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Speeding spacecrafts, Thrown tongues Q: I have been tracking the speed of the New Horizons spacecraft in the early stages of its journey to Pluto. In March, its speed continued to increase from 29,000 mph (13 km/s) to its current (26 March) speed of 39,000 mph (17 km/s). What is causing this steady increase, and will it continue? (Ryan, Gillette, Wyoming)
Drawing of New Horizons when it probes far beyond Pluto, and perhaps encounters an icy Kuiper Belt object. Courtesy of Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute A: Increased speed does seem mysterious since her launch rockets finished firing months ago. The increase, however, is only relative to Earth; Earth changes direction as she orbits the Sun. If, instead, we consider the spacecraft’s velocity relative to the Sun, then her velocity has decreased steadily from 95,700 mph (43 km/s) on 20 Jan. (shortly after launch) to 78,300 mph (35 km/s) on March 26, says mission design specialist Yanping Guo of Johns Hopkins University Applied Physics Laboratory. Such a decrease is reasonable because the Sun's gravity pulls on the spacecraft, slowing her.
But when we look at the spacecraft's velocity, relative to Earth, we get a different picture because Earth's direction changes as she orbits the Sun. Look at the snapshot of New Horizon's trajectory (green path). The spacecraft had zero velocity relative to Earth as she sat on the launch pad (time 1 in the drawing); her speed and Earth's were the same. Then she launched, achieved escape velocity (25,000 mph, relative to Earth) and soon left Earth's orbit. During the intermediate period (time 2) in March, New Horizon's velocity increased towards its September maximum. It will continue to increase until September when the velocity reaches maximum. In September (time 3), about nine months after launch, Earth will head directly away from the spacecraft, says Guo. So, Earthlings will see the spacecraft moving away fastest as we zoom apart. The spacecraft has an apparent increase in velocity but only apparent on and relative to Earth. After September, the spacecraft's velocity relative to Earth will decrease, as Earth again flies in the same direction as the spacecraft. "The rate of New Horizon's velocity will appear to increase and decrease as the Earth moves around the Sun," says Michael Buckley, Johns Hopkins University Applied Physics Laboratory spokesman. Further Reading: Where is New Horizons? JHUAPL Q: How long is an average frog's tongue? (Maureen, Austin, Texas) A: Frogs' tongues are typically about a third of the length of the frog, "as if our own tongue reached our belly button," says Barry Shell of the GCS Research Society of Canada.
But frogs vary — ranging in length from the foot-long (30-cm) Goliath frog of West Central Africa to the half-inch (1-cm) Cuban Pygmy frog. Moreover, not just their size varies, but the way they catch their prey, and the size of the tongue that does the job. Consider three examples: Not all frogs have tongues. Tongueless frogs such as the African Clawed Frog suck food from the pond into their mouths. Further Reading: Kiisa's video and image gallery, Northern Arizona University Frog's tongues by Barry Shell, GCS Research Society of Canada African Clawed frog, Animal Diversity of the University of Michigan Museum of Zoology (Answered May 30, 2006)
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The drawing shows Earth (blue dots in the
figure) and the spacecraft (green squares) at three different times, represented by the numbers: 1
(launch in January), 2
(an intermediate time in April) and 3 (September when Earth and the spacecraft zoom
off in opposite directions). Drawing courtesy of Johns Hopkins University Applied Physics
Laboratory, modified by author.
White's
treefrog, a 4-inch (10-cm) frog found in Australia and Indonesia. Courtesy of
Wikipedia.