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A Heliophysics Gateway to Deep Space

Are missions to the Sun particularly relevant to our interstellar ambitions? At the current state of our technology, the answer is yes. Consider Solar Cruiser, which is the planned NASA mission using a solar sail that could maintain non-Keplerian orbits, allowing it to investigate the Sun’s high latitudes. And throw in the European Space Agency-led Solar Orbiter, which left our planet early Monday (UTC) on a United Launch Alliance Atlas V rocket, lifting off from Launch Complex 41 at Cape Canaveral Air Force Station in Florida. Herewith the gorgeous arc of ascent:

Image: Launch of the ESA/NASA Solar Orbiter mission to study the Sun from Cape Canaveral Air Force Station in Florida on Feb. 9, 2020. Credit: Jared Frankle.

Missions to the Sun allow us to explore conditions close to a star and, significantly, deep in its gravity well, where interesting things can happen. When we discuss one way of propelling a sail beyond the heliosphere, the irony is that an Oberth maneuver, which takes place at a few solar radii, can bring additional chemical propulsion online at perihelion to extract the maximum push. So in propulsive terms, we go to the Sun in order to get flung from the Sun at highest speed. If we want to get beyond the heliosphere fast and with today’s tools, the Sun is a major factor.

Solar Orbiter is not, of course, designed around interstellar matters, but the synchronicity here works well for us. The more data about conditions near the Sun, the better for what we will want to do in the future. Günther Hasinger is the European Space Agency’s director of science:

“As humans, we have always been familiar with the importance of the Sun to life on Earth, observing it and investigating how it works in detail, but we have also long known it has the potential to disrupt everyday life should we be in the firing line of a powerful solar storm. By the end of our Solar Orbiter mission, we will know more about the hidden force responsible for the Sun’s changing behavior and its influence on our home planet than ever before.”

And, I would add, we’ll know a great deal more about how spacecraft operate inside Mercury’s orbit. Moreover, think about all the interesting maneuvers that have to take place to make this happen. Three gravity assists come into play as Solar Orbiter goes for the Sun, two of them past Venus in late 2020 and August of 2021, and one past Earth in November of 2021. The first close pass of the Sun will be in 2022, at about a third of an AU, with the gravity of Venus being used to push Solar Orbiter up out of the ecliptic plane. Ulysses achieved an inclined orbit in 1990, but Solar Orbiter will be carrying cameras allowing us to directly image the Sun’s poles, a role for which Ulysses was not equipped. The spacecraft is to reach an inclination 17 degrees above and below the solar equator.

Solar Cruiser and Solar Orbiter have much to teach us about interstellar possibilities, as does, for that matter, the continuing Parker Solar Probe mission. Along the way we learn, in addition to the significant science return about the Sun itself, about how spacecraft cope with being subjected to the solar wind and the temperatures of passage near the Sun. We learn about heat shielding and how to minimize what is needed so as to maximize payload. Solar Orbiter will face temperatures of up to 500º C, 13 times that experienced by satellites in Earth orbit.

So if we’re thinking deep space today, we should also be thinking about heliophysics. Our best bet at getting a successor to the Voyager missions well beyond the heliosphere and at significantly higher speeds that Voyager 1 is a close solar pass and propulsive kick that will demand deep knowledge of conditions at perihelion. Solar Orbiter’s 10 scientific instruments will measure electric and magnetic fields, passing particles and waves, solar atmospheric conditions and the outflow of material.

All these are factors as we contemplate the close approaches that will fling solar sails into the Kuiper Belt. In not many years, we could build a ‘sundiver’ mission that would make for great heliophysics as well as data from deep space — two missions in one.

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{ 14 comments… add one }
  • Charley February 11, 2020, 13:20

    Will this result in a real speed increase? Don’t you have to have a fair amount of propellant on board to make maximum use of the ‘Oberth maneuver’ ? And what would be the additional shielding mass that would have to be carried to protect the craft from the extreme heat?

    • Paul Gilster February 11, 2020, 14:21

      The concept is being explored in various places. The last analysis I saw was that Voyager 1’s speed could be roughly tripled if the maneuver were executed properly, so that’s a useful increase. I don’t have any figures on the mass of the shield, but it would certainly have been factored into this preliminary result.

      • Charley February 11, 2020, 20:39

        I’d like to add to the comment about the speed issue. While they view the idea that Voyager 1’s speed could be tripled in such a maneuver there’s always in my mind at least a downside to additional speed: slowing down. If you only have perhaps 10 minutes to do all the observations that you wish to do, even if you got there a lot faster, have you gained anything?

