This little planet1 with the adorable heart-shaped tramp stamp is Pluto. The photo was taken around 4 pm EDT 13 Jul 2015 by the Long Range Reconnaissance Imager (LORRI) + the Ralph Instrument aboard the New Horizons spacecraft from a distance of 476,000 miles. As of today it is the best image ever taken of Pluto.
The Boston businessman-turned-astromomer Percival Lowell (13 Mar 1855 – 12 Nov 1916) was among the first to suggest that the perturbations in the orbits of Uranus and Neptune were caused by the gravitational pull of a massive, trans-Neptunian planet – Planet X. In 1894 he founded the Lowell Observatory in Flagstaff, Arizona and spent the rest of his life searching in vain for his elusive prize.2 After a protracted legal battle with Lowell’s widow Constance, the Observatory resumed the search for Planet X with the unlikely choice of a Kansas farm boy.
Clyde Tombaugh (4 Feb 1906 – 17 Jan 1997), the oldest child of Muron and Adelle Tombaugh, was born in Streator Illinois. As a child he became interested in astronomy and after the family moved to a new farm in Burdett, Kansas, he began building telescopes and sending his observations to Lowell Observatory. The observatory’s director, Vesto Slipher, was so impressed with his drawings that he offered Tombaugh a job.
The offer could not have been more fortuitous; a hailstorm had destroyed the family’s crop and dashed any hope of college. So instead of studying astronomy Tombaugh would now be doing astronomy.3
Tombaugh spent ten months taking thousands of dry-plate photographs of the heavens using the Observatory’s 13-in astrograph then repeated the same images several days later. The plates were then compared using a Zeiss Blink Comparator to look for differences, suggesting a body in orbit rather than a fixed star. On 18 Feb 1930, after 10 months of searching he found a difference: comparing the images of 23 and 29 Jan 1930 taken in the Gemini constellation Tombaugh found an moving body far outside the orbit of Neptune. He found Planet X.
The Observatory announced its findings on 13 Mar 1930 (Lowell’s birthday) and it created a public sensation.4 The New York Times reported that this new planet was “possibly larger than Jupiter and four billion miles away.” Every succeeding measurement, however, made Pluto smaller: in 1931 Nicholson estimated it to be about the size of the Earth, in 1948 Kuiper estimated it to be 1/10 the size of the Earth and in 1976 James Christy estimated it to be 1/500 the size of the Earth, or about half the size of the Moon.
Pluto’s distance (aphelion: 48.871 AU) small size (radius: 1184±8 km) and low magnitude (avg. 15.1) has made it difficult to image from even the largest terrestrial telescopes. Here, for example, are the film negatives from the 61-in Kaj Strand Astrometric Reflector used by James Cristy in 1978 to discover Pluto’s first satellite, Charon: 5
This infrared image taken by the 8.2 m Subaru Telescope in 1999 was the best image ever taken from the ground: 6
In fact, the only way to get a better picture of Pluto would be to go outside the Earth’s atmosphere, which was precisely the point of the the 2.4 m Hubble Space Telescope (HST). After it was placed in orbit by the Space Shuttle Discovery (STS-31) in Apr 1990 the HST instantly revolutionized astrophysics. On 21 Feb 1994 R. Albrecht of the European Space Agency took this photograph using the ESA Faint Object Camera:
Marc Buie of the Southwest Research Institute used these photos taken by the HST Advanced Camera for Surveys (ACS) between 2002–2003 to create this photo series:
Buie’s photos allowed him to render the first ever map of Pluto's surface.7 They were also the limit of the HST’s capabilities: the only way to get a better picture of Pluto would be to actually go to Pluto, which was precisely the point of New Horizons.
New Horizons was launched on the largest available Atlas V rocket on 19 Jan 2006. It passed the Moon in 9 hours, Mars in less than three months and got a gravity assist from Jupiter a year later.8 But Pluto was still a long way off so the satellite spent most of the next seven years asleep. It woke up on 7 Dec 2014 and began sending images of Pluto back the next month. By 15 May 2015 it was returning images better than the HST.
Today (14 Jul 2015) at 7:49:58 am EDT New Horizons passed within 7800 miles of Pluto. It will take months to receive all of the telemetry, but it’s likely that a new best picture of the planet is coming soon.
1. On 24 Aug 2006 the International Astronomical Union, meeting in Prague, reclassified Pluto from a planet to a dwarf planet designated 134340 Pluto. There was – to say the least – an outcry from the public who, let’s be honest, wouldn’t know a planet if they were standing on one (Oh, wait...) For an analysis see: Messeri, Lisa. The Problem with Pluto: Conflicting Cosmologies and the Classification of Planets. Social Studies of Science. Apr 2010: 40(2); 187–214 (online).
2. See: Strauss, David. Percival Lowell: the Culture and Science of a Boston Brahmin. Cambridge: Harvard University Press, 2001. (WorldCat).
3. After his discovery of Pluto Tombaugh went to school, eventually receiving an M.S. from the University of Kansas. He taught astronomy at the New Mexico State University until his retirement in 1973. A vial of his ashes was glued to the New Horizons satellite and in a fitting tribute his remains will be the first to leave this solar system. See: Tombaugh, Clyde. Out Of Darkness: The Planet Pluto. Harrisburg, PA.: Signet, 1980 (WorldCat).
4. Pickering, WH. The Discovery of Pluto. Monthly Notices of the Royal Astronomical Society. May 1931: 91(7); 812–817 (online).
5. Cristy, JW, Harrington, RS. The Satellite of Pluto. Astronomical Journal. Aug 1978: 83; 1005–1008 (online).
6. Cruikshank, DP, wt. al. Ethane on Pluto? Science. 27 Aug 1999: 285(5432); 1355 (online)
7. Buie calls Pluto “Cold and dark, but still a great vacation spot.” See: Buie, MW, et al. Pluto and Charon with the Hubble Space Telescope: I. Resolving changes on Pluto's surface and a map for Charon. The Astronomical Journal. 2010: 139; 1128–1143 (online).
8. For a description of the mission see: Guo, Y, Farquhar, RW. New Horizons Pluto–Kuiper Belt Mission: Design and Simulation of the Pluto–Charon Encounter. Acta Astronautica. Feb 2005: 56(3); 421–429.
14 Jul 2015 ‧ Photography