New Horizons recently observed the Kuiper Belt object Quaoar (“Kwa-war”), which – at 690 miles or 1,100 kilometers in diameter – is roughly half the size of Pluto. This animated sequence shows composite images taken by New Horizons’ Long Range Reconnaissance Imager (LORRI) at four different times over July 13-14: “A” on July 13 at 02:00 Universal Time; “B” on July 13 at 04:08 UT; “C” on July 14 at 00:06 UT; and “D” on July 14 at 02:18 UT. Each composite includes 24 individual LORRI images, providing a total exposure time of 239 seconds and making the faint object easier to see.
New Horizons’ location in the Kuiper Belt gives the spacecraft a uniquely oblique view of the small planets like Quaoar orbiting so far from the sun. When these images were taken, Quaoar was approximately 4 billion miles (6.4 billion kilometers) from the sun and 1.3 billion miles (2.1 billion kilometers) from New Horizons. With the oblique view available from New Horizons, LORRI sees only a portion of Quaoar’s illuminated surface, which is very different from the nearly fully illuminated view of the Kuiper Belt object from Earth. Comparing Quaoar from the two very different perspectives gives mission scientists a valuable opportunity to study the light-scattering properties of Quaoar’s surface.
Like the planet Pluto, Quaoar dwells in the Kuiper Belt, an icy debris field of comet-like bodies extending 5 billion kilometers beyond Neptune's orbit. Over the past decade more than 500 icy bodies--Kuiper-Belt Objects or "KBOs" for short--have been found there. With a few exceptions all have been significantly smaller than Pluto.
Previous record holders are a KBO called Varuna, and an object called 2002 AW197, each approximately 540 miles across (900 kilometers). Those diameters were deduced by measuring the objects' temperatures and calculating a size based on assumptions about the KBOs' reflectivity. Such estimates are less certain than Hubble's direct measurements.
In June the New Horizons mission received the go-ahead to fly onward to 2014 MU69 -- considered one of the early building blocks of the solar system -- with a planned rendezvous of Jan. 1, 2019.
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