CubeSats to Mars and beyond
by Jeff Foust
|The MarCO cubesats, flying by Mars at an altitude of 3,500 kilometers, will receive InSight’s UHF signals and retransmit them on X-band frequencies back to Earth in real time.|
All of that activity and interest involving CubeSats, though, has been primarily limited to spacecraft in Earth orbit. Some scientists and engineers have started to explore what CubeSats could do beyond Earth orbit, including missions to the Moon and near Earth asteroids, with mission concepts being developed for launch later in the decade (see “CubeSats to the Moon”, The Space Review, August 4, 2014). But that interest is accelerating, with the first interplanetary CubeSat mission now under construction for launch next year, and concepts for increasingly ambitious missions under study.
That first interplanetary mission is now slated to be a JPL project called Mars Cube One, or MarCO. A pair of 6U CubeSats roughly 12 by 24 by 36 centimeters in size, the MarCO spacecraft will hitch a ride on next March’s Atlas V launch of the InSight Mars lander mission. Deployed from the upper stage of the rocket, MarCO-A and -B will coast in interplanetary space on the heels of InSight.
MarCO’s mission unfolds when the spacecraft arrive at Mars in September 2016. As InSight enters the Martian atmosphere, it will transmit telemetry at eight kilobits per second on a UHF band too weak to be detected back on Earth. The MarCO cubesats, flying by Mars at an altitude of 3,500 kilometers, will receive those UHF signals and retransmit them on X-band frequencies back to Earth in real time.
MarCO won’t be the only relay for InSight during landing: the Mars Reconnaissance Orbiter (MRO) will also listen for the UHF signals for transmission back to Earth. However, MRO can’t simultaneously receive and transmit, so it will store InSight’s entry, descent, and landing telemetry for later retransmission back to Earth, a delay of an hour or more. MarCO will be able to relay the telemetry without delay.
MarCO is considered an experimental mission: one that provides a useful capability in support of InSight, but not an essential one. “MarCO is an experimental capability that has been added to the InSight mission, but is not needed for mission success,” said Jim Green, head of NASA’s planetary science division, in a June press release about the mission.
MarCO has come together quickly, JPL’s Andrew Klesh said in an August 11 presentation at the annual Conference on Small Satellites at Utah State University in Logan. It passed a mission concept review last October, allowing work on MarCO to begin in earnest. “We have a launch delivery date of December 1st of this year,” he said, with assembly of flight hardware starting the week of the conference.
For a CubeSat, MarCO is rather complex. “It’s quite a deployable mission,” Klesh said. Two solar panels deploy from either side of the spacecraft, generating 35 watts of power at launch, decreasing to 17 watts at Mars. A UHF antenna deploys from the bottom of the spacecraft to receive the InSight transmissions. A large “reflectarray” antenna unfolds from the top to transmit the high-gain X-band signal back to Earth.
MarCO is rather large for a CubeSat: the 6U form factor is only now starting to be used for missions, and at a mass of 14 kilograms, it’s far heavier than the typical 1U or 3U CubeSat. However, it makes use of much of the technology, from spacecraft subsystems to launch vehicle deployers, developed to support the broader CubeSat community.
“I keep saying ‘we’ are sending a spacecraft to Mars, because it’s more than NASA, it’s more than JPL. It’s all of the partners who are taking part in the MarCO mission,” Klesh said. “It’s also the broader community we’ve really built up from the advanced CubeSat technology that Aerospace [Corporation], Clyde Space, and many others have been developing over the last 15 years.”
Engineers at JPL hope that MarCO is just the beginning of the use of CubeSats for interplanetary exploration. A team at the lab has been studying what other missions that such small spacecraft could perform.
|“You can send these things to dirty, dangerous, unknown environments almost as precursor missions prior to sending our big billion-dollar spacecraft to those places,” Spangelo said.|
After MarCO “we have really big plans of taking these CubeSats really far into the solar system, either them being able to travel themselves with some of the emerging propulsion systems, or being carried as secondaries on primary missions,” said Sara Spangelo of JPL during an August 9 presentation at a CubeSat workshop held just before the main smallsat conference at Utah State.
JPL has been developing a roadmap of potential interplanetary CubeSat missions, she said, that could work in conjunction with a larger mission, including serving as landers or penetrators, or independently. Those missions would last from a few days to a few years.
CubeSats, Spangelo said, could also serve as scouts for larger, more expensive missions, given the low cost of such small spacecraft and a greater acceptance of risk. “You can send these things to dirty, dangerous, unknown environments almost as precursor missions prior to sending our big billion-dollar spacecraft to those places,” she said.
One example of an interplanetary CubeSat mission she discussed is one that would fly as a secondary payload on a proposed Discovery-class mission called Kuiper, a space telescope that would operate at the Earth-Sun L-2 point. The 6U CubeSat would carry out some initial technology demonstrations at L-2, then fire its thrusters to gradually drift to the L-5 point, which has yet to be explored but could host dust clouds or small asteroids. “We think this is a great opportunity to potentially explore that space with a low-cost spacecraft that has already completed its primary tech demonstration,” she said.
Performing such interplanetary missions requires developing spacecraft subsystems with different capabilities than those traditionally used for Earth-orbiting CubeSats. Spangelo said JPL is developing a “library” of parts available today, including radiation-hardened processors and UHF and X-band communications systems, designed for use on 3U and 6U interplanetary CubeSats.
