Rosetta Boosts Hopes of Deflecting Earth-Killing Asteroids


The primary goal of the pioneering spacecraft that landed on a comet last week is to search for clues about the composition of the early solar system.

But the European Space Agency’s historic Rosetta mission might also have a futuristic defense application. It adds weight to the idea that small spacecraft may be able to protect Earth from big objects like asteroids and comets using gravitational force alone.

The concept, known as a gravity tractor, has been around for almost a decade. Now, Rosetta could bring more data to the science.

“If you were aware of a potentially hazardous object like a comet or an asteroid that was on an Earth-crossing orbit, you don’t necessarily have to do the Bruce Willis-option of putting a nuclear device on it and blowing it up,” said Alastair Reynolds, a science-fiction author and former scientist at the ESA who played a small role in publicizing the Rosetta mission. He was referring to the plot of the 1998 movie, “Armageddon,” starring the Hollywood actor.

“You don’t even have to go there and, say, put a rocket motor on it to steer it into another orbit,” he added. “Really, all you have to do, if you have sufficient notice, is put another spacecraft near it in orbit, and the mutual gravitational attraction of the spacecraft and the rock or the comet will be enough to perturb the orbit, and if you get out there early enough and you have enough of a head start, a tiny perturbation in the orbit sufficiently far away will be enough to avoid a collision.”


Reynolds, whose 2005 novel, “Pushing Ice,” depicted a team of astronauts who mine comets in the year 2057, said he doubted that investigating the concept of a gravity tractor was “any part of the thinking behind the mission.” Even so, he said, “having shown that we can put a spacecraft near a remote body in the solar system and then land another spacecraft on it is obviously a useful capability to have demonstrated.”

The ESA made history on Nov. 12 when its Rosetta spacecraft — after a decade of flying through space — rendezvoused with the comet 67P/Churyumov-Gerasimenko some 300 million miles away and dropped a washing machine-sized lander, called Philae, onto the 2.5-mile-wide block of rocky ice.

The lander actually touched down on the surface three times after its harpoons and ice screws failed to deploy and secure the craft in place. As a result, it bounced back into space and finally came to rest in the shadow of a cliff or crater, where there wasn’t enough sunlight for its solar panels to recharge its batteries.

Before going into hibernation on Nov. 15, Philae conducted a series of experiments. It recorded a surface temperature of -115 degrees Celsius and collected data that led scientists to conclude that the upper layers of the comet surface consist of 10-20 centimeters of dust overlaying “mechanically strong ice” or “ice and dust mixtures,” according to a blog post by ESA.

It wasn’t immediately clear how much Rosetta and its Philae lander have impacted the comet’s orbit around the sun. A spokesperson didn’t respond to a request for comment.


The idea for a gravity tractor was devised by two former NASA astronauts, Edward Lu and Stanley Love, who in a 2005 article in the science journal Nature, argued that a spacecraft orbiting an asteroid could use the small gravitational force between the two bodies as a “towline,” according to an article by Stefan Lovgren in National Geographic News.

They calculated that, given a lead time of about 20 years, NASA could launch a craft that could safely deflect an asteroid measuring 650 feet wide in about a year of “towing,” the article reported.

Lu didn’t immediately respond to telephone and e-mail requests for comment.

One of the biggest differences between comets and asteroids is their composition. Comets are made of mostly ice and dust, while asteroids are composed of mostly metal and rocky material. Comets have elongated orbits, while asteroids have shorter, more circular routes around the sun. There are more than 3,500 comets and millions of asteroids in the solar system.

Exploring the celestial bodies with spacecraft isn’t new. In 2001, NASA’s NEAR Shoemaker spacecraft conducted a soft-landing on the surface of the near-Earth asteroid Eros. In 2005, Japan’s Hayabusa rendezvoused with and landed on the asteroid Itokawa. Rosetta, however, is the first time a scientists conducted a controlled landing of a spacecraft on a comet.


Reynolds, the author, said details of the mission have caused him to rethink how he might portray the subject in future work. He will probably use thrusters rather than ballasted spacesuits to keep astronauts grounded on a comet’s surface, for example.

And while he’s thrilled for his former colleagues and their historic achievement with the Rosetta mission, Reynolds also said he’s excited for the possibility of manned flights to such orbital outposts.

“It is a tremendous achievement, but at the same time it’s a vindication of all the amazing things you could do if you had a human being out there,” he said.

“Imagine if there had been a human being close enough to that comet to actually just control the lander in real-time, then there would have been no question about where it landed and you wouldn’t have ended up stuck in the shadows,” he said. “Until we come up with artificial intelligence that’s as adaptive and intelligent as ourselves, there’s always going to be a good case for getting a human being into the loop somehow.”

Asked whether he thought comet and asteroid mining would become a reality in coming decades, Reynolds said, “A tremendous amount can happen in 50 years, much more than people are generally aware of. I always say, 50 years is the difference between the first crossing of the Atlantic by an aircraft, which was [flown by the British aviators] Alcock and Brown, and then the first landing on the moon.”

About the Author

Brendan McGarry
Brendan McGarry is the managing editor of He can be reached at Follow him on Twitter at @Brendan_McGarry.
  • rtsy

    Comets are ice and dust while asteroids are mostly metal and rock, not the other way around

  • OriginalK

    Computational celestial mechanics is a mature field well validated by experiment. So I doubt very much that the results of the comet mission will divert from earthborn analysis enough to add any significant new knowledge about asteroid diversion via gravity tractors. OTOH, it’s a good subject to impress the politicians with who hold the grant money so it’s all good.

  • Captain Obvious

    Probably a cover story for an actual mission to deflect this comet

  • LeoC

    The mention of more than 3,500 comets in the solar system should be clarified. That number is based on known comets discovered by astronomers. The number is rising as more comets appear in the inner solar system. I saw one count last August that went over 5,000. The comet that recently flew by Mars came from the Oort Cloud which is thought to be the home of billions of icy bodies.

  • Brian B. Mulholland

    I am deeply skeptical that we would be able to identify an incoming object, be it comet, asteroid or anything else, in time to position a given mass to change its’ orbit and thereby avert a collision with Earth. The theory is perfectly sound, but we are generations away from mapping an object’s trajectory that well, and from being able to use the tiny gravity of a plausible vehicle to protect this planet. The elephant in the living room that many scientists wish to ignore is the use of a hydrogen bomb, and the light pressure of its’ detonation to change its’ orbit. Assuming that we detect an incoming comet in time, which is not a given, putting one or more h-bombs on a controlled near miss is something that’s not far out of the state of the art. Gravity tractors are as far away as Larry Nivens’ “scrith.”