For the third time this year, a large spacecraft in Earth orbit will likely fall from orbit. Unlike the previous spacecraft, UARS and ROSAT, the Russian spacecraft, Phobos Grunt, is a probe that was launched into a parking orbit on November 9, 2011 in preparation for an injection into a trajectory that would have allowed the spacecraft to reach Mars. However, a yet-to be determined anomaly prevented the spacecraft from performing the burn that would have placed it on a course for Mars. The window for Phobos Grunt to reach Mars passed on November 21st, and with ground controllers still unable to contact the spacecraft, it will likely reenter the atmosphere at a yet-to be determined date.
The impending reentry of Phobos Grunt presents the same issue of debris surviving similar to both UARS and ROSAT. However, Phobos Grunt presents a potentially more hazardous situation because it contains several tons of toxic hydrazine fuel in tanks aboard the spacecraft and in a large expendable external tank that was intended to fuel Phobos Grunt's escape from Earth orbit and then be jettisoned. Russian space officials state with certainty that the hydrazine fuel will explode some 50 miles in the atmosphere during reentry and does not pose a danger should debris from the spacecraft reach the surface of the Earth.
However, the position Russian space officials have taken with regards to the threat of debris has evolved since the possibility arose that Phobos Grunt would reenter the atmosphere. Russian space officials initially stated with certainty that no debris would survive reentry, but that position has evolved to the point where they admit that pieces of the spacecraft could survive reentry. The evolution of this position brings into question the true concerns that the Russian space agency have about the hydrazine fuel aboard Phobos Grunt. If that fuel has frozen or will do so before the spacecraft reenters the atmosphere it could survive reentry and potentially contaminate the environment or cause harm if it lands in a populated area. This possibility has led Phobos-Grunt being analogized with the scenario of the United States reconnaissance satellite designated USA-193.
USA-193 suffered a complete failure of its electrical systems shortly after launch. Without the ability to manuver, its orbit rapidly decayed and threatened to reenter the atmosphere with a full complement of the same type of hydrazine fuel contained within the fuel tanks of Phobos-Grunt. The United States, concerned that the hydrazine fuel aboard USA-193 was frozen and might possibly survive reentry, responded by planning and executing an intercept of the satellite. The United States planned to use an ancillary capability of its anti-ballistic missile system (ABM), which employed a modified SM-3 missile launched by the USS Lake Erie to intercept USA-193. The missile struck the hydrazine tank directly and fragmented the satellite. The debris from intercept, which was performed when USA-193 was in a very low orbit, reentered and burned up in the atmosphere within a few weeks, thus not only eliminating the hydrazine threat but also the possibility that the debris would not create a hazard to other spacecraft in orbit.
Considering the hydrazine question of USA-193 is similar to the situation with Phobos Grunt, the question is will a similar intercept be performed to prevent the potential of the hydrazine fuel reaching the ground. The answer depends on several factors. First, it is unclear that the Russian Federation has the technical capability to perform such an intercept. The Russian Federation has no publically acknowledged capability to perform an intercept, and even if they do possess an undisclosed capability, it is uncertain whether they would reveal it to perform an intercept of their errant spacecraft.
The alternative to the Russian Federation performing an intercept is to turn to the People's Republic of China. The PRC has a proven anti-satellite (ASAT) capability, which it demonstrated on its aging weather satellite FY-1C. The test, which was performed in a high polar orbit, resulted in a substantial debris field that continues to grow and resulted in international discord, which the PRC has yet to distance itself. While it is uncertain they would agree to perform an intercept either for technical or political reasons, the fact remains that they do possess the capability to perform it.
The less politically favorable alternative for the Russian Federation is to approach the United States to perform an intercept of Phobos Grunt using the same ancillary capability demonstrated with its ABM system, which intercepted USA-193. The circumstances would have to be grave indeed for the Russian Federation to do so given that the Russian Federation publically and diplomatically chastised the United States' intercept of USA-193 as a test of an ASAT and publically denounced the concerns about the hydrazine fuel. Furthermore, engaging the United States to perform the intercept might damage the Russian Federation's position in the stand-off it has with the United States over the proposed ABM system that the United States is proposing to install in Europe.
The other alternative even though it would certainly be liable under international law for any damage Phobos Grunt might cause once it reenters,is for the Russian Federation to do nothing and allow Phobos Grunt to fall to Earth with the hope that it does not impact land or cause damage to person and property. The question is what route will the Russian Federation take? Only as this situation plays out in the weeks to come will we be certain how events will unfold.