‘Oumuamua is a Hawaiian term for scout or messenger. And it’s a perfect name for a chunk of space rock that was discovered last month by the Pan-STARRS 1 telescope. Sitting atop the summit of Haleakalā in Maui, the telescope spotted what first looked like a regular fast-moving small asteroid. Follow-up observations over the next couple of days gave scientists a good idea of the rock’s orbit. This asteroid doesn’t call our solar system home.
Here’s a look at the orbital path courtesy of the European Southern Observatory.
‘Oumuamua was already cruising away from our solar system by the time Pan-STARRS 1 spotted it. “We had to act quickly,” says Olivier Hainaut from the ESO. “‘Oumuamua had already passed its closest point to the Sun and was heading back into interstellar space.”
First designated a comet, the asteroid exhibited no signs of outgassing as it passed the Sun. Once scientists discovered it wasn’t from the solar system, it was reclassified as an interstellar object and given the official designation 1I/2017 U1 (‘Oumuamua). Being the first asteroid from interstellar space, the asteroid is also the first to receive the new interstellar object designation.
So, what do we know about this chunk of interstellar space rock? ESO’s Very Large Telescope (VLT) was tasked with measuring the object’s brightness, color, and speed as it cruised away from Earth. Images captured by the VLT shows a dramatic change in brightness as ‘Oumuamua spins on its axis every 7.3 hours.
Karen Meech explains what this change in brightness is telling us. “This unusually large variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape. We also found that it has a dark red color, similar to objects in the outer Solar System, and confirmed that it is completely inert, without the faintest hint of dust around it.”
‘Oumuamua is most likely a dense asteroid made up of either rock, or high metal content. It probably does not have any large amounts of water or ice, and the red color is the result of cosmic ray bombardment over millions of years.
Observations suggest the asteroid measures at least 400 meters long. Based on Meech’s comments, that would put the width at about 40 meters.
An artist drew up the asteroid with these measurements in mind.
Astronomers believe outside visitors like ‘Oumuamua cruise through the inner solar system about once per year. Spotting them is the tricky part. Survey telescopes, like Pan-STARRS, soak in huge parts of the night’s sky and can spot their faint reflections as they get closer to Earth. Once discovered, astronomers can turn to more powerful telescopes to learn more.
While ‘Oumuamua is already on its way out, astronomers are still observing it. “We hope to more accurate pin down where it came from and where it is going next on its tour of the galaxy,” says Hainaut. “And now that we found the first interstellar rock, we are getting ready for the next ones!”