Why will JUICE take 8 years to reach Jupiter, longer than any earlier spacecraft?

Question

Galileo took six years from Earth to Jupiter.

Cassini-Huygens took three years to reach Jupiter and six years to reach Saturn.

New Horizons took one year to reach Jupiter.

Juno was launched in 2011 and is planned to arrive at Jupiter in 2016, after five years.

In 2022, the ESA mission JUICE will depart for Jupiter. Arrival is not expected before 2030, eight years after launch. Or, more precisely: ESA JUICE Operations: 7.6 years.

Why is JUICE so slow? Are agencies using a slow technology to save costs and energy, are the planets poorly aligned^{heh heh}, or there some other boundary conditions?

Answer 1

From the Yellow Book:

"06/2022 - Launch by Ariane-5 ECA + EVEE-type Cruise"

EVEE means Earth Venus Earth Earth - a long series of gravity assist maneuvers to fit more payload into the Ariane 5 ECA launcher capabilities.

Why an Earth slingshot comes first:

The fact that the first arc includes an Earth gravity assist for both launch opportunities removes the dependency of the relative declination of Venus, and therefore the baseline and backup launches have the same escape velocity, resulting in similar launch mass performance.

ECA performance:

The ECA version is able to carry a satellite weighing 4100 kg towards the following earth escape orbit:

• infinite velocity V∞ = 3475 m/s

• declination δ = - 3.8°

JUICE:

The mass injected into the Earth escape trajectory would be 4800 kg (without launch adapter), with a hyperbolic escape velocity of 3.15 km/s.

References:

• Ariane 5 ECA user's manual: http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf

Answer 2

Deer Hunter's covered JUICE's flight itself, so I'm limiting my answer to addressing why New Horizons and Cassini were so much faster. There is a de facto upper bound in how long it can take for a probe to reach its destination because the scientists who do all the work to design and build a mission also want to be around for the payoff when it arrives. From initial design to arrival is already typically 10-15 years which is a large fraction of a career; if you add the years of lobbying and advocacy needed to get a mission approved it can easily use up half a scientists career just to get a probe in place to begin studying its target.

Cassini had to travel ~2x as far from the Earth as the Jupiter missions, New Horizons has ~8x as far to go. Total transit times from Earth to destination are almost 7 years for Cassini and 9.5 for New Horizons. The length of time New Horizons is taking has been described as a challenge for building and maintaining the team; and is why it's only doing a flyby not entering into orbit: The later would have required either a far larger probe to decelerate by rocket thrust on arrival, or a much slower transit to keep the total mass down. The first option wasn't realistic since it already was using the most powerful Atlas variant ever flown; and no other readily available rocket with significantly more power was available.

Answer 3

Because Ariane 5's upper stage is not reignitable, and thus the craft has to eject Earth in a single burn. This removes the possibility of launching towards Venus first, which would have saved some time, because the ejection angle to get to Venus is not compatible with the single burn the rocket can currently provide.

Ariane 6 is supposed to get a reignitable upper stage.