On June 3rd, two dozen teams of observers in South America and South Africa tried to learn something about distant 2014 MU69, which New Horizons will visit in early 2019.
|Update (July 6): None of the observing teams involved in the June 3rd event described below witnessed an occultation. This suggests that 2014 MU69 is more reflective and thus smaller than had been assumed — or perhaps it's a binary object. Here is more information.
January 1, 2019, is still 19 months away — but for members of NASA's New Horizons team, it's hurtling toward them like a freight train. That's when the spacecraft will fly past the distant Kuiper Belt object c at close range.
Astronomers guess that it's between 25 and 45 km (15 and 30 miles) across, but the exact size depends on the reflectivity of its surface — and that's unknown. In fact, they needed the Hubble Space Telescope to discover this incredibly dim speck (magnitude 27½) at all. It's some 6½ billion km (43.3 astronomical units) from the Sun, a third farther out than Pluto is.
"This object has so far proven to be impossible to detect from the ground," laments Marc Buie (Southwest Research Institute). "100% of the data we have directly on 2014 MU69 is from HST, starting with the discovery images and then onward to additional images for astrometry."
To learn more and guess less, Buie and the New Horizons team have turned to an observing technique that can be even more powerful than HST: stellar occultations. It turns out that 2014 MU69, currently drifting among the rich star fields of Sagittarius about 5° northeast of the Teapot, passes directly in front of three stars this year: June 3rd, July 10th, and July 17th. That's the good news. The bad news is that these stars are themselves very faint, and (as the maps below show) getting into the predicted tracks presents plenty of challenges.
But any extra information about the character of 2014 MU69 will be a boon to planning the 2019 flyby. In South America, the predicted track for the June 3rd event passed very near Santiago, Chile, so it missed all the world-class observatories perches in the Andes farther north. But some of them (on Cerro Tololo and Cerro Pachón, for example) watched anyway. In South Africa, the track skirted very close to the South African Astronomical Observatory site in Sutherland.
"Very close" wasn't good enough, so the project funded a massive occultation campaign. Led by Buie, the effort dispatched 22 two-person mobile observing teams in Argentina and South Africa. Among them were nine amateur volunteers from the International Occultation Timing Association and from the Research and Education Collaborative Occultation Network.
The target star was a dim 15th magnitude, so the teams needed portability and a lot of aperture. Ultimately Buie turned to Sky-Watcher USA and purchased a battalion of its 16-inch truss-tube Dobsonian reflectors. He also acquired two ultralight Dobs, with 20- and 24-inch apertures, from Hubble Optics, and then paired all of these scopes with QHY174M-GPS CCD cameras. Buie tested other cameras with more sensitivity, but he notes, "Having the timing internal to the camera is something that makes a lot of sense for occultation work."
Because 2014 MU69 is no more than 50 km wide, the team relied heavily on the new Gaia star catalog to make the path's prediction as accurate as possible. Still, Buie deployed his teams in Argentina 10 to 25 km apart perpendicular to the path to create a "fence" of observers. Across the Atlantic in South Africa, Anne Verbiscer (University of Virginia) coordinated the teams in South Africa. As Buie stresses, "Both HST and Gaia have been tremendously supportive of the New Horizons extended mission as well as the occultation prediction work, including HST images of the occultation stars themselves."
The Results are In (Sort of)
According to early reports gleaned by Sky & Telescope, June 3rd's effort couldn't have gone much better. Buie says that every team collected usable data, though some might have been partially impacted by clouds. "That is quite remarkable," he says, "and it took some heroics on the part of the South African teams." (Bad weather forced many of them to relocate.)
The stellar "shadow" cast by 2014 MU69 took about 11 minutes to sweep across Earth, so from any given location the star would disappear for no more than about 2 seconds. Since the cameras were making ½-second-long exposures, at most four frames will show the star missing. But even with such beefy telescopes, the expected signal-to-noise ratio (even in optimal conditions) won't be high.
So did anyone see the star disappear? Anja Genade (SAAO) reports good data but no disappearance with the 74-inch Radcliffe reflector in Sutherland. As for all those mobile teams, Buie isn't saying — at least just yet. Look for him to announce those results early next week.
Meanwhile, back on May 10th, the IAU's Minor Planet Center quietly bestowed the number 486958 on 2014 MU69. The latter had been assigned as a provisional designation until the object had passed through three oppositions. And since the third anniversary of its discovery (on June 26, 2014) is just a few weeks away, the change to something more permanent was due.
What the appellation 486958 means is that this object can now be named. Alan Stern, the mission's principal investigator, notes that NASA and New Horizons will conduct a naming contest later this year.