I found the answer in two different books. One is "Discovering the Planets", by Jacqueline Mitton. It's in the school library. It says Pluto is 3,670 million miles away, which is about 5,900 million kilometres.

This is the average distance. Pluto takes about 250 years to orbit around the Sun, and sometimes it's closer to the Sun (and the Earth) and sometimes it's farther away. There's a book called "Pluto", by Denis Brindell Fradin, in the Carlingwood public library, and it says that the closest Pluto comes to Earth is around 4,300 million kilometres. That's pretty far!

Pluto is the only planet that hasn't been visited by a spacecraft from Earth. However, plans are underway to send two small spacecraft in 1999 or the year 2000. Here is a description that we got from NASA.PLUTO FAST FLYBY FACT SHEET

March 1993

Solar System Exploration Division

* MISSION SUMMARY *

Pluto, the smallest planet in our solar system, has remained enigmatic since its discovery by astronomer Clyde Tombaugh in 1930. Pluto is the only planet not yet viewed close-up by spacecraft, and given its great distance and tiny size, study of the planet continues to challenge and extend the skills of planetary astronomers. Most of what we know about Pluto we have learned since the late 1970s. Such basic characteristics as the planet's radius and mass were virtually unknown before the discovery of Pluto's moon Charon in 1978. Since then, observations and inferences about Pluto-Charon, now considered a "double-planet" system, have progressed steadily to a point where many of the key questions about the system must await the close-up observation of a space flight mission.

For example, there is a strong variation in brightness, or albedo, as Pluto rotates, but we do not know if what we are seeing is a system of varied terrains, or areas of different composition, or both. We need a much closer look to understand these features and the chemical, geological and perhaps orbital history they represent. We know there is a dynamic, largely nitrogen and methane atmosphere around Pluto that waxes and wanes with the planet's elliptical orbit around the sun, but we need to understand how the atmosphere arises, persists, is again deposited on the surface, and how some of it escapes into space. Telescopic studies indicate that Pluto and Charon are very different bodies, Pluto being more rocky, Charon more icy. How and when the two bodies in a double-planet system could have evolved so differently is a question that awaits data from close-up observation.

More fundamentally, beyond our basic interest in Pluto and Charon, is the likelihood that these bodies hold important keys to our understanding of the giant planets and comets and their role in the formation of the solar system. From the Voyager missions to the outer planets and their moons, we have a basic inventory of the characteristics of the icy and rocky bodies of the outer solar system. We have learned much about such planet-like bodies as the moons Triton and Titan, and are beginning to understand Pluto as a third member of this triad of small outer "planets." Data about Pluto and Charon, gathered using ground-based and Earth-orbiting observatories like the Hubble Space Telescope, continually improve our understanding of these bodies and have helped define the important questions about PlutoCharon. To address these questions, NASA is now studying a robotic reconnaissance mission to Pluto-Charon called PLUTO FAST FLYBY.

Pluto Fast Flyby will be unique in its approach. In order to minimize cost, while containing the risks associated with lower cost, Pluto Fast Flyby is being conceived as a pair of very small spacecraft, using, where possible, lightweight advanced-technology hardware components. The baseline Pluto Fast Flyby mission, based on a 1996 new start authorization, calls for launch of the two ~110-150 kg spacecraft in 1999-2000 toward encounters with Pluto and Charon around 2006-8. Pluto began receding from the Sun in 1989, and its thin atmosphere is condensing out into surface frost as it cools. Therefore, minimizing flight time and launching at an early opportunity is important for the mission's atmospheric and surface science objectives (see below). There is a direct relationship between spacecraft weight and flight time, so spacecraft design tradeoff analyses are particularly critical for this mission.

* PLUTO FAST FLYBY SCIENCE OBJECTIVES *

Characterize Pluto's and Charon's global geology and geomorphology.

Map the surface composition of both sides of each body.

Characterize Pluto's neutral atmosphere, measuring its composition, thermal structure, and aerosol content.

* CANDIDATE EXPERIMENTS *

Visible Imaging System: a charge-coupled device (CCD) imaging camera to map surface features and geomorphology of Pluto and Charon, and to search for small satellites.

Infrared Mapping Spectrometer (perhaps sharing foreoptics with the CCD camera) to study the surface composition of Pluto and Charon.

Ultraviolet Spectrometer to measure atmospheric composition.

Radio Science Uplink Occultation Experiment to profile temperature and pressure of the atmosphere from the surface through the ionosphere.

* MISSION CHARACTERISTICS *

TRAJECTORY:

Two spacecraft, on direct trajectories (i.e., no gravity-assists)

LAUNCH VEHICLES:

Titan IV/Centaur or Proton; both would entail kick stages

LAUNCH DATES:

1999-2000, assuming a 1996 new start

CRUISE: 6.5-8.5 years, depending on mass

CRUISE SCIENCE: None planned, but asteroid flyby, other imaging, H/He detection, and radio science are possible

ARRIVAL AT PLUTO-CHARON: 2006-2008, depending on mass and assuming a 1996 new start

FLYBYS:PFF-1 @ 10,000 km; PFF-2 TBD based on PFF-1 results; both flybys @12-18 km/sec

DATA RETURN: Onboard storage capability of at least 400Mb per spacecraft; science data downlink at 2540 bps to 34-meter ground stations

* BASELINE SPACECRAFT CHARACTERISTICS *

TYPE:Highly miniaturized descendant of the present class of outer solar system platforms: composite structure bus (14.6 kg), no deployables

MASS:Less than 150 kg; goal is 110-120 kg (7 kg total instrument allocation)

Power: RTG source providing 65 watts at Pluto

Communication: X-Band transponder; 1.47 meter high-gain antenna

Propulsion: Pressure-fed hydrazine monopropellant design delivering 350 m/s delta-V

Attitude Control: Widefield star sensor and three solid-state rate sensors

Pointing Knowledge: Will exceed 1.5 mrad; stability of 10 urad over 1 sec

Slewing Ability: 90 in 3 minutes via cold nitrogen gas thrusters

THE PLUTO FAST FLYBY CHALLENGE:

A BIG MISSION FOR SMALL SPACECRAFT

Recent interplanetary spacecraft like Galileo and the upcoming Cassini have been designed relatively large and heavy in order to bring a maximum exploration payload (including probes) through gravity assists and the intense radiation of Jupiter. A large mission of this type to Pluto had been under consideration since the Voyager 2 encounter with the frigid Neptunian moon Triton in 1989. The encounter revealed to a surprised science community that Triton had polar ice caps, evidence of seasonal changes, active volcanism, and an atmosphere. The implications for Pluto and Charon were recognized immediately, and spurred plans for a Cassini-class mission. But recent emphasis at NASA on smaller, cheaper, and faster missions pointed toward the possibility of a much smaller mission to Pluto-Charon. The key for such a mission is to deliver a scientifically useful payload to the distant system at minimum cost, and to do so before Pluto's atmosphere collapses (in about 2020).

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