Chasing ‘Oumuamua—unfortunately human technology isn’t up to the task

"Chemical propulsion just doesn’t close the case in this scenario."

A little more than a month ago, an interstellar visitor now known as 'Oumuamua passed within 24 million kilometers of Earth. It is now moving rapidly away from our planet at a velocity of approximately 26km/s. That is considerably faster than, say, the Voyager 1 spacecraft, which is hurtling beyond the Solar System at a velocity of 17km/s.

The first interstellar object is doubly intriguing because humans have never been able to study something from beyond the Solar System up close. Moreover, recent observations have shown that 'Oumuamua has a reddish color, astronomers say, and unexpected oblong shape, like that of a giant, 400-meter-long cigar. Already, the object is fading from view, and we will never see it again as it zooms away.

But what if we could? NASA is building what will be the world's most powerful rocket, the Space Launch System. Would it be capable of launching a small probe to catch 'Oumuamua? Do we have any technology that can catch this interstellar interloper?

Catch me if you can

To find out, Ars turned to the Advance Concepts Office at NASA's Marshall Space Flight Center in Huntsville, Alabama. The office's manager, Mark Rogers, spoke to Ars, as did one of his mission planners, Larry Kos. It turns out they were curious, too, and had done some preliminary calculations on the possibility of intercepting 'Oumuamua.

The short answer is, unfortunately, we are too late now with our existing technology. Although 'Oumuamua is moving at a velocity of 26km/s, factoring in Earth's velocity vector, the delta-v between a spacecraft in Earth orbit and the object is closer to 60km/s. "Chemical propulsion just doesn’t close the case in this scenario," Rogers said. "It’s not feasible."

But what if NASA had worked feverishly after detection of the object on October 19 and already sent a probe into space? The problem with our primary propulsion methods is that, while chemical rockets are very good at getting stuff out of Earth's orbit, they're gas guzzlers in space. Most of our existing in-space propulsion systems are based on chemicals, and they need a lot of fuel—often hydrogen—to move a spacecraft about. In this case, Kos calculated the specific impulse needed to catch 'Oumuamua at about 450 seconds.

That's actually good news, as conventional rocket engines can reach that level of specific impulse. But there's also some bad news. Sustaining that specific impulse to catch up to 'Oumuamua would require a lot of fuel. How much? Well, according to Kos, NASA would need to launch the equivalent of about two International Space Stations, or about 800 tons, of fuel into Earth orbit. And with all of that fuel, we could push a 10kg spacecraft—a GoPro with some rudimentary avionics—to catch up with 'Oumuamua eventually.

Alas, the first iteration of the Space Launch System rocket, although powerful, will only launch 70 tons. It should fly by 2020. A more powerful version, capable of lifting 130 tons, may be ready to fly by the end of the 2020s. We simply have no capacity to launch that much fuel now—or in the near future.

Another option, if we'd been able to see 'Oumuamua sooner, would have been to fly a spacecraft out to the closest intercept point, about 60 times the distance between the Earth and Moon. Unfortunately, 'Oumuamua was not found until five days after this closest flyby. Because of the object's hyperbolic orbit, it did not move much crosswise relative to the stellar background as it made its closest approach to the Sun on September 9 and then moved toward Earth.

Even though astronomers think such interstellar flybys are probably fairly frequent and estimate that an interstellar object similar to 'Oumuamua passes inside the orbit of the Earth several times a year, such flybys are tricky to find. In other words, we'd have to get very, very lucky to spot a similar interstellar object much earlier, and, then, we'd have to have a spacecraft on the pad and ready to go. (The average lead time for a NASA science mission, alas, is five to seven years prior to launch.)

Future flybys

We've probably missed our chance with 'Oumuamua with current technology. But assuming the Space Launch System—or some other reasonably powerful rocket, such as the Falcon Heavy or New Glenn boosters—come online within the next decade, might we be able to catch another interstellar object on short notice with a different means of in-space propulsion?

Electric propulsion, which sips fuel by comparison to chemical rockets and uses solar cells and an ion thruster, would not require a huge mass of propellant to be launched into orbit. However, current systems don't have the capacity to go fast enough, and even scaled-up electric propulsion would take years to gain the velocity to match 'Oumuamu—and many years more to chase it down. Moreover, the further our probe got from the Sun, the less solar energy it would be able to collect.

One answer to this is nuclear thermal propulsion, Rogers said. A nuclear thermal engine, such as the NERVA project developed in the 1960s and early 1970s, might have barely enough energy to chase down an object like 'Oumuamua with a 10kg probe. But for this, we would need Earth to be traveling in roughly the same direction as an interstellar object, and that is basically the luck of the draw. (Oh, and there's the fact that due to costs and nuclear concerns, NASA has not worked in depth on a nuclear engine since 1972.)

Starshot

A final possibility for catching up to 'Oumuamua comes through technology being studied by the Breakthrough Starshot project, which is essentially a spacecraft on a gram-scale chip. This miniature technology would include cameras, photon thrusters, power supply, navigation and communication equipment. And these chips would carry a light sail, with an Earth-based, phased array of lasers in the 100GW range providing the sail with enough push to get the craft moving at roughly 20 percent the speed of light.

If such a tiny, 1-gram spacecraft could be launched by 2021, a recent paper found, it could reach ‘Oumuamua in about seven years. That would be delightful, except for the fact that the tiny spacecraft don't yet exist, they rely on unproven miniaturization technologies, and no one has yet figured out the geopolitics of building a huge Earth-bound array of lasers aimed into space.

In short, 'Oumuamua may be lost to us forever. Perhaps it will spur humans to up our spaceflight game so that we will not miss the next interstellar interloper.