My first Magazine Sky & Telescope - 04.2019

SOLAR SYSTEM Voyager 2 Enters Interstellar Space Helio VOYAGER 2 HAS BECOME the second (named for the Chinese Moon goddess) has landed softly on the farside of the Moon — the ﬁ rst mission to accomplish this milestone. The spacecraft launched on Decem- ber 7, 2018, and touched down in Von Kármán crater on January 3rd at 2:26 UT. The 180-km-wide (110-mile- wide) crater is one of the few ﬂ at areas within the South Pole-Aitken Basin. This basin was created in one of the solar system’s largest impacts, which might have smashed through the crust and exposed the lunar mantle. Explor- ing this region may reveal information about the formation and structure of the Moon. The lander snapped images of the terrain around it, then the Yutu 2 rover (Chinese for “jade rabbit”) rolled down its ramp later that same day. ck THE CHINESE SPACECRAFT Chang’e 4 Heliosphere Voyager 1 Chang’e 4 Lands on Lunar Farside Te r MOON e probe to break through to interstellar space, mission scientists announced December 10th at a meeting of the American Geophysical Union in Wash- ington, D.C. A plasma detector onboard Voyager 2 recorded a sharp decline in the speed of the solar wind on November 5th. Around the same time, the spacecraft also saw a sharp uptick in cosmic rays — high-speed atomic particles that whiz around the galaxy — as well as an increase in the ambient magnetic ﬁeld. This conﬂuence of events gave mission scientists conﬁdence that the probe had ﬁnally broken out of the heliosphere, a bubble of space surrounding the Sun in which the solar wind reigns supreme. This marks the second time that a spacecraft has crossed this threshold. Voyager 1 crossed the heliopause in 2012 (S&T: Dec. 2013, p. 10). However, paus Voyager 2 200 au Voyager 2 crosses the heliopause. its plasma detector had stopped work- ing back in 1980. Now, with Voyager 2 joining its twin in interstellar space, scientists will be able to obtain the ﬁ rst direct measurements of the ionized gas that drifts between the stars. Voyager 2 The spacecraft landed around local lunar sunrise, giving the solar-powered crafts roughly two weeks of illumina- tion before sunset in late January; heating units keep both of them warm during lunar night. Throughout the three-month mission, the Queqiao Yutu 2, from the Chang’e 4 lander will also provide a second measure of the ﬂ ux of galactic cosmic rays that impinges on the solar system. Mission scientists anticipate another ﬁ ve to 10 years of operations for the aging probes. ■ CHRISTOPHER CROCKETT (Chinese for “magpie bridge”) orbiter will relay the probes’ data from its halo orbit around the L2 Lagrangian point 60,000 km past the Moon. In addition to examining its new home with a battery of instruments, Chang’e 4 carries a small container with three species of plant seeds, fruit ﬂ y larvae, and yeast. This student exper- iment will measure how an enclosed mini-ecosystem fares in low gravity. The lander will also carry out low-frequency radio observations. The Chinese space agency has committed to an open data policy for this mission, though it’s not yet clear how the data will be released. Later this year, the heavier Chang’e 5 lander and sample return capsule is expected to join its smaller sibling on the Moon. However, this mission still awaits a return to ﬂ ight for the Long March 5 heavy-lift rocket. ■ DAVID DICKINSON See videos of Chang’e 4 and Yutu 2 at https://is.gd/Change4landing. sk yandtele scope.com • A PR I L 2 019 9

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