National Geographic News
Ceres is shown in relation to Vesta, Earth's Moon, Mercury and Mars.    Image credit: NASA/JPL-Caltech/UCLA

Ceres is shown in relation to Vesta, Earth's moon, Mercury, and Mars in an illustration.

Photograph by NASA/JPL-Caltech/UCLA

Charles Q. Choi

National Geographic

Published November 18, 2013

Rocket scientists who have plotted a course for a human mission to the largest known asteroid, Ceres, say that such a voyage may not be much more challenging than sending people to Mars, according to a new study.

Research investigating human missions to asteroids blasted off in 2010, when President Obama proposed a human mission to an asteroid by 2025. NASA's Asteroid Initiative plans to use a robotic spacecraft to tow an asteroid to a stable orbit just beyond the moon, which would enable astronauts to visit the space rock as soon as 2021.

However, "we wanted to look beyond the small asteroid that President Obama's plan wants to send people to," said aerospace engineer James Longuski of Purdue University in Indiana. "Let's take a bigger step to the biggest asteroid."

Ceres was the first asteroid discovered, and is roughly 605 miles (975 kilometers) wide, or as big across as Texas. This makes it the largest asteroid in the asteroid belt between Mars and Jupiter—it accounts for more than a third of the belt's mass.

"It's so large, it has enough of a gravitational field to pull itself into a round shape, unlike most other asteroids, which just look like potatoes and funny-shaped rocks," Longuski said.

Ceres is also the smallest and closest dwarf planet at about 257 million miles (415 million kilometers) from the sun, and is the only one in the inner reaches of the solar system. NASA's Dawn spacecraft is scheduled to reach Ceres in 2015. (Related: "NASA's Dawn Spacecraft to Reach Asteroid This Weekend.")

Intriguingly, Ceres may have vast amounts of frozen water beneath its crust—if it was composed of 25 percent water, it would have more water than all the fresh water on Earth. Some researchers even think Ceres may have an ocean of liquid water under its surface, potentially making it of interest to scientists looking for signs of extraterrestrial life, since there is life virtually wherever there is liquid water on Earth.

"There's also a lot we could learn about the birth of the solar system from Ceres, since it's essentially a large leftover from the solar system's formation," said  aerospace engineer Frank Laipert, also of Purdue University. "And a human could be a lot more effective as a scientist on Ceres than a robotic probe."

Going Nuclear

Longuski and Laipert calculated the best, most efficient way for humans to fly to Ceres would be with engines that electrically thrust out matter for propulsion.

The team calculated that nuclear power is a better source of electricity than solar for such propulsion for this mission because it weighs less and can provide the constant power needed for continual thrust to Ceres.

In their design for a human mission to Ceres, the scientists limited the crew time spent on the asteroid-bound and Earth-bound legs of the mission to 270 days each and the total crew time spent away from Earth to two years. Their aim was curbing astronaut exposure to hazardous deep-space radiation, which can cause cancer.

Unlike Mars, Ceres' gravity is too weak to help capture any spacecraft hurtling toward it. Moreover, it has no atmosphere to help slow down a lander. As such, any mission to the dwarf planet needs a substantial amount of propellant to get close to the asteroid and then brake its approach. (See pictures of asteroids and comets.)

The researchers divided the flight to Ceres into three parts. First, a supply vehicle would arrive at Ceres, bringing all the supplies needed to sustain the crew while on the asteroid as well as any equipment and propellant required to return to Earth.

Next, the crew vehicle is assembled in low Earth orbit and travels in elliptical spirals away from Earth. However, it travels without astronauts on board for about two years, to limit crew exposure time to deep-space radiation.

Finally, once the massive spacecraft builds up enough speed to reach Ceres, the astronauts take a small capsule to rendezvous with the crew vehicle and fly to the asteroid.

"As Easily as Mars"

All in all, the scientists calculated the crew mission would need an initial mass of 289 metric tons and the supply mission 127 metric tons, or 458 metric tons combined.

In comparison, the International Space Station weighs about 450 metric tons. The researchers said four heavy-lift rockets would suffice to carry the entire mission into orbit, comparable with what current plans for a human mission to Mars require.

"We can go to Ceres as easily as Mars," Longuski said.

The key technology that a human mission to Ceres needs to develop is a nuclear power plant capable of generating nearly 12 megawatts of power.

"It's probably the most futuristic piece of technology we relied on for our study," Laipert said. A mission could be feasible with just a 8- to 9-megawatt system, although more propellant would be required, he added.

The researchers noted that better, more efficient trajectories to Ceres may exist, such as ones using the moon's gravity to slingshot a spacecraft outward. They also noted the water on Ceres could be electrically split apart into hydrogen and oxygen for propellant, potentially cutting down the amount of supplies the mission needs to deliver to the asteroid.

Still, the investigators said that even without these options, their work demonstrated that a human mission to the dwarf planet was feasible.

The investigators suggested the supply missions could begin launching in October 2026, with the crew mission departing in August 2030 and returning in May 2032. Launch opportunities should repeat about every 2.3 years, according to the study, published online November 2 in the journal Acta Astronautica.

Speedy and Efficient

"They've identified a relatively speedy and efficient way to get people to Ceres and back," said planetary scientist and former NASA astronaut Tom Jones, a senior research scientist at the Florida Institute of Human and Machine Cognition, who did not take part in this study.

"It could be a part of an exploration plan after we go to Mars and want to expand farther out in the solar system." (See "After Space Shuttle, Does U.S. Have a Future in Space?")

Still, Jones did not see any reason for a human mission to Ceres. A robotic lander using the trajectories to Ceres that Laipert and Longuski developed "will likely tell us all we need to know about Ceres without the cost and risks of a human mission deep into the asteroid belt," Jones said.

