This article originally appeared on Van Kane's blog and is reposted here with permission.

One of the major revolutions in planetary science that I’ve seen in my lifetime is the discovery that the solar system contains not just one ocean world—our Earth—but several ocean worlds. Unlike our planet, which has its oceans on the surface, these other worlds trap their oceans beneath a surface layer of ice (or in the possible case of the asteroid Ceres, beneath a rocky shell). For several of these worlds, such as Jupiter’s moons Ganymede and Callisto, the oceans appear to be locked between layers of ice and therefore would be unlikely candidates for abodes of life. For two of the moons, Jupiter’s Europa and Saturn’s Enceladus, the oceans appear to lie directly on top of a rocky core that would provide key elements needed to support life as well as energy from possible hydrothermal vents. Saturn’s moon Titan is a unique case, with seas of liquid ethane, methane, and propane on the surface and a water ocean in the interior that may or may not be in contact with the rocky core and occasionally interact with the surface. (This article and poster give more background on these worlds and their oceans.)

NASA’s managers, at the direction of Congress, have begun to put together an Ocean Worlds program to explore Europa, Titan, and Enceladus. At a recent meeting of an advisory group for NASA, the Committee on Astrobiology and Planetary Science (CAPS), Jim Green, the head of NASA’s Planetary Science Division, and Barry Goldstein from the Jet Propulsion Laboratory, provided updates on plans to explore these worlds. In this post, I’ll report on the highlights of their talks (the presentations will be posted to this site (scroll down to the March 29–31 meeting) sometime in the future).

Europa Multi-flyby Mission

The only currently approved mission in the Ocean Worlds program is the Europa multi-flyby spacecraft. This mission, estimated to cost ~$2 billion, will orbit Jupiter and take approximately 45 toe dips into the high radiation belts surrounding this moon to make close flybys. In between flybys, the spacecraft will have time to transmit the volumes of data it collected up close back to Earth. (This presentation gives a good overview of the mission design and science goals while this presentation summarizes the instrument payload.)

The mission is well into its design phase. At the CAPS meeting, the project manager, Barry Goldstein with the Jet Propulsion Laboratory, updated the committee members on refinements to the design.

Until recently, NASA’s managers had hoped that the main spacecraft could carry a 250 kilogram free flying daughter spacecraft to conduct complimentary studies. Ideas ranged from a simple Europa lander, to a spacecraft that would divert to flyby the volcanic moon Io, to a spacecraft dedicated to flying through any plumes ejecting material from the surface of Europa. NASA had invited the European Space Agency to propose (and pay for) a daughter spacecraft. In addition, a group at NASA’s Goddard Space Flight Center had developed a proposal for a free flyer that would swoop even lower to the surface than the main spacecraft to fly through any plumes while carrying a mass spectrometer more tuned to identifying bio signatures than the main spacecraft’s instruments.

Unfortunately, it appears that NASA has decided to drop the idea of a daughter spacecraft. I’m told that ESA’s managers determined that they had no way to fund such a spacecraft on the timeline for the Europa mission. NASA’s managers may have also decided they lacked the funding to build their own daughter spacecraft.

Dropping the daughter spacecraft opens up new possibilities for launching the multi-flyby spacecraft to Jupiter. NASA’s primary plan for sending this spacecraft on its way is the Space Launch System (SLS) that would enable a direct launch. This extremely large booster could launch the spacecraft directly to Jupiter with a flight time of 2.1 to 2.5 years. It will have sufficient heft to give the project a 33–35% mass margin, providing a cushion should the actual spacecraft as finally implemented weigh more than its designers currently think it will (which usually happens).

The SLS, however, is still in development and its reliability will only be proven through one or more future flights. In addition, this program is something of a political football, and so assuming it will be funded through development and into the time period of the Europa launch is a risk. It’s also unclear what an SLS launch would cost and whether or not the planetary science program could afford it. The project’s managers, therefore, are designing the spacecraft to also be capable of being launched on a less powerful commercial launch vehicle.

Currently that backup would be either an Atlas V 551 or Delta IV Heavy booster followed by three Earth and one Venus flybys to receive gravity assist boost that would enable the final flight to Jupiter (known as the EVEEGA trajectory). This extended looping flight would take 7.4 years to reach the Jovian system.

Dropping the 250 kilogram free flyer (plus supporting equipment on the main spacecraft) opens up an alternative launch plan. By enlarging the spacecraft’s propellant tanks to allow a large deep space maneuver to set up a single Earth gravity assist (known as the ∆v/EGA trajectory), a Delta IV Heavy vehicle could deliver the Europa mission in just 4.7 years while still providing a healthy mass margin of 34%. (For Falcon Heavy fans, NASA’s managers will consider this booster, too, once they have its final specifications, but they believe it would have similar performance to the Delta IV Heavy.)

This new launch backup is not yet an official plan as engineers and NASA’s managers examine it in more detail. If they decide they can adopt it, the net savings in flight time if the SLS launch is unavailable is 2.7 years.