept:

It really is very well done. Based in part on our paper with Slava Turyshev, it coherently explains how this concept would work and what the challenges are. Thank you, Jimiticus.

But the biggest challenge… this would be truly a generational effort. I am 54 this year. Assuming the project is greenlighted today and the spacecraft is ready for launch in ten years’ time… the earliest for useful data to be collected would be more than 40 years from now, when, unless I am exceptionally lucky with my health, I am either long dead already, or senile in my mid-90s.

 Posted by at 11:27 pm
Dec 242016
 

Once again, I feel compelled to use the same image and same words that I have been using for many years, to wish all my family, all my friends, indeed everyone on the good Earth a very merry Christmas: the words of the astronauts of Apollo 8.

I know, I know, it’s the same thing every year. But there really aren’t any better words. Just imagine: three human beings, for the first time in human history, far from the Earth, in orbit around another celestial body. And back on Earth, one of the most troubled years in recent history: 1968. So on Christmas Eve, with about a billion people listening—a full one quarter of the Earth’s population at the time—they greeted us Earthlings with the opening passages from the Book of Genesis, the common creation mythology of several major religions.

And then Frank Borman ended the broadcast with words that are as appropriate today as we are heading towards more troubled times as they were back then: “And from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.”

 Posted by at 9:10 am
Apr 092016
 

This beautiful image is a frame capture of the latest SpaceX first stage rocket, moments after its successful landing on the drone ship Of Course I Still Love You (yes, that really is the drone ship’s name) last night:

The landing was a little sloppy. I mean, look how far off-center the rocket happens to stand.

Still… I am seriously beginning to believe that Elon Musk may accomplish his ultimate goal within my lifetime: the beginning of the human colonization of Mars.

To live long enough to see the first human set foot on Mars… now that’s a dream worth living for.

 Posted by at 10:21 am
Mar 292016
 

Until recently, this used to be one of my favorite deep space images:

It is a frozen lake in the Ruach Planitia region of Neptune’s Moon Triton: an incredibly distant, dark and desolate world.

OK, the image is still one of my favorites, but on my list of favorites, it’s just been taken over by this one:

That, ladies and gentlemen, is a large (about 30 km) frozen lake (most likely frozen nitrogen), in the Sputnik Planum region of the planet Pluto.

Who would have thought that Pluto, the recently demoted ex-planet, a frozen world at the edge of the solar system, would have such complex climate and such a fascinating geological history?

Wow.

 Posted by at 9:35 am
Jan 292016
 

Eons ago, back when dinosaurs still roamed the Earth, George W. Bush was still a first-term president, there were only five Star Wars films and Java applets were still cool, I created an applet that showed what Mars would look like if its surface was covered by oceans.

I liked what I did so I added the capability to use other data sets, including data sets for the Earth.

The applet is worthless now, or almost so. Java applets are no longer supported in Google’s Chrome browser. They were never really supported on mobile platforms. Even in browsers that do still support Java, the user has to go through hoops and add my domain as a security exception (not recommended) to allow my unsigned applet to run; all this a result of vain attempts to address the security risks inherent in Java and its implementations.

Anyhow, the applet still works if you can run it. And this is what the Earth looks like today:

Someone recently asked what our planet would look like if it was devoid of oceans. If sea levels were 5000 meters below the present value, the planet would still have a shallow ocean in place of the Pacific. Otherwise, though, it would be mostly dry land with only some inland seas where the Atlantic and the Indian oceans used to be.  It would be possible to walk from pole to pole without wetting your feet; however, you might get a tad thirsty along the way, and there’d not be much rain either.

Decrease ocean levels by another 1000 meters to 6000 below present sea levels, and the last remaining ocean is gone:

Finally, at 7000 meters, the only open water that remains would be in places of the deepest ocean trenches. (Mind you, even then, some of these seas would still be up to four kilometers deep.)

I was also asked what things would look like if the seas rose. There is a surprising amount of change to coast lines by an increase of a mere 50 meters:

Florida is gone; Western Europe looks noticeably different. Increase the sea level rise to 200 meters, and now the change is rather more dramatic:

earth+0200

India is now an island or almost so (there may be some land bridges connecting it to the Asian continent that are too narrow to be visible at this map’s resolution). Much of Europe, Russia, Australia, South America, and the eastern parts of North America, gone.

Finally, at 1000 meters, only mountain ranges remain:

With this little dry land left, there is not much in the way of storms; like Jupiter with its Great Red Spot, the Earth might also develop long-lived storms that circumnavigate the planet many times before dissipating.

 Posted by at 3:30 pm
Jan 282016
 

This is NASA’s week of tragedy.