Follow Charles Q. Choi on Twitter and Google+.

David Torney
David Torney

They'd have more volunteers if an asteroid were made of gold.

Sean Deany
Sean Deany

Getting to Ceres via the methods discussed may be a good option and while it is optimistically not terribly far into the future I anticipate the proposed mission strongly. In my own lifetime the prospects for manned missions to Mars has disappointingly got further and further into the not so foreseeable future. Although the general consensus now is that it should be achievable even with restrictive budgets by the mid 2030s. Unfortunately the GFC has shifted own optimism quite a lot and there isn't huge interest from the general public currently in space faring. While Mars does gets attention I think therefore and naturally it will be visited first and numerously before other options are put in place for human flight to Ceres. Discoveries made by successfully studying Ceres with the DAWN mission will certainly determine if "we" go to Ceres first or not.       

I have been working on some conceptualisations for human flight / mission to the dwarf planet Ceres based on science and a scenario for a short sci-fi story which can be viewed by going to my web site:

El Gabilon
El Gabilon

If humans are to survive they are going to have to get off this planet a lot sooner than they think. Our first step should be to colonize the moon. Chasing after asteroids that are not a threat to us is a waste of money.  A lot of this action should be done by private industry which can do the job much cheaper than government projects.  Americans are one of the hardest working peoples on the planet, and the government through projects such as this, and the grand give away to foreign countries of our tax money is leading us into bankruptcy. Private industry has already shown that it can do the job for less, and profit rather than loss is their goal.  Our government cannot (1) provide  a single payer health care system (2) educate our children (3) keep us out of needless wars (4) provide for our elders or the poor who are in need (5) eliminate crime (6) end corruption of bankers and wall street scumbags etc.  There is sufficient evidence that the government and its agencies are incompetent and there is no reason to think our space program is not. We will admit that in order to get our space industry into place the government was needed, but now the profit motive should be the goal. Let private industry go to this asteroid and after it gets there extract its wealth if there is any, if not we shouldn't be going there.

Gaylon Arnold
Gaylon Arnold

Before humans get far in the Solar System we need to solve several problems:  1.  cosmic radiation    2. the inevitable degradation of health due to weightlessness  3. long travel time.  The only solution to these problems in the short run is:  a one way trip.  Otherwise, robotic exploration will be the only way go.  It's likely that AI capabilities will increase faster than human capacity to travel.  When humans finally get to other planets, we'll be greeted by our robotic ambassadors working in AI communities.  They'll take time off from their busy schedules to try and make us feel welcome, but will be more productive once we go back home.

Dusty Rockets
Dusty Rockets

Does it HAVE to be nuclear power? Is there just NO other efficient way to get us there without building another nuclear power plant?

Eric Raymond Baluyut
Eric Raymond Baluyut

@Dusty Rockets Right now, I'm pretty sure that's what every other super-power country is working on -- more efficient energy source. If we had any new and advance energy source then we'd be closer to the era of star-ships and mechs. But alas the most powerful humanity has to offer right now is nuclear. Other than that, solar, but as the article above mentioned -- nuclear equipment tends to have lighter payload than solar.

Swiftright Right
Swiftright Right

It would be wasteful to send a manned mission to any planet unless you plan on colonizing it and by the time we can terraform planets well enough to make colonizing worth while we will have AI probes who will be far , far more effective explorers then even the most intelligent human.

Swiftright Right
Swiftright Right

@Bart Odom So would you advocate building a massive space station  to house hundreds of photographers with Kodaks when a cheap robotic could do the same job with better results?

Where exactly did I say exploration is a waste? For the cost of ONE manned mission to Mars we could send 2-3 probes to every planet in the Solar system. Whats more important to you? Planting a stupid little nylon flag on a largish rock or discovering what lays under the clouds of Neptune and Uranus (2 planets we know close to zero about) Or finally exploring/ ruling out water oceans on Europa.

Randy Coots
Randy Coots

@Swiftright Right @Bart Odom We need to explore the solar system with human occupants. Robots are very limited in their ability to change scope of mission. It is extremely likely that there will be newly discovered information on such a mission, a robot can't make changes to mission to explore any newly discovered information. 

Its also necessary for man to learn how to live in space as we expand our reach into the solar system. Without manned missions we are limiting this, and/or preventing it entirely.

There are more important things than cost cutting and cutting corners because its inconvenient to pay the price up front.

Ronald Fazar
Ronald Fazar

@Swiftright Right @Bart Odom Thinking in absolutes and arguing over slivers of the big picture just make people angry.  There are obviously advantages to both manned and unmanned missions.  The single biggest obstacles however is - politics.  

Swiftright Right
Swiftright Right

@Sarah Dellinger @Swiftright Right @Bart Odom Sarah, water does not just disappear. When you drain a water table your simply spreading that water around in a different way. It would be far more economically and ecologically sound to simple pump and treat water from our own oceans then it would be to import ice from rocks billions of miles away.

Even in your scenario how would sending a manned mission to collect water be better then sending robots which could collect more of the same water while costing less and doing less ecological damage?

Bart Odom
Bart Odom

So exploration just for the sake of scientific discovery is a waste? Is that your view of all science? How sad...

Sarah Dellinger
Sarah Dellinger

@Swiftright Right @Bart Odom This isn't just about photography. This asteroid could hold the water reserves necessary to overcome the agricultural crisis we will face when the water tables run out.. but just sending a robot won't tell us that. Real scientists need to be there to study and ACCURATELY determine if there is hope. Why send probes to every planet in the solar system? We already know they aren't inhabitable. By your own reasoning, why waste the money to have robots explore them when we can't benefit from it? 


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