New Horizons cleared for optimal approach to Ultima Thule
NASA’s New Horizons spacecraft has been officially cleared to embark on what mission engineers determined to be the optimal path for observation after three weeks of hazard searches near its second target, Kuiper Belt object Ultima Thule, turned up no debris that could pose a danger to it.
Because the probe is speeding to Ultima Thule at 31,500 miles (50,700 kilometers) per hour, even tiny particles of rock or ice could destroy it on impact.
A similar search for nearby hazards was conducted by the New Horizons team in 2015 in anticipation of the Pluto flyby. No hazards were found then, allowing the probe to make the close approach outlined by the mission’s hazard watch team, which had been formed back in 2011.
The Long Range Reconnaissance Imager (LORRI), a powerful telescopic camera and one of seven science instruments aboard New Horizons, searched for rings around the KBO and orbiting moons, both of which could produce potentially hazardous debris in the spacecraft’s path. From the probe’s current distance, LORRI is capable of detecting moons 2 miles (3 kilometers) in diameter and ring particles that reflect just five ten-millionths of the sunlight hitting them.
The announcement that New Horizons will take the optimal path to Ultima Thule was made Dec. 18, the last day a decision about whether to alter the spacecraft’s trajectory due to potential hazards could be made. Closest approach will take place on Jan. 1, 2019, at 12:33 a.m. EST (05:33 GMT).
“Our team feels like we have been riding along with the spacecraft, as if we were mariners perched on the crow’s nest of a ship, looking out for dangers ahead. The team was in complete consensus that the spacecraft should remain on the closer trajectory, and mission leadership adopted our recommendation,” said hazards team leader Mark Showalter of the SETI Institute.
Closest approach will bring the spacecraft within 2,200 miles (3,500 kilometers) of Ultima Thule. In contrast, New Horizons’ closest approach to Pluto was 7,800 miles (12,500 kilometers). A closer flyby is necessary because Ultima Thule is much smaller than Pluto and is about one billion miles further from the Sun.
In contrast to Pluto, which has a diameter of 1,473 miles (2,370 kilometers), Ultima Thule is estimated to have a diameter of just 23 miles (37 kilometers). It is not large enough to be spherical and is irregularly shaped. Observations conducted on several occasions when the KBO passed in front of a background star indicate it is either double-lobed or a binary composed of two objects orbiting one another.
Occultation observations also suggested Ultima Thule, which is composed of the building blocks of the solar system that never became part of a larger object, such as a dwarf planet, is dark and reddish colored. Mission principal investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado, expects its temperature to be between 40 and 50 degrees above absolute zero, the temperature at which molecules within a substance are nearly motionless.
A final package of commands will be uploaded to the spacecraft just one week ahead of the flyby.
One day before New Horizons was given the “all clear,” a team of scientists led by Scott Sheppard of the Carnegie Institution for Science announced the discovery of the most distant dwarf planet to date, located about 120 astronomical units (one AU is equal to the average Earth-Sun distance — about 93 million miles) from the Sun. Nicknamed “Farout,” and located beyond dwarf planets Eris and Sedna, the distant object is estimated to have a diameter of approximately 300 miles (483 kilometers), making it large enough to be spherical.
New Horizons will not be able to observe “Farout” even from a distance as the dwarf planet is in a different direction than the one the spacecraft is traveling.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.