JPL is also studying advanced technologies that could provide additional capabilities for interplanetary CubeSats, particularly in the area of propulsion. “The emerging electric propulsion systems we’ve been hearing about over the last few years are extremely exciting,” Spangelo said in a separate talk at the smallsat conference August 11. “They’re game-changing in terms of what CubeSats will be able to do.”
Those systems, she said, would give CubeSats capabilities ranging from formation flight to interplanetary transit. “You can do interesting things, like hover on Saturn’s rings,” she said. “There’s all sorts of cool things you can do with this capability.”
One electric propulsion system in particular being studied is one under development at the University of Michigan called the CubeSat Ambipolar Thruster. In her talk, Spangelo discussed how that system, used on a 6U or 9U CubeSat, could allow both flyby and capture missions throughout the inner solar system. When applied to a Mars mission, a 20-kilogram spacecraft (which includes 12 kilograms of propellant) could get to Mars orbit in just under one year.
Another technology under development at JPL for interplanetary CubeSats is a deployable Ka-band antenna. “One of the limiting factors for CubeSats, as you start to go out beyond low Earth orbit, is the issue of data communications rates,” said JPL’s Jonathan Sauder in a conference talk August 11.
|“We hope these microlanders can be to Mars missions kind of the way CubeSats have been to Earth-to-orbit launches,” said Staehle.|
The antenna under development at JPL is a scaled-down version of the large deployable reflectors used on some communications satellites. The antenna fits into a volume of about 1.5U when stowed inside the CubeSat, deploying to provide a parabolic reflector 0.5 meters in diameter. That antenna, he said, could support data rates 10,000 times that provided by an X-band patch antenna mounted on the side of a spacecraft.
While enabling high data rates for CubeSats going to Mars and other destinations in the solar system, the antenna’s first application could be in Earth orbit. Sauder said the antenna is being considered for a mission concept called RainCube, which would put a Ka-band radar inside a 6U CubeSat for Earth sciences applications.
A model of a MarsDrop microlander on display at JPL’s booth at the Conference on Small Satellites in Logan, Utah, earlier this month. (credit: J. Foust)
One of the most interesting interplanetary mission concepts discussed at the conference does not strictly use the CubeSat technical architecture, but does adopt the CubeSat ethos of low-cost missions willing to accept higher degrees of risk, in this case for landing very small spacecraft on the surface of Mars.
“We hope these microlanders can be to Mars missions kind of the way CubeSats have been to Earth-to-orbit launches,” said Robert Staehle of JPL in a presentation on the MarsDrop concept at the smallsat conference August 13.
Staehle said he was motivated by the excess capacity available on many Mars missions, in much the same way CubeSats have taken advantage of excess launch capacity to fly as secondary payloads. While Mars landers are often very mass constrained, the cruise stages that accompany them often have significant excess capacity: several hundred kilograms in the case of the Mars Science Laboratory mission that carried the Curiosity rover. “The question was, could we make a lander small enough that we could carry a few of them on subsequent Mars missions?” he said.
The MarsDrop lander is based on the Earth Reentry Breakup Recorder, a device developed by the Aerospace Corporation to provide data on the breakup of spacecraft as they reenter the Earth’s atmosphere. The recorder, about 30 centimeters across and weighing 3 kilograms, wasn’t designed to survive all the way to the Earth’s surface, yet has done so on three flights.
Modeling shows that, on Mars, it would decelerate to subsonic velocities tens of kilometers above the surface. That aspect of the recorder’s design opens up the use of a variety of drag devices, Staehle said, to further slow the vehicle. One concept that has been studied is a “parawing” that deploys once the vehicle goes subsonic, which not only further slows the vehicle but enables precision landings: within hundreds of meters initially, and potentially later down to tens of meters.
That design would allow for a scientific payload of up to 1 kilogram. That seems small, but Staehle said it could be enough for a variety of different instruments, including cameras and spectrometers. The lander, deploying solar panels and a UHF antenna on petals after touchdown, could operate for 90 Martian days, returning at least 20 megabytes of data.
|“In the last 15 years or so, we’ve been operating CubeSats 300 to 800 kilometers above the Earth,” said JPL’s Klesh. “Next year, with MarCO, we’re sending two spacecraft 157 million kilometers to Mars.”|
MarsDrop, Staehle said, could work well for focused investigations in areas of the planet not accessible by rovers. “There are all kinds of exciting places to go on Mars,” he said. “Many of those places are excluded to a large rover or expensive lander because of site safety concerns, because of astrobiology concerns, and because there simply isn’t enough money to put landers in enough places to investigate all the exciting places if you have to a billion dollars, or even a few hundred million dollars, per lander.”
A single MarsDrop lander would initially cost about $20 million, he estimates, a cost that could go down in time to less than $10 million per spacecraft. That would allow each large Mars mission to carry several MarsDrop landers for a few percent of the mission’s overall cost.
MarsDrop, Staehle added, could be used for other solar system destinations with significant atmospheres. “It also could work at Venus and Titan, destinations of particular interest,” he said.
Those working on both advanced concepts and missions under active development like MarCO feel that CubeSat technology, and its underlying philosophy of small, low-cost missions, is ready to be extended to missions to Mars and elsewhere in the solar system.
“In the last 15 years or so, we’ve been operating CubeSats 300 to 800 kilometers above the Earth,” said JPL’s Klesh. “Next year, with MarCO, we’re sending two spacecraft 157 million kilometers to Mars.”