— How to spot an alien spaceship
HOW TO RECOGNISE AN ALIEN SPACESHIP
Knowing we are not alone in the universe might depend on identifying an alien emissary millions of kilometres away. LAUREN FUGE explains how we could do so.
THE OBJECT HAD BEEN hurtling through the Solar System for years by the time astronomers spotted it, just 33 million km away – 20 million km closer than Mars ever comes to the Earth. Highly elongated and about the size of a WWII battleship, its trajectory proved it was an interstellar interloper – our Solar System’s first identified visitor from deep space.
Astronomers named it `Oumuamua – Hawaiian for “a messenger from afar arriving first” – because Hawaii’s PAN-STARRS telescope was the first to spot it, in October 2017. Was it, as some armchair scientists speculated, an alien emissary? Such wild conjecture turned out to be a stretch. Analysis pointed to it simply being an oddly shaped asteroid. Ultimately there was nothing to hint it was more than “a big chunk of rock”, says astronomer Olivier Hainaut, of the European Southern Observatory.
Too bad for alien enthusiasts. So how did scientists work out what it was? If it had been an alien spaceship, how would we have known?
ORBIT
The first thing that stood out about ‘Oumuamua was its orbit. Though passing through the Solar System, it was not captured by the Sun. “It is the only object seen so far with a strongly hyperbolic orbit,” says David Jewitt, an astronomer at the University of California, Los Angeles, “meaning it is travelling so fast that the Sun’s gravity cannot hold it back.”
This indicated it could be something novel, says Jonti Horner, an astrobiologist at the University of Southern Queensland. But “extraordinary claims require extraordinary evidence, so people across the planet went into a frenzy to get more observations and lock things down”.
COMA
A key observation is to determine if an object is surrounded by a fuzzy cloud, or ‘coma’, of dust and gas: this is the signature of a comet heating up and releasing gas as it approaches the Sun. ‘Oumuamua didn’t show any such activity, ruling out it being a comet, though that didn’t prove it was an alien spacecraft.
BRIGHTNESS
The next thing to look at is how an object’s brightness changes over time. Asteroids have irregular shapes and tend to spin, so they appear brighter or dimmer as they tumble in the sunlight. The brightness of a spaceship, on the
ROTATION SPEED RADIO WAVES
other hand, would be more stable. ‘Oumuamua showed significant fluctuations in brightness, suggesting it was an asteroid. . An object that is rotating might be a hint it is creating artificial gravity – think the rotating ring of the spacecraft in Andy Weir’s Hermes The Martian, or Discovery One 2001: A Space Odyssey.
in Spin produces a centrifugal force that can mimic the effect of gravity. The faster the spin, the greater the force. Astronomers could see ‘Oumuamua was rotating but each rotation took seven to eight hours – way too slow to replicate any meaningful gravitational effect for an object its size. To produce artificial gravity similar to what we experience on Earth, it would need to rotate more like once a minute. An obvious giveaway could be found by listening for radio transmissions across a range of wavelengths. Says Hainaut: “Narrow radio emissions, especially if they are modulated in some way, don’t really happen in nature.” Listening for signs of alien civilisations is not a new idea – programs like SETI have long been monitoring distant solar systems for life – but we rarely have cause to tune into our own. In December 2017 the Breakthrough Listen program focused the 100-metre Green Bank Telescope on ‘Oumuamua but found no indication of artificial signals.
COLOUR
Astronomers can also learn about the object’s surface by analysing the spectrum of reflected light. Unexpected signatures could point to materials such as spacecraft paint. Seeing bright, short flashes might indicate an artificial polished surface. ‘Oumuamua was found to have a dark reddish hue, perhaps indicating a surface covered with dense, metal-rich rock, reddened from cosmic ray bombardment.
HEAT SIGNATURE
A spacecraft might give off a heat signature from an engine or an internal energy source, visible to us in the thermal infrared. Its engine could also give off detectable emissions. Another indication of an engine might be an object straying off the path of a natural gravitationally driven orbit. However, outgassing can also disturb the orbits of comets, so it would take a large variation to signal an artificial spacecraft.