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The blots.nasa.gov article NASA’s SunRISE Mission Studying Solar Particle Storms Moves Toward Launch says that the upcoming

SunRISE mission — short for the Sun Radio Interferometer Space Experiment — passed a mission review on Sept. 8, 2021, moving the mission into its next phase.

Later it goes on to say:

Consisting of six miniature solar-powered spacecraft known as CubeSats, the SunRISE constellation will operate together as one large telescope — forming the first space-based imaging low radio frequency interferometer — to create 3D maps pinpointing how giant bursts of energetic particles originate from the Sun and evolve as they expand outward into space. The mission will also map, for the first time, how the Sun’s magnetic field extends into interplanetary space — a key factor that drives where and how storms move throughout the solar system. Data from SunRISE will be collected and transmitted to Earth via NASA’s Deep Space Network. The six CubeSats will span roughly six miles across and fly slightly above geosynchronous orbit at 22,000 miles from Earth’s surface.

Question: Why put SunRISE in the graveyard? Why will the SunRISE mission "fly slightly above geosynchronous orbit"?

  1. Why anywhere near geosynchronous at all?
  2. Why "slightly above" rather than at? Why not slightly below?

one frame from a GIF at "SunRISE measures radio bursts from solar events and transmits the data to NASA’s Deep Space Network. Credit: NASA"

SunRISE measures radio bursts from solar events and transmits the data to NASA’s Deep Space Network. Credit: NASA (single frame from the GIF)

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    $\begingroup$ Just a guess - because there are a lot of opportunities for a rideshare launch. Big satellite is launched to geostationary orbit, and then the cubesats need just a little orbit ajustment. Low orbit is not convenient for this mission, it should fly above Earth's ionosphere. And further trajectories (Lagrange points, or heliocentric orbits) can't provide enough downlink bandwidth because of higher distance. I have no links to prove this, however. $\endgroup$
    – Heopps
    Sep 20 '21 at 12:10
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    $\begingroup$ Upd: yes, it's planned as rideshare with commercial satellite by Maxar launched to GEO. jpl.nasa.gov/missions/sun-radio-interferometer-space-experiment $\endgroup$
    – Heopps
    Sep 20 '21 at 12:14
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Why will the SunRISE mission "fly slightly above geosynchronous orbit"?

There are few advantages and practical reasons for doing so. The first advantage is trying to minimize Earth-based radio noise from contaminating the signal without having to send the spacecraft to some far away place like L1. If you are slightly above geosynchronous orbit, you can reduce a lot of the radio noise between spacecraft and ground (not all but a lot).

The mission launch requirements state that the

...SunRISE Observatory shall achieve a mean orbit equivalent in science return to the nominal 37,027 km altitude circular orbit with an eccentricity of $0^{\circ}$ and and inclination of $0^{\circ}$... As this orbit is above Earth's ionosphere and plasmasphere, it ensures that SunRISE can observe over the required frequency range (0.26 MHz < $\nu$ < 11.3 MHz...) and obtain the required observatory availability... This orbit is easily accessible on rideshares with telecommunication satellites. The notional orbit is known as a supersynchronous orbit and is where GEO spacecraft are maneuvered to be decommissioned, hence the common phrase "GEO graveyard."

Heopps is also correct that the current contract for the ride share is with Maxar/SSL.

The exact location of any given SunRISE CubeSat need not be well known, but the relative distance between each of the six CubeSats will be known to an extremely precise value using GNSS.

See also:

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  • $\begingroup$ Thanks for the answer! I now see that it must be in a high orbit to avoid the ionosphere, but any thoughts on "2. Why 'slightly above' rather than at? Why not slightly below?" Why exactly is "(t)he notional orbit... known as a supersynchronous orbit and is where GEO spacecraft are maneuvered to be decommissioned, hence the common phrase 'GEO graveyard'" better than subsynchronous or synchronous? $\endgroup$
    – uhoh
    Oct 27 '21 at 15:47
  • $\begingroup$ I guess my "notional" title could have been "Why put SunRISE in the graveyard?" In fact I like that so much I'm going to add it, since Halloween is coming! :-) $\endgroup$
    – uhoh
    Oct 27 '21 at 15:48
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    $\begingroup$ @uhoh - Slightly above, I think, helps reduce radiation hazards (i.e., less intense outer radiation belt fluxes) and helps reduce radio noise from other spacecraft (since most are at or inside geo). The graveyard part is an end-of-mission plan requirement from NASA, I think. $\endgroup$ Oct 27 '21 at 15:52
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    $\begingroup$ I see. So when the SunRISE spacecraft reach their end-of-mission, they are already where they need to be. This way they don't need a reliable system to move there, they can just be decommissioned in place. Putting it in the graveyard orbit from the start achieves this (requirement of EOM in graveyard) $\endgroup$
    – uhoh
    Oct 27 '21 at 20:21
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    $\begingroup$ @uhoh, GSFC states: "The altitude is generally increased, as opposed to decreasing, to prevent an accumulation of debris that future GEO missions would need to pass through to get to GEO, and to prevent potential signal interference. " - I guess referring to disposing above rather than below GEO $\endgroup$ Jan 16 at 15:29
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SunRISE will deploy a constellation of cubesat spacecraft which together will synthesize a radio-frequency antenna large enough to give the angular resolution required to study the solar phenomena of interest.

They will need to move those individual spacecraft around from time to time to optimize the geometry for aperture synthesis. If they were at geosynchronous altitude, they would need to coordinate each and every move with nearby satellites. That alone would be time consuming.

On top of that, the owners of communications satellites worth hundreds of millions of dollars would take a dim view of little cubesats flitting around their darlings.

Better to stay out of geosynchronous altitude entirely. They decided to go to the graveyard orbit because they can just turn off the cubesats at end of mission rather than having to save propellant to transfer there.

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For the specific part about why not in GEO, the short answer is because it doesn't need to be in GEO. Given the undesirability of a satellite collision in GEO, anything that doesn't need to be in GEO shouldn't be there. Putting it in the graveyard orbit from the start achieves this.

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Partial answer:

Why "slightly above" rather than at? Why not slightly below?

not directly related to SunRISE but https://ntrs.nasa.gov/api/citations/20130000278/downloads/20130000278.pdf had this to say regarding design end of mission passivation/moving to graveyard orbit:

The altitude is generally increased, as opposed to decreasing, to prevent an accumulation of debris that future GEO missions would need to pass through to get to GEO, and to prevent potential signal interference.

This post adds some more:

Why are obsolete geostationary satellites re-orbited above the geostationary belt?

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