October 30, 2013
The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft is currently circling the Moon. With the spacecraft safely settled into its observation orbit, the mission science team is busy testing and calibrating its instruments. This U.S. mission was designed to characterize the lunar “atmosphere” – the extremely tenuous zone of gases that vary in time in the space above the Moon. Technically called an exosphere, this region contains extremely low concentrations of a variety of elements and compounds, of varied origins and a largely unknown life cycle. LADEE is designed to monitor and characterize these species, with the goal of identifying the process and sources of the gases and how they vary with time.
Initially a precursor to human lunar return, LADEE was selected for development early in 2008, as we wanted to understand the lunar exosphere before the lunar environment was contaminated by humans. The LADEE spacecraft is designed to observe the Moon in its natural, pristine state. However, the very act of going to the Moon inadvertently (though briefly) modifies the lunar atmosphere. When a spacecraft arrives at the Moon, it uses its on-board rocket engines to brake into lunar orbit or to descend to the surface. These rockets spew large quantities of exhaust gas into space and as the vehicles get captured into the Moon’s gravity field, so too does this exhaust product.
From estimates drawn on the Apollo landings, the rocket exhaust expended from each Lunar Module temporarily doubled the total mass of the natural lunar atmosphere. This artificial addition of gases eventually dissipates, driven off by solar interactions and other complex effects. In time, the Moon resumes its normal state of near-vacuum. The creation of a temporary artificial atmosphere created by rocket effluent and its subsequent dissipation is imperfectly understood, except to the extent that we know that it happens. The one-month “commissioning phase” that the LADEE mission is currently experiencing was largely designed to ensure that the exhaust from the orbital braking burn of the spacecraft (and subsequent low-rate out-gassing from the spacecraft) is largely complete. We want to measure the Moon’s environment, not the products of the craft that brought us there.
But the U.S. will not be the only one conducting a mission at the Moon for the next few months. The long-planned Chinese robotic mission Chang’E 3 is scheduled for launch to the Moon in early December. Their lander mission will place a fairly large (1200 kg) spacecraft on Sinus Iridum in the northwestern quadrant of the near side, deliver a small roving vehicle and examine and measure the properties of the lunar surface over the course of several months. But before it begins its surface mission, the Chang’E 3 spacecraft will burn roughly 2600 kg of rocket fuel in the vicinity of the Moon’s exosphere. I have not seen any documentation on the fuel this spacecraft will use, but it is highly likely that it will be the chemicals unsymmetrical dimethylhydrazine (UDMH; H2NN(CH3)2) and nitrogen tetroxide (N2O4). These propellants are widely used in spacecraft because they are liquid at room temperature and can be easily stored in tanks for long periods of time (a requirement for long-duration spaceflight to destinations beyond low Earth orbit).
When UDMH and nitrogen tetroxide are burned in a rocket engine, they produce a variety of exhaust gases; the dominant combustion products are water (H2O), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), and a few trace species, including hydrogen (H2) and hydroxyl (OH). Expelled by a rocket nozzle, these gases rapidly expand in all directions in the vacuum of space. Because most of the burn occurs after the spacecraft has been “captured” by the gravity of the Moon, this rocket exhaust is also captured by the Moon. Thus, exhaust from an orbital or a landing vehicle becomes (temporarily) part of the lunar atmosphere.
If you’re thinking that this “rude” addition of alien gases will mess up the very delicate phenomena that LADEE was designed to map and measure, you’re correct – it does. You might even expect the scientists of the LADEE team would be very upset at this disruption of their carefully planned measurement strategy. But you would be wrong. This problem is actually an opportunity.
The coincidence of Chang’E 3 arriving at the Moon after LADEE has begun observations has developed into a serendipitous occurrence for lunar science. Because we don’t understand very well how exospheric gases are added to and removed from the Moon, what has landed in our laps is an unplanned (but controlled) experiment. A known quantity of gases – of known composition – will be added to the lunar atmosphere at a precisely known time, in a precisely known place. One could have not designed a better experiment to measure how this addition of material is distributed, how its distribution evolves over time, and how these expelled gases dissipate into cislunar space. Even better, LADEE will have almost a full month to monitor and characterize the lunar atmosphere before Chang’E arrives, thus allowing us to first observe the “natural” Moon and then the “contaminated” Moon and how the lunar atmosphere recovers from its defilement.
None of this was prearranged – the Chinese schedule their missions on the basis of their own time-table and programmatic needs (just as NASA’s lunar goals have changed over the last 5 years). But because of a fortuitous alignment of schedules, we have a unique opportunity to observe in real time how the Moon works. Hopefully, the Chinese will provide us with detailed mass numbers of their spacecraft and exactly what variety of fuel it carries, but even if they don’t, physics dictates a certain mass and volume of the exhaust gas and its composition will be measured by LADEE (allowing us to know the type of fuel used). China’s December lander mission to the Moon will provide our U.S. mission with a welcome bit of “traffic exhaust,” giving scientists the opportunity to learn more from LADEE than we’d originally envisioned.
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