It used to be said that the Sun never set on the British Empire. Those days may be long gone, but there is still a place where the Sun forever shines, and it’s on the Moon. The Peak of Eternal Light is a mountain at the lunar south pole that is always in view of the Sun. Its year-round temperature is a comparatively mild (by lunar standards) -20C, making it possibly useful as a site for a future lunar base. The possibility of water ice in nearby craters, though not proven, could be an attractive bonus.
No wonder the European Space Agency is fascinated with the Peak of Eternal Light. Fascinated enough to make it a prime survey target for SMART-1, the ion-powered spacecraft that entered lunar orbit on Monday. SMART-1’s studies of the Moon’s south pole will surely be fascinating, as will its look at the South Pole-Aitken Basin, a huge impact crater that punches deep into the Moon’s mantle. At stake may be new theories about the Moon’s formation.
But for deep-space enthusiasts, the SMART-1 mission is equally valuable as a shakeout of ion engine technologies. To adjust the spacecraft’s orbit for the long haul, its engine will burn almost continuously for the next four days before a set of shorter burns achieves the final orbit some time in January. SMART-1’s engine uses electricity from solar panels to heat xenon gas, creating ions that are accelerated by electrical forces to emerge as thrust. So far the engine has performed flawlessly. SMART-1 saved fuel at the expense of speed and time, and its 82 liters of xenon (the stuff that used to be found in flash bulbs) proved more than adequate for a spiralling, 84-million kilometer cruise to the Moon.
From an article on SMART-1 by David McAlary for the Voice of America, quoting Marc Rayman, deputy mission manager for NASA’s Deep Space 1:
“Ion propulsion is applicable to any place we want to get to in the solar system that previously has been unaffordable or physically impossible to get to with what we had,” said Marc Rayman. “The set of technologies we have, by allowing us to make the spacecraft less expensive, launch on more affordable launch vehicles, means that now we can be more rapid in responding to scientific questions that we devise and sending spacecraft to more places in the solar system and conducting more detailed studies.”
Deep Space 1’s mission was the first time an ion engine had been used for primary propulsion. What Rayman is talking about are trips to the outer Solar System and perhaps the Kuiper Belt. Early NASA concept studies for missions to nearby interstellar space, one called the Interstellar Precursor, the other the Thousand Astronomical Unit mission, were designed around nuclear-electric ion engines. These would use nuclear power to ionize the xenon fuel because sunlight at those distances would no longer be adequate for the job. The ion option won’t get us to Centauri, but it’s a key player in the expansion to the outer planets that will one day spread to nearby stars.