Greetings from the Atlanta Airport — I am off to the Republic of Korea this week. Before I go, I wanted to share another great talk I heard from the meeting George Lewis hosted at Cornell.
Yousaf Butt, now at the Harvard Smithsonian Astrophysics Center, gave a wonderful talk athat leaves me further doubting the Bush Administration’s claim that USA-193 posed a risk to human health and needed to be shot down.
We’ve gone round and round on this one, both on this blog and elsewhere. (You should, if you have a chance, read the exchange between Yousaf and Andrew Higgins in Space Review — starting with Yousaf’s letter, Higgins’s reply and their final exchange).
Since Yousaf and Higgins exchanged letters, Yousaf has succeeded in having NASA release, through the Freedom of Information Act, one of the studies in question: Robert L. Kelley and William C. Rochelle, Atmospheric Reentry of a Hydrazine Tank.
The study concludes — not surprisingly — that the tank (and hydrazine) would survive re-entry to pose a risk to human health. But a more interesting question is whether the study would have survived peer review.
As Yousaf notes in a technical commentary for the Bulletin of the Atomic Scientists, the study makes three assumptions that the authors acknowledge are unrealistic. What is particularly disturbing, is that the simplifications all push in one direction — to inflate the likelihood that the hydrazine tank will survive:
1. NASA modeled the remaining hydrazine as as uniform mass along the interior surface of the sphere with a void in center. (The tank was 3/4 full). In fact, the tank contains a bladder that pushes the hydrazine toward the fuel out-take.) Here are Yousaf’s diagram.
By evenly distributing the hydrazine around the tank, NASA can assume that tank would tumble and that the heat, therefore, will be uniformly distributed around the entire tank. If the mass is off center, the tank would stabilize during reentry, like a shuttlecock, loading the heat on the front-end.
2. NASA modeled the hydrazine as a single finite-element. This means that before any of the hydrazine is assumed to melt, it must all begin melting. “The simplification in this model which could have the largest impact is also difficult to address,” Kelley and Rochelle wrote. “It is extremely likely that the N2H4 in contact with the Ti wall will melt away in layers.”
3. NASA modeled the temperature of the frozen hydrazine as 214 K, even though temperatures in excess of 250 K are more realistic. “The initial temperature of the tank and the N2H4 was dictated to be 214 K for this study,” Kelley and Rochelle wrote, “although the actual temperature would very likely be higher.” Dictated?
Simplification is, of course, necessary in modeling. I am clearly out of my depth in refereeing this dispute, but the one-directional nature of the bias is disconcerting, as is the fact the hydrazine tank exploded and burned for tens of seconds unexpectedly. Such things do not inspire confidence in NASA’s model.
At issue — other than the truthfulness of certain public officials — is how NASA models the reentry of spacecraft and debris like tanks of hydrazine. NASA uses a one-dimensional model called the Object Reentry Survival Analysis Tool (ORSAT). The European Space Agency has developed a multi-dimensional code called SCARAB — Spacecraft Atmospheric Reentry and Aerothermal Breakup.
Both are good for simple object that like tumbling spheres. But many spacecraft, like all cows, aren’t spherical. There is some reason to think SCARAB is going to be more accurate for objects, like the hydrazine tank on USA 193, that deviate from these simplifications.
Now that doesn’t prove that the tank and hydrazine would have burned off. Fortunately, Yousaf and his colleagues at Hyperschall Technologie Göttingen, an ESA contractor, are using the SCARAB code to build a three-dimensional model of the tank of hydrazine.
It will be interesting to see if the hydrazine, and the Administration’s story, survive re-entry.