Jupiter’s icy moon Europa (above left in a reprocessed color view made by images taken by NASA’s Galileo spacecraft in the late 1990) is thought to have a warm ocean shrouded beneath a layer of ice. Saturn’s icy moon Enceladus (shown in cross section above right) is also believed to have a liquid ocean between its rocky core and icy crust. Credit: NASA

A version of this opinion piece originally appeared in the Oct. 8 issue of SpaceNews as “Europa or Enceladus? Why Choose?”

Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, are very similar worlds. Both distant moons are thought to have warm water oceans shrouded with a layer of ice.

The oceans of Europa and Enceladus are warmed by the tidal forces of their respective planets and may contain lifeforms that have never seen the sky. Therefore, both moons are prime targets for further exploration. Which one we should explore first?

Three main differences exist between Europa and Enceladus. The Cassini space probe, which encountered  Enceladus some 22 times during its 13 years orbit Saturn, flew through the geysers of water that erupt from underneath the ice layer through fissures. Cassini found evidence of complex organic molecules that could indicate life beneath the surface of Enceladus. Europa has similar geysers, but the evidence of organic molecules is far less certain. Europa is orbiting in the middle of a zone of intense radiation emanating from Jupiter.

Any space probe that spends too much time in that region would quickly find its electronics fried unless it were heavily shielded. Enceladus’ environment is relatively clear of hard radiation. Nevertheless, Europa is the current first target for NASA, with the Europa Clipper due to launch in the early 2020s and a Europa lander to follow a few years later.

Rep. John Culberson (R-Texas), the chair of the House subcommittee that funds NASA, is a moving force for exploring Europa. Europa Clipper will orbit Jupiter, flying by Europa frequently, before moving out of the radiation zone. Europa Lander will follow once its predecessor maps the Jovian moon and locates some landing sites.

With the planning for the Europa Clipper and the Europa Lander in advanced stages and a powerful member of Congress supporting the twin missions, re-tasking the probes to Enceladus is likely not in the cards.

However, a way may be found to do both. The Europa Clipper and Europa Lander are envisioned to be launched to Jupiter space by NASA’s planned heavy-lift Space Launch System. The SLS will be able to lob huge payloads toward Jupiter on a direct flight path, avoiding the time-consuming gravity assist maneuvers that previous probes to the outer planets have had to use.

The problem with the Space Launch System is that it is a fully expendable rocket that could cost between $1.5 billion to $2.5 billion to launch. NASA is struggling to make the SLS more affordable to operate, but the sad fact is that using the heavy-lift rocket is a great expense for the missions to Europa. NASA does have the option of using a commercial rocket, say the SpaceX Falcon Heavy, to launch the twin missions to Europa.

The Falcon Heavy has a slightly lower lift capacity than the Space Launch System, 64 metric tons to low Earth orbit as opposed to 70 metric tons. And the SLS has a larger fairing that can accommodate a wider payload. Enhancements down the line will increase the Space Launch System’s capabilities even more.

However, the Falcon Heavy has two distinct advantages over the Space Launch System. Even in the totally expendable mode the SpaceX rocket costs just $150 million to launch. Just as important, Falcon Heavy has already flown.

Switching to the Falcon Heavy may cause some trade-offs in designing both the Europa Clipper and the Europa Lander to fit the smaller rocket. However, the cost savings could be plowed into an Enceladus orbiter. A probe could be sent to the icy moon of Saturn and orbit it for as long as necessary to ferret out its secrets.

Indeed, enough money might be left over to land on Enceladus, near one of the fissures, to attempt to ascertain what resides beneath its icy surface. Two icy moons for the price of one sounds like a pretty good deal for NASA and the planetary science community.

A recent NASA Inspector General Report that detailed the continuing cost overruns and schedule slippages experienced by the Space Launch System further makes the case for using commercial rockets to send probes to Europa and Enceladus.

Mark Whittington, who writes frequently about space and politics, has published a political study of space exploration entitled “Why Is It So Hard to Go Back To The Moon?” He blogs at Curmudgeons Corner.