X-37B, Tiangong-1, and Bad Orbital Math
Bloopers in Space, Jim Oberg, IEEE Spectrum
“A horrendous error appears to have been made by a well-respected British spaceflight society, which reportedly is about to publish an article claiming that the X-37B, the secret Pentagon space plane, is shadowing the recently launched Chinese prototype space station, Tiangong-1, to spy on it in flight. But that’s ridiculous. The American plane’s orbit is at a steep angle with respect to that of the Chinese space station. When the two vehicles pass, they do so at speeds of up to 8000 meters per second, making it practically impossible for one to gather intelligence on the other.”
Good pick by Jim. What’s more astounding was the BBC publishing the story without checking the facts. Similar inclinations do not mean similar orbits. And of course the idea that the X-37 is a spy satellite is also laughable. Let’s try to think critically, folks!
Same orbital inclination , same altitude, but the X37 and Tiangong are 105 ° out of phase. Oops. The British publishers of the “Spaceflight ” journal are, after all, amateurs.
Having said that , the reporters failure to fact check and get second sources of attribution says a lot about what passes for journalism these days , and the general state of affairs in basic science education
I would point out that the RAAN (longitude of ascending node) precesses, and the rate of precession depends on the size of the orbit. X-37B orbit is about 30km smaller than that of Tiangong. So that means that, given some time (I’m not sure how much), the orbits WILL be coplanar. Now, I don’t think the Spaceflight folks were considering that, but it means that if the USAF wants to get sort of near something in a Tiangong (or Shenzhou, for that matter) orbit, putting a long duration facility at that inclination is not a totally crazy idea.
Now, Jim makes other good points about why X-37B was probably not sent up specifically to look at Tiangong, but the fact that the RAAN is different is probably not the best one.
The oblateness (non-spherical nature) of the Earth does indeed cause the RAANs of the two orbits to change at slightly different rates: that of X-37B decreases by about 6.23 deg/day, while that of Tiangong by about 6.13 deg/day. However, this implies that the orbits will shift relative to each other at a very low rate: less than a tenth of a degree per day. At this rate, the planes will coincide in about 2.7 yr, far beyond the end of the X-37B lifetime. Oberg’s RAAN-based argument against the spying story therefore appears solid.
Thanks for the specifics. That would indeed seem to rule out X-37B as targeting Tiangong. But I do find it curious that X-37B was launched into an inclination that at least covers both the Tiangong and previous Shenzhou ground tracks exactly. It’s an unusual orbital inclination that is quite China-specific. Wonder why it was chosen.
I was suspicious of this story when it first broke, partly for the reasons cited by Oberg (though since I was busy, I didn’t do the math), but mostly because we have assets better suited to the job. Unfortunately, the Internet being what it is, blogs are still picking up the story as if it were true, and news, and viewing logical debunking like Oberg’s as attacks on their conspiracy theories. This may be about as bad as the smaller number of sites that stated without fear of contradiction that the X-37B was testing a weapon capable of triggering earthquakes and had caused the Japanese tsunami of 2011. Oy…
As a high-tech specialist of ancient vintage, I usually operate under the old reporting rule of “If your mother says she loves you, check it out.” I find this sort of sloppy acceptance distressing; didn’t *anyone* bother to compare the orbits of the two? Meanwhile, we can depend on this misinformation continuing to spread across the net like a fungus for months to come as bloggers with more enthusiasm than sense stumble across it and recapitulate it. I sometimes think that if all the misinformation and ‘crankery’ were erased, the Internet’s storage requirements would shrink by half.
Only half? I’d be inclined to apply Surgeon’s law to this one — “ninety percent of everything is crap.“
Steve