        I’m in no way pretending that I have the answer to any or all of these questions. It’s just that if one goes to a place to make observations, you got to kind of stay in the area to do so. I remember when they sent the Cassini mission to Saturn, the mission took seven years to get to the planet and the lander that landed on Titan lasted one half hour.

    • Alex Tolley February 11, 2020, 15:10

      The Parker Solar Probe will get to within 8.5 solar radii. The heat shield masses a mere 160 lbs, (72.5 kg). Therefore the propellant is the main extra mass. Bear in mind that the extra propellant is only needed to drop the craft into a close elliptical orbit of the sun (and even there, gravity assists can reduce the needed propellant). The propellant used to propel the craft from the sun is going to be the same as that which would have been used to propel the spacecraft outwards from Earth.

      This is the sort of mission where a solar sail can place the probe in the low perihelion, benefitting from the Oberth maneuver. Depending on sail performance, decisions can be made on whether to add any extra rocket propulsion for the Oberth maneuver.

      Vulpetti et al “Solar Sails: A Novel Approach to Interplanetary Travel” shows that pure sailing using sundiver trajectories can achieve velocities over 120 km/s, reaching the solar focal point at 550 AU in about 25 years. The key to all this is very low aerial densities of sails coupled with thermal stability at or near perihelion. Depending on the solar environment, a magsail or electric sail might be good technologies too. As the Parker Solar Probe’s heat shield can keep the probe at a comfortable 85F. there should be no problem is using even conventional chemical rockets behind a heat shield to execute such a maneuver.

      As the captain says in Bradbury’s “The Golden Apples of the Sun”, after the maneuver a ship will be traveling “North”.

      • Paul Gilster February 11, 2020, 16:21

        Love the reminiscence of “Golden Apples of the Sun”!

  • Thomas Goodey February 11, 2020, 17:15

    Surely, for a close-to-Sun Oberth maneuver, you don’t even need chemical propellant? Such a maneuver must be ideally suited to the use of a solar thermal rocket engine:
    https://en.wikipedia.org/wiki/Thermal_rocket#Solar_thermal_rocket

    • Alex Tolley February 11, 2020, 23:55

      The issue I see for a solar thermal rocket is that the concentrating reflector must be exposed to solar conditions. It will require very high reflectivity to prevent degradation. If we have that good of reflective material, then a heatshield could be a lot lighter. A chemical rocket can operate fully shielded.

      Having said that, I am a “fan” of solar thermal rockets. The concentrator can be a fixed solar reflector or fresnel lens, a beamed laser reflector or lens, or even a solar sail that is reconfigured as a concentrator when high thrust is needed. As always, the propellant can be any volatile, from water to hydrogen, suitable for a range of other propulsion systems (e.g. nuclear thermal). Solar thermal propulsion could even be made into a hybrid – solar thermal/electric for higher Isp.

      • Paul Gilster February 12, 2020, 8:07

        I think that’s right, Alex. Solar thermal in this environment would demand extensive cryo-cooling, heat exchange and a heat shield substantial enough to produce a significant mass penalty over a solid rocket engine.

  • Tim Kyger February 11, 2020, 21:28

    I have to make this observation.

    They launched to the Sun….they went at *night*!

    (((shaaaa-boom)))

  • Harry R Ray February 12, 2020, 10:55

    Paul Gilster: Matt Caplan has designed a SERIOUS UPGRADE to the Shkadov Thruster that can move the Sun FAST ENOUGH to escape an impending nearby supernova. Go to https://www.freethink.com/articles/stellar-engine/ ASAP for all the details.

    • Paul Gilster February 12, 2020, 11:41

      An upgrade to Shkadov gets my attention. Thanks for the tip.

    • Andrei February 12, 2020, 14:55

      All you need to do is to build a Dyson sphere to collect all that solar wind. Piece of cake, just remember to get a spare or even better a sixpack of Dysons on your way out from the mall.

  • Bruce D. Mayfield February 12, 2020, 12:53

    This discussion prompted me to learn more about the Oberth effect, and I copy this from wikipedia:

    “The Oberth effect is strongest at a point in orbit known as the periapsis, where the gravitational potential is lowest, and the speed is highest. This is because firing a rocket engine at high speed causes a greater change in kinetic energy than when fired at lower speed. Because the vehicle remains near periapsis only for a short time, for the Oberth maneuver to be most effective the vehicle must be able to generate as much impulse as possible in the shortest possible time.”

    That last sentence made me think, what gives the greatest possible thrust in the shortest possible time? Seems like a job for a well timed H-bomb, no?

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