Wednesday, November 18, 2009

Phoenix Phone Home?

Now that we're finally coming out of the harsh and deep Martian arctic winter, the engineers at JPL will soon be making an attempt to contact the Phoenix Lander. Sometime in January or February (at the time of writing) the Goldstone 70-m antenna will be trained skyward to tune into the Deep Space Network, hoping for a signal relayed off of the Mars Odyssey or Mars Reconnaissance Orbiter (MRO). But how likely is a successful contact? And even if contact is successful, what then?

The big question on everyone's mind is whether or not the lander even survived the winter. In the Martian arctic, just like in the arctic here on earth, winter is greeted with increasingly short days and eventually the long night of the solstice. And on Mars, the winter lasts almost twice as long as on the Earth. As a consequence, winter temperatures above 60 degrees of latitude plummet to below the frost point of Carbon Dioxide. Since the bulk of the martian atmosphere is composed of this gas, it condenses out into a layer up to several meters thick. This layer is called the Seasonal Polar Cap and contrasts with the permanent or perennial polar cap which is made of water ice. You can see the progression in this series of HST images: http://www.nasaimages.org/luna/servlet/detail/nasaNAS~4~4~16147~119568:Seasonal-Changes-in-Mars--North-Pol.

This seasonal cap is thought to be made up partly of a fluffy frost, but also contains thick slab ice in places. Such a deposit could mechanically damage parts of the lander, especially in more delicate places like the solar panels. Furthermore, deprived of so-called keep-alive heating in the long, dark polar night, most of the electrical components will have dropped far below their temperature design tolerances. Optical components are particularly vulnerable, as are electrical connections not rated for this degree of cold.

But this is not the worst that slab ice can do. This transparent layer can also cause severe disruptions in the underlying bed. By trapping incoming solar radiation which is readily absorbed by the dark regolith, evaporation can occur at the base. The resulting high pressure layer of carbon dioxide gas is unstable and eventually will cause the overlying ice to buckle and crack explosively. This geyser-like eruption of gas and entrained regolith is called a sublimation spider (http://hirise.lpl.arizona.edu/PSP_003114_0930) and if one formed near the lander it would be bad news. The lander could be mechanically disrupted, or the resulting regolith streak could cover the solar panels, leaving a till that would prevent them from capturing solar radiation once the ice sublimated away.

Our current state of understanding of the lander's condition is sketchy. While the HiRISE camera on MRO has recently obtained some imagery of the lander and area (http://hirise.lpl.arizona.edu/ESP_014393_2485), the contrast is still insufficient to tell if Phoenix is mechanically sound. We also won't know much more for several weeks, since MRO has been chasing a computer bug which has prevented science operations since late August.

But let's, for argument's sake, assume that the solar panels are intact and still generating power come January. What can we expect? For starters, the spacecraft will likely be hurting. Many of the instruments may be damaged, some fatally. The SSI and RAC optics may be cracked, allowing dust into their interiors. The LIDAR may also be internally damaged and unable to produce a beam or analyze the results. The Robotic Arm Joints may no longer function. MECA and TEGA may be more robust, but most of their cells are already used up, and they probably would require more power to be run then will be available in the spacecraft's weakened state. Remember that when the lander ceased communications in 2008 after 152 sols, it was not yet even northern equinox. Still, power levels and temperatures had already fallen below what was sustainable for a solar-powered mission.

Additionally, the spacecraft will be highly confused. With the cold and lack of power, it is likely that the on-board computers have rebooted countless times. Thus it is unlikely that the spacecraft has any idea what the time or date will be. Since Phoenix was the first interplanetary spacecraft designed without a dedicated direct-to-earth connection, it must rely on orbiter overflights for communicating with Earth. Each of these lasts only a few minutes every couple of hours. But since the lander has no idea where and when it is, Phoenix will not know when these overflights will occur. Thus it will start transmitting in a search pattern, saving up enough battery power to transmit, trying for contact, and then shutting back down. To make matters worse, given the high latitude of the lander, not all overflights present a good communications opportunity. Often, the orbiters barely rise above the horizon.

In the best of cases, we will make contact. But do not expect a return to science operations. While I am available, I don't expect a call from Pasadena to take up my strategic science planner's job on Phoenix again. However, that simple beep of recognition is a valuable sign, and if that's all we get, it will still be greeted by smiles and celebration. It shows that it is possible for a spacecraft to hunker down and survive at high latitudes on Mars. That information alone opens up new possibilities for exploration.

If we are extraordinarily lucky, it might be possible to get some data on the state of health, or even some SSI images. These simple command sequences, called "runouts" can be run with modest manpower as there will not be time to assemble many people before winter closes in again. Thus this represents the absolute best we can hope for. We've got our fingers crossed: here's to hoping that our baby made it through.

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