CE-2 flyby of Toutatis Options

[JimOberg]

"China kept this encounter secret -- even the date -- with the intent, arguably, to cover up any failure. "

Jim, post #5 at the start of this thread suggests to me that China did not keep anything secret.

Phil

Good point, and I'd seen that paper -- so was puzzled by total lack of run-up and real-time coverage of the encounter.

To modify my complaint: despite disclosure to the space science community of the planned intercept date [a fact independently discovered and published by foreign astronomers], there was no Chinese media attention to the progress of the mission for months leading up to, and days AFTER, the encounter.

I'm still dismayed by that behavior. And to discourage it happening again, I urge those who share my dismay to pass it on to their Chinese contacts.

It really was out of character of recent Chinese public information release policies, which have been satisfyingly candid and open, even about future plans and actions. We had gotten accustomed to an open publicity policy. Seeing this one exception to it startled me.

[Paolo]

Is it possible that the aim point was much farther out and this is just an accidental close pass?

apparently, they aimed a point 15 km from the asteroid (its center of mass, I think). ephemeris uncertainties of the two objects probably summed (in the worse possible direction) to yield 3.2 km (I don't know whether this is measured relative to the surface or center of mass. in the former case, it would translate in a miss distance from the center of mass of some 5 to 6 km)

[JimOberg]

apparently, they aimed a point 15 km from the asteroid (its center of mass, I think). ....

Source, please?

[machi]

This one, I suppose.

[JimOberg]

This one, I suppose.

You've picked a language I can't handle, please translate the relevant portion. Hsieh hsieh ni.

[tedstryk]

Here is what most of us are using. I know the Google translation is rough, but it is what most of us non-Chinese speakers are using. Not clear as to why you couldn't do this.

Two stars collided probability of less than one ten-millionth Intersection speed of nearly 11 kilometers per second, Chang E II will Tutadisi [Toutatis] asteroid collision? Wu Weiren smiled and said: "If you hit, it created a miracle in order to avoid causing trouble Chang E II track detuning by some, and let them pass."

Zhou Jianliang said: "through the precise measurement and control, we can control the distance between the Chang-e II and Tutadisi asteroid if they collided, the impact of the asteroid is small but we should not have caused it any impact. "he said, need to take pictures on the Tutadisi" Chang E on the 2nd design orbit is to make them as close as possible without collision. Our current monitoring and control capability, the minimum distance if they rendezvous designed for 15 km, the collision probability is less than one ten-millionth.

According to reports, Chang E II with Tutadisi asteroid rendezvous when the distance is only 3.2 km.

[JimOberg]

Here is what most of us are using. I know the Google translation is rough, but it is what most of us non-Chinese speakers are using. Not clear as to why you couldn't do this.

Thanks! Since the article mentions the actual 3.2 km fly-by, does this mean it was published AFTER the actual fly-by -- or is it dated before the encounter?

ADMIN - Jim, UMSF is a place for discussion and collaboration. It's not a short order grill where you can show up at the counter and make demands. That is exactly how your previous three posts have come across. You have the same resources available to you through the Internet that the rest of us have. Investigate it yourself and let us all know.

[JimOberg]

Unnecessary quoting removed

Acknowledged and agreed!

[jgoldader]

The success of this flyby makes one wish for a few spacecraft stationed at one of the Earth-Moon Lagrange points that we could dispatch, with sufficient notice, for flyby inspections of other closely approaching NEOs. A visual camera and perhaps a near-IR imager with well-chosen filters or an imaging near-IR spectrometer, maybe even a magnetometer, would make for a nice payload. I suspect that in some cases, navigation strategies might exist that could eventually return the spacecraft back to the Lagrange point for additional encounter opportunities.

Jeff

[dvandorn]

The success of this flyby makes one wish for a few spacecraft stationed at one of the Earth-Moon Lagrange points that we could dispatch, with sufficient notice, for flyby inspections of other closely approaching NEOs...

Good concept. However, since you'd have a wide variety of approach trajectories for these various NEOs, any such on-demand intercept spacecraft would likely need to start out its mission with an awful lot of delta-V available in its fuel reserves. With enough energy packed into the fuel tanks, and with possibly a small fleet of such interceptors, you could use some to perform high-delta-V intercepts once or twice in their lifetimes, and others to perform low-delta-V intercepts many times.

Not only would this be a good way to collect data on the various NEOs that pass through the neighborhood, it would be really good operational experience for deploying a small fleet of interceptors whose purpose is last-minute trajectory deflection on objects that are spotted late and could impact Earth. We obviously don't have good or proven strategies for last-minute deflection technology at the present time, of course... but by the time we decide what measures would be effective, we would have a lot of good operational experience at deploying on-demand spacecraft to intercept approaching objects.

-the other Doug

[elakdawalla]

Michael Khan from ESA has done a little trigonometry using the distance information on that graphic and came up with the head-scratching result that, assuming the distance and time information on the graphic is correct, Chang'E 2 passed the asteroid at 59 kilometers:



The other thing that has me a little puzzled is that the images on the multi-view montage must not all be shown at their original pixel scale; the range to the target decreases by only a little more than half, yet the apparent size enlarges by more than a factor of 10. Examining them closely, the smallest image looks fairly clean, while the largest one has artifacts that suggest it's been enlarged.

[machi]

I did some calculations based on two highest resolution images (47 km/16:30:05, 93 km/16:30:09) and resulting distance is anywhere between 0 - 33 km. Time for closest approach is 16:30:00-01. Errors are caused by inaccuracies in time information.
It looks that some informations are definitely incorrect. Because we have evidently images with 5 and 10 meters resolution, I think that 240 km and 16:30:24 are wrong informations.
Another possibility is that flyby distance wasn't 3.2 km, but 32 km. 32 km can be result in all calculations (based on informations 47 km/16:30:05, 93 km/16:30:09, 240 km/16:30:24).

[mcaplinger]

Examining them closely, the smallest image looks fairly clean, while the largest one has artifacts that suggest it's been enlarged.

Isn't this just the same image replicated at different sizes, as was pointed out earlier?

If it was a full sequence, one would expect to see a big change in phase angle from inbound to outbound since the geometry changes rapidly around C/A.

[Unless the whole image sequence was taken a fair time out from C/A, which doesn't make a lot of sense. Still don't know what the camera FOV or IFOV was or how the spacecraft was pointed during the fyby. Was it just inertially fixed?]

[Phil Stooke]

The illustration might just be an 'artist's impression' of the approach, not the actual image sequence. We don't really know what audience it was intended for. It might not support too much analysis.

Phil

[machi]

If it was a full sequence, one would expect to see a big change in phase angle from inbound to outbound since the geometry changes rapidly around C/A.

If flyby distance was really 3.2 km, then changes in phase angle were minimal even few seconds after flyby .
I did simple graphic with two images, for which we know (?) distance. For comparison I've added 240 km distance.

Still don't know what the camera FOV or IFOV was or how the spacecraft was pointed during the fyby. Was it just inertially fixed?

If Paolo is right, then FOV is 7.2° and camera has 1024×1024 CMOS detector (or maybe 1280×1024). This roughly corresponds to published resolution 10m/93 km, 5m/47 km.

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