Doesn't the Pluto New Horizons spacecraft now own the fastest manmade object record?
- question from Chris Rawlins

Artist concept of New Horizons during its study of Pluto

As the New Horizons launch approached, many news reports hailed the new probe as the fastest spacecraft ever. However, that is not quite correct. We have previously written about two vehicles called Helios launched to study the Sun during the 1970s. Both of these probes attained maximum speeds of around 150,000 mph (250,000 km/h) at closest approach to the Sun in their highly elliptical orbits. Helios 2 was slightly faster than its twin craft, and this probe still holds the speed record as not only the fastest spacecraft but also the fastest manmade object in history.

New Horizons does hold a speed record of its own, however, but it is much more specific than simply the "fastest spacecraft." New Horizons is instead the fastest spacecraft launched from Earth to date. In other words, New Horizons was traveling faster as it left Earth orbit than any previous vehicle launched into interplanetary space. New Horizons attained an escape velocity of about 35,800 mph (57,600 km/h) as it departed Earth orbit. This speed is so fast that the probe reached the distance of the Moon in only nine hours (compared to three days for the Apollo missions) and will reach Jupiter in just 13 months. Previous holders of this speed record included the European/NASA probe Ulysses, designed to study the Sun, that was launched at 34,450 mph (55,400 km/h) in 1990 and NASA's Pioneer 10 launched to Jupiter at 32,400 mph (52,100 km/h) in 1972.

Artist concept of the Pioneer 10 and 11 probes

During its mission, New Horizons will pass within 1.4 million miles (2.27 million kilometers) of Jupiter to increase its speed thanks to a gravity-assist maneuver. This flyby will accelerate the craft to 52,000 mph (83,700 km/h) at its closest approach to the giant planet before flinging the probe outward on its long journey to distant Pluto. While this speed is faster than the probe's launch speed, it is still far from the maximum speed attained by both of the Helios vehicles. Indeed, it is even much slower than the fastest planetary flyby ever attempted. This record appears to belong to Pioneer 11 that came within a mere 26,725 miles (43,000 km) of Jupiter's atmosphere in 1974 and reached a top speed of 107,500 mph (173,000 km/h) in the process.

One final record that New Horizons could potentially have a shot at is the fastest interstellar spacecraft. This record currently belongs to Voyager 1 that is exiting our solar system at a rate of about 38,600 mph (62,100 km/h). The final speed of New Horizons is not yet known for certain since it has not been decided what trajectory the craft will follow as it heads out of the solar system. Mission controllers will not need to make this decision until shortly before the craft reaches Pluto in 2015. Either the craft will continue along its current path or it will be redirected to make a closer inspection of one or more icy bodies inhabiting an outer region of our solar system called the Kuiper Belt. Nevertheless, New Horizons will only be traveling at about 31,300 mph (50,370 km/h) when it encounters Pluto, and any change in direction to explore other bodies will almost certainly reduce the probe's speed further. Given the small size of Pluto and other Kuiper Belt objects, it seems doubtful that any gravity-assist maneuvers would have much of an impact on the probe's speed, so it is unlikely that New Horizons will ultimately leave the solar system at a higher velocity than Voyager 1.

In addition to the confusion over the speed record of New Horizons, another case of poor reporting in recent space news concerns the probe Stardust. Stardust was launched in 1999 to intercept the comet Wild 2 and collect dust particles for return to Earth. The probe's sample return capsule successfully re-entered Earth's atmosphere and landed in Utah on 15 January 2006.

Stardust sample return capsule after landing and streaking across the sky

It has been incorrectly reported in some news sources that the Stardust capsule became the fastest manmade object to ever make atmospheric entry since it was traveling at 29,000 mph (46,660 km/h). Just as in the case of New Horizons, this is not quite correct since the speed record Stardust achieved is not that general. Stardust does now hold the record for the fastest craft to enter Earth's atmosphere that had previously been held by Apollo 10. The Apollo 10 command module entered Earth's atmosphere in 1969 at a speed of 24,790 mph (39,885 km/h) while carrying a crew of three astronauts.

The fastest atmospheric entry of all-time, however, belongs to a different vehicle that plunged into the atmosphere of a planet other than Earth. The Galileo spacecraft spent eight years studying Jupiter and its moons before ending its mission by purposefully diving into the planet's atmosphere. During its suicide plunge in 2003, Galileo reached a top speed of about 108,000 mph (173,770 km/h). Galileo narrowly topped the previous record that had been held by its own atmospheric probe that dove into Jupiter's atmosphere at a rate of 106,000 mph (170,000 km/h) in 1995.

Artist concept of Galileo burning up in Jupiter's atmosphere

As we can see, spacecraft speed records can become a complicated subject since there are so many different ways the records can be measured. It is important to read all the fine print that describes a specific record in order to better understand how it compares to speeds of other craft. The following table compares the various records we have spoken of above and will hopefully help to clarify them.

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