Today is the 30th anniversary of the loss of the space shuttle Challenger with seven souls on board. One of my notable memories of this event is that it was the first time that I recall that the national broadcaster in then still communist Hungary didn’t dub a speech of Ronald Reagan. I think the speech was actually carried live (it took place at 5 PM EST, which would have been 11 o’clock at night in Hungary; late, but not too late) and it may have been subtitled, or perhaps not translated at all, I cannot remember. For me, it was also the first disaster that I was able to record on my VCR; for days afterwards, my friends and I replayed and replayed the broadcasts, trying to make sense of what we saw. (Sadly, those tapes are long lost. My VCR was a Grundig 2000 unit using a long-forgotten standard. After I left Hungary, I believe my parents used it for a while, but what ultimately happened to it and my cassettes, I do not know.)

Yesterday marked the 49th anniversary of the Apollo 1 fire that claimed the lives of three astronauts who were hoping to be the first to travel to the Moon. Instead, they ended up burned to a crisp in the capsule’s pure oxygen atmosphere, with no chance of escape. Arguably though, their tragedy resulted in much needed changes to the Apollo program that made it possible for Apollo 11 to complete its historic journey successfully.

And finally, in four days it will be exactly 13 years since the tragedy of Columbia, which disintegrated in the upper atmosphere at the conclusion of a successful 16-day mission. I remember that Saturday all too well. I was working, but I also had CNN running on one of my monitors. “Columbia, Houston, comm check” I heard many times and I knew something already that those in the mission center didn’t: CNN was already showing the multiple contrails over Texas, which could only mean one thing: a disintegrating vehicle. And then came the words, “Lock the doors”, and we knew for sure that it was all over.

Of course the US space program was not the only one with losses. The Soviet program had its own share of tragedies, including the loss of Vladimir Komarov (Soyuz 1 crash, April 24, 1967), three astronauts on boar Soyuz 11 (depressurization after undocking while in space, June 30, 1971), and several deaths on ground during training. But unlike the American cases, these Soviet deaths were not all clustered around the same date.

 Posted by at 3:35 pm
Dec 302015
 

It is nice to have a paper accepted on the penultimate day of the year by Physical Review D.

Our paper in question, General relativistic observables for the ACES experiment, is about the Atomic Clock Ensemble in Space (ACES) experiment that will be installed on board the International Space Station (ISS) next year. This experiment places highly accurate atomic clocks in the microgravity environment of the ISS.

How accurate these clocks can be depends, in part, on knowledge of the general relativistic environment in which these clocks will live. This will be determined by the trajectory of the ISS as it travels through the complex gravitational field of the Earth, while being also subject to non-gravitational forces, most notably atmospheric drag and solar radiation pressure.

What complicates the analysis is that the ACES clocks will not be located at the ISS center-of-mass; therefore, as the ISS is quite a large object subject to tidal accelerations, the trajectory of the ACES clocks is non-inertial.

To analyze the problem, we looked at coordinate transformation rules between the various coordinate systems involved: geocentric and terrestrial coordinates, coordinates centered on the ISS center-of-mass, and coordinates centered on ACES.

One of our main conclusions is that in order for the clock to be fully utilized, the orbit of the ISS must be known at an accuracy of 2 meters or less. This requirement arises if we assume that the orbits are known a priori, and that the clock data are used for science investigations only. If instead, the clock data are used to refine the station orbit, the accuracy requirement is less stringent, but the value of the clock data for scientific analysis is also potentially compromised.

It was an enjoyable paper to work on, and it is nice to end the year on a high note. As we received the acceptance notice earlier today, we were able to put the accepted version on arXiv just in time for it to appear on the very last day of the year, bearing the date December 31, 2015.

Happy New Year!

 Posted by at 8:57 pm
Dec 242015
 

It has become a habit of mine. On Christmas Eve Day, I like to offer my best wishes to all my friends, members of my extended family, and indeed to all good people on this Earth with the words of the first three human beings in history who left our planet and entered orbit around another celestial body: The astronauts of Apollo 8, who accomplished their historic mission at the end of one of the most tumultuous years since World War 2, 1968.

And as they emerged from the dark side of the Moon and reestablished radio contact with the Earth, they greeted their fellow humans by quoting from the Book of Genesis. They then finished their broadcast with these unforgettable words: “And from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas and God bless all of you – all of you on the good Earth.

 Posted by at 5:00 pm
Dec 162015
 

The reason for my trip to China was to participate in the 3rd workshop on the TianQin mission.

TianQin is a proposed space-borne gravitational wave detector. It is described in our paper, which was recently accepted for publication in Classical and Quantum Gravity. The name, as typical for China, is poetic: it means a zither or harp in space or perhaps (sounds much nicer in English) a celestial harp. A harp that resonates in response to continuous gravitational waves that come from binary pulsars.

Gravitational waves are notoriously hard to detect because they are extremely weak. To date, we only have indirect confirmation of gravitational waves: closely orbiting binary pulsars are known to exhibit orbital decay that is consistent with the predictions of Einstein’s gravity.

