The thrusters have fired, the clock is ticking. Humanity’s envoy to the outer solar system, the same New Horizons probe that flew over Pluto in the summer of 2015, will meet a faint speck of light called MU69 on New Year’s Day, 2019.
There’s just one problem: we still don’t know what MU69 looks like. It might be a misshapen potato, two rocks touching each other like Chinese stress balls, or even a binary, a pair of objects separated by a wide gap.
Last month, this seemingly innocuous question ate up seven straight presentations at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Provo, Utah – and for good reason.
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If MU69 is a binary, that would be exciting because it would quite literally offer two space rocks for the price of one. But it is also risky. When New Horizons zips past, the probe’s high-resolution camera has only a short window for taking precious snapshots.
Normally, these photos would be of MU69’s centre of mass. But in a binary, that centre would be empty space. “If we didn’t account for this,” said team member of the Southwest Research Institute in Texas, “we could miss it.”
What we do know of MU69 comes from two main sources.
First is the Hubble Space Telescope, which saw MU69 as a faint point of light in 2014, when the New Horizon’s team was searching for somewhere to visit after Pluto. It seems to be between about 20 and 40 kilometres across, only a bit bigger than a large city.
This July, the team used Hubble again, straining to see anything else that tags along with MU69 on its voyages through the Kuiper belt. They came up empty. If MU69 does have a companion, it must be close by.
The second method uses the universe like a backlight. When MU69’s path on the sky crosses a star, MU69 blocks out that star’s light, casting a weak shadow over Earth like the Batman symbol spotlighted on the clouds over Gotham.
The nature of that shadow – exactly where it falls on Earth, and when – should allow the New Horizons team to suss out MU69’s shape.
So far, those results aren’t painting a clear, unequivocal picture. Observations of one such shadow from 17 July support the idea that MU69 is a contact binary, shaped like a peanut shell or stress ball. But a single tantalising data point, taken a week earlier, hints that it is a binary.
Getting a close-up
The New Horizons team may still solve this mystery in advance of their probe’s blind date. With more data to analyse, and another chance to see MU69’s shadow – it will cross a star on 4 August 2018 and be visible from Senegal.
But if they can’t figure it out, the probe itself obviously will. Within weeks of its arrival, or in the case of an especially close pair, just days, New Horizons should be able to see whether its target is in one or two pieces and then adjust accordingly.
Either way, the answer will help refine our creation stories of the outer solar system. From looking at larger objects, we know that the Kuiper belt has egalitarian tastes: it hosts more binary partners with near-equal sizes than anywhere else. If smaller MU69 follows the same trend, it should help winnow down the possible theories of how this region formed, and how stable it has been since.
These pairs could have formed separately and captured each other. More likely, they coalesced out of the same protoplanetary disk that gave birth to the solar system and survived since then.
Imagine the still-unseen surface of MU69, time-lapsed over billions of years in the cold recesses of the solar system as it collects impact scars; as chemical smudges spread out and brown like sliced apples.
Don’t you hope, just a little, that it has spent all these eons with a friend?
