Why has no one launched a probe that goes into a highly elliptical orbit around the sun ( on a path that goes beyond Pluto without escaping the sun's gravitational field) and capture images as it tours the deep space and deliver all the data when it's on it's approach back to Sun?

  • $\begingroup$ Define "Highly Elliptical". $\endgroup$
    – PearsonArtPhoto
    Jul 16, 2015 at 17:52
  • 2
    $\begingroup$ Not on purpose, I hope. $\endgroup$
    – LocalFluff
    Jul 16, 2015 at 17:55
  • $\begingroup$ I suspect that Cassini was on a similar orbit to the one proposed while it was on it's way to Saturn. Probably the Pioneer/ Voyager probes as well. Usually if a space probe makes it out to Jupiter, it never gets closer to the Sun, and never if it gets to Saturn. $\endgroup$
    – PearsonArtPhoto
    Jul 16, 2015 at 18:00
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    $\begingroup$ What would be the point of such a mission? Making photos "of deep space" isn't very interesting, and as Voyager and New Horizons have shown, we can return data from Pluto if we want to, no return trip necessary. $\endgroup$
    – Hobbes
    Jul 16, 2015 at 18:15

4 Answers 4


The closest to this design was the Ulysses probe, which ended up in a heliocentric orbit with a semi-major axis of 3.37 AU and an eccentricity of 0.603, and an orbital period of 6.2 years. (Orbital elements available here.) It was launched in 1990, and operated until about 2008.

However, the point of this probe wasn't to take pictures from deep space, but rather to get pictures of the Sun—specifically, the sun's poles. To get a spacecraft into a polar orbit, you would have to get rid of a large amount of momentum in the plane of the Solar System and add a bunch of momentum perpendicular to the plane; and as we all know, the best way to get a big delta-v is to head out to Jupiter. So the craft went out to Jupiter, and used its gravity to slingshot itself out of the plane of the Solar System. (If you do the math, the perihelion of Ulysses's orbit is at about 5.4 AU, which you might recognize as the orbital radius of Jupiter.)

Ulysses did some science when it swung by Jupiter the first time, but most of its subsequent data collection was confined to the times (over three orbits) when it was over the Sun's poles. When it got back to Jupiter after its first orbit, of course, Jupiter had gone on around its orbit, and there wasn't much there to see. (It did apparently do some "distant observations" of Jupiter at the time, but it wasn't a close flyby—about 0.8 AU away.)


A 30-year mission is difficult to engineer for. The Voyagers are still alive (more or less) after 30 years, but that's as much luck as skill.

The probe would also have to be large:

  • it needs enough fuel to maneuver close to anything interesting (e.g. a KBO or comet) .
  • it needs a radar to find those objects. Radars are large and use lots of power (for the distances you need, dozens of kilowatts if not megawatts) so you need a nuclear reactor.
  • sample return (the biggest reason for designing a probe to return) is complicated and we've only done it a few times using unmanned probes.

Voyager is just about the largest object we can send to the Kuiper Belt at the moment, this mission would need to be many times heavier.

So we're talking about a massively expensive mission, with a mission objective that's really vague and has a low probability of success.

Given the realities of planetary exploration (only a few missions per year, tight budget constraints), only the missions that are most likely to succeed will be funded.

  • $\begingroup$ And you have to consider the PI Team -- PIs with enough seniority to lead up an expensive mission might not be alive by the time they've actually got their data back. (I think I heard that New Horizons specifically had some younger folks as Co-Is) $\endgroup$
    – Joe
    Oct 13, 2020 at 15:22
  • $\begingroup$ Yes, NH specifically recruited some young researchers with this in mind, and other long-term missions are doing the same. $\endgroup$
    – Hobbes
    Oct 13, 2020 at 15:39

There is not a spacecraft that does exactly as you suggest. The closest was probably Cassini, which was in an elliptical orbit to achieve orbit around Saturn, or Ulysses, which was used to orbit the Sun's poles, as @MichaelSeifert explained. Why not? There are quite a few reasons. Let's look at a few.

  1. The way that you have an elliptical orbit instead of an escape trajectory is a result of speed. An escape trajectory is a fast trajectory. For New Horizons to just meet Pluto and return home, it would take about a hundred years (About half of it's orbital period, which is the general rule of thumb for such). That's a really long time, and people wouldn't want to wait so long for their data.
  2. While sending data back quicker is nice, it is not essential. New Horizons will be able to send it's data back over 18 months. Even if it could somehow bounce and return home at the same speed that it went there, it wouldn't be able to send the data back much quicker, as it would still be far away during that time.
  3. There are other potentially useful missions for New Horizons to accomplish afterward.

Bottom line, it's faster, cheaper, and better to just let the spacecraft keep going it's way.


I'm guessing it's because 'delivering the data when it returns' isn't as good as it sounds.

We're receiving data just fine from New Horizons (albeit slowly), and I think this will always outweigh the benefits of a spacecraft physically coming closer. Light travels very, very fast, and having the point of transmission closer doesn't make a huge difference to the data downlink.

The main point, however, is that the spacecraft will eventually become useless in it's heliocentric orbit, because Pluto would have moved on when the probe returns to aphelion on it's second orbit. A flyby probe like New Horizons can go on and collect data from other Kuiper Belt Objects, and later outside the heliosphere.

New Horizons

The trajectory of New Horizons, on a path to escape the Solar System after flying past Pluto, and hopefully exploring new worlds beyond. (Courtesy NASA)

  • $\begingroup$ Thanks. The aim of the probe would be to discover objects that lie around the apogee areas of asteroids/comets... If we could embed a probe on asteroid and it goes on a ride into the deep space and it returns with some startling find.. who knows ? May be there are some small objects that can't be seen though Hubble that would be visible to the probe .. What scientific value does it add ? Not much. but something that shed light on unknown is always fascinating.. $\endgroup$ Jul 16, 2015 at 18:40
  • $\begingroup$ @SRudrabhatla You have to weigh the benefit of what can potentially be found against the increased risk, complexity and cost of the mission. New Horizons isn't exactly trivial. Look at the difficulties of the Rosetta/Philae mission to 67P. You are basically suggesting to, at least, combine the two. Doing anything like that would lead to an exponential increase in mission complexity, and that is assuming we even have boosters that would be capable of pushing the hardware onto an appropriate transfer orbit. You also have (relatively) very limited maneuvering capability out there. $\endgroup$
    – user
    Jul 17, 2015 at 13:09

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