Gravitational radiation is quadrupole radiation. It means basically that it simultaneously squeezes spacetime in one direction and stretches it in a perpendicular direction. This leads to the preferred method of detection: two perpendicular laser beams set to interfere with each other. As a gravitational wave passes through, a phase shift occurs as one beam travels a slightly longer, the other a slightly shorter distance. This phase shift manifests itself as an interference pattern, which can be detected.

But detection is much harder in practice than it sounds. Gravitational waves are not only very weak, they are also typically very low in frequency. Strong gravitational waves (relatively speaking) are produced by binaries such as HM Cancri (aka. RX J0806.3+1527) but even such an extreme binary system has an orbital period of several minutes. The corresponding gravitational wave frequency is measured in millihertz, and the wavelength, in tens or hundreds of millions of kilometers.

There is one exception: inspiraling neutron star or black hole binary systems at the very end of their lives. These could produce detectable gravitational waves with frequencies up to even a kilohertz or so, but these are random, transient events. Nonetheless, there are terrestrial detectors such as LIGO (Laser Interferometer Gravitational-wave Observatory) that are designed to detect such events, and the rumor I heard is that it may have already happened. Or not… let’s wait for the announcement.

But the continuous waves from close binaries require a detector comparable in size to the wavelength of their gravitational radiation. In short, an interferometer in which the laser beams can travel at least a few hundred thousand kilometers, preferably more. Which means that the interferometer must be in space.

This is the idea behind LISA, the Laser Interferometer Space Antenna project. Its current incarnation is eLISA (the “e” stands for “evolved”), a proposed European Space Agency mission, a precursor of which, LISA Pathfinder, was launched just a few days ago. Nonetheless, eLISA’s future remains uncertain.

Enter the Chinese, with TianQin. Whereas eLISA’s configuration of three spacecraft is designed to be in deep space orbiting one of the Earth-Sun Lagrange points with inteferometer arm lengths as long as 1.5 million kilometers, TianQin’s more modest proposal calls for a geocentric configuration, with arm lengths of 150,000 km or so. This means reduced sensitivity, of course, and the geocentric orbit introduces unique challenges. Nonetheless, our colleagues believe that it is fundamentally feasible for TianQin to detect gravitational waves from a known source with sufficient certainty. In other words, the primary mission objective of TianQin is to serve as a gravitational wave detector, confirming the existence of continuous waves emitted by a known binary system, as opposed to being an observatory, usable to find previously unknown sources of gravitational radiation. Detection is always easier: in radio technology, for instance, a lock-in amplifier can be used to detect the presence of a carrier wave even when it is far too weak to carry any useful information.

Theoretical sensitivity curve of the proposed TianQin mission.
Theoretical sensitivity curve of the proposed TianQin mission.

The challenges of TianQin are numerous, but here are a few main ones:

  • First, precisely controlling the orbits of shielded, drag-free test masses such that their acceleration due to nongravitational forces is less than \(10^{-15}~{\rm m}/{\rm s}^2\).
  • Second, precisely controlling the optical path such that no unmodeled effects (e.g., thermal expansion due to solar heating) contribute unmodeled changes more than a picometer in length.
  • Third, implementing time-delay interferometry (TDI), which is necessary in order to be able to compare the phases of laser signals that traveled different lengths, and do so with sufficient timing accuracy to minimize the contributions due to fluctuations in laser frequency.

Indeed, some of the accuracy requirements of TianQin exceed those of eLISA. This is a tall order for any space organization, and China is no exception. Still, as they say, where there is a will…

Unequal-arm Michelson interferometer
Unequal-arm Michelson interferometer.

One thing that complicates matters is that there are legal barriers when it comes to cooperation with China. In the United States there are strong legal restrictions preventing NASA and researchers at NASA from cooperating with Chinese citizens and Chinese enterprises. (Thankfully, Canada is a little more open-minded in this regard.) Then there is the export control regime: Technologies that can be utilized to navigate ballistic missiles, to offer satellite-based navigation on the ground, and to perform remote sensing may be categorized as munitions and fall under export control restrictions in North America, with China specifically listed as a proscribed country.

The know-how (and software) that would be used to navigate the TianQin constellation is arguably subject to such restrictions at least on the first two counts, but possibly even the third: a precision interferometer in orbit can be used for gravitiational remote sensing, as it has been amply demonstrated by GRACE (Gravity Recovery And Climate Experiment), which was orbiting the Earth, and GRAIL (Gravity Recovery And Interior Laboratory) in lunar orbit. Then there is the Chinese side of things: precision navigation requires detailed information about the capabilities of tracking stations in China, which may be, for all I know, state secrets.

While these issues make things a little tricky for Western researchers, TianQin nonetheless has a chance of becoming a milestone experiment. I sincerely hope that they succeed. And I certainly feel honored, having been invited to take part in this workshop.

 Posted by at 5:32 pm