Exhibit 5.1

September 12, 2025

Momentus Inc.

3901 N. First Street

San Jose, CA 95134

(650) 564-7820

Ladies and Gentlemen:

We have acted as counsel to Momentus Inc., a Delaware corporation (the “Corporation”), with respect to certain legal matters in connection with the filing with the Securities and Exchange Commission (the “Commission”) of a registration statement on Form S-1 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Securities Act”).

The Registration Statement relates to the issuance by the Corporation of up to (i) 4,862,058 shares of the Corporation’s Class A common stock, par value $0.00001 (“Common Stock”), issuable upon exercise of that certain common stock purchase warrant issued by the Corporation to Armistice Capital, LLC on August 14, 2025 (the “Inducement Warrant”); (ii) 8,696 shares of Common Stock issued to a vendor to settle outstanding debt of $10,000 under a debt settlement agreement (the “Debt Settlement Agreement”); (iii) 298,327 shares of Common Stock issuable upon conversion of the secured convertible promissory note dated July 12, 2024, by the Corporation in favor of Space Infrastructure Venture, LLC “SIV”), as amended and restated (the “Initial Convertible Note”); (iv) 3,125,462 shares of Common issuable upon conversion of the secured convertible promissory note dated October 24, 2024, by the Corporation in favor of SIV, as amended and restated (the “Subsequent Convertible Note” and, together with the Initial Convertible Note, the “Convertible Notes”); (v) 2,000,000 shares of Common Stock issuable upon the exercise of that certain common stock purchase warrant issued by the Corporation to SIV on September 8, 2025 (the “Additional Warrant”); (vi) 269,950 shares of Common Stock issuable upon the exercise of that certain common stock purchase warrant issued by the Corporation to SIV on November 14, 2024, as amended (the “First Warrant”); and (vii) 193,273 shares of Common Stock issuable upon the exercise of that certain common stock purchase warrant issued by the Corporation to SIV on December 8, 2024, as amended (the “Second Warrant” and, together with the Inducement Warrant and the First Warrant, the “Warrants”) (the shares of Common Stock registered pursuant to the Registration Statement, collectively, the “Securities”).

In connection with this opinion, we have examined and relied upon the originals, or copies certified or otherwise identified to our satisfaction, of (i) the Second Amended and Restated Certificate of Incorporation of the Corporation, as amended to date (the “Certificate of Incorporation”); (ii) the Amended and Restated Bylaws of the Corporation, as amended to date (the “Bylaws”); (iii) certain resolutions of the Board of Directors of the Corporation; (iv) the Warrants; (v) the Debt Settlement Agreement; (vi) the Convertible Notes; (vii) the Registration Statement; and (viii) such records, documents, certificates, and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below.

For purposes of this opinion, we have assumed, and express no opinion as to, the authenticity of original documents and the genuineness of all signatures, the conformity to the originals of all documents submitted to us as copies, the truth, accuracy and completeness of the information, representations and warranties contained in the instruments, documents, certificates and records we have reviewed, the legal capacity of all natural persons or entities (except for the Corporation), the absence of any undisclosed termination, modification, waiver or amendment to any document reviewed by us, the absence of any other extrinsic agreements or documents that might change or affect the interpretation or terms of documents we have reviewed, and the due authorization, execution and delivery of all such documents where due authorization, execution and delivery are prerequisites to the effectiveness thereof. As to any facts material to the opinions expressed herein that were not independently established or verified, we have relied upon oral or written statements and representations of officers and other representatives of the Corporation.

Based upon and subject to the foregoing, and the further limitations set forth below, we are of the opinion that the Securities have been duly and validly authorized for issuance and, when issued and delivered by the Corporation upon valid conversion of the Convertible Notes or exercise of the Warrants, as applicable, in accordance with and in the manner described in the Registration Statement, the Warrants, the Debt Settlement Agreement, and the Convertible Notes, as applicable, the Securities will be validly issued, fully paid and nonassessable.

Our opinion expressed above is subject to the qualifications that we express no opinion as to the applicability of, compliance with, or effect of any laws except the Delaware General Corporation Law (“DGCL”).

We do not find it necessary for the purposes of this opinion, and accordingly we do not purport to cover herein, the application of the securities or “Blue Sky” laws of the various states to the issuance of the Securities.

This opinion is limited to the specific issues addressed herein, and no opinion may be inferred or implied beyond that expressly stated herein. Our opinion expressed above is made as of the date hereof and is subject to, and may be limited by, future changes in the factual matters set forth herein, and we undertake no duty to advise you of the same. The opinion set forth herein is based upon the law in effect (and published or otherwise generally available) on the date hereof, and we assume no obligation to revise or supplement this opinion should such law be changed by legislative action, judicial decision or otherwise. In rendering our opinion, we have not considered, and hereby disclaim any opinion as to, the application or impact of any laws, cases, decisions, rules or regulations of any other jurisdiction, court or administrative agency.

This opinion is furnished to you in connection with the filing of the Registration Statement in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act.

We hereby consent to the filing of this opinion as Exhibit 5.1 to the Registration Statement. We also consent to the reference to our firm under the heading “Legal Matters” in the Registration Statement. In giving such consent, we do not believe that we are “experts” within the meaning of such term as used in the Securities Act or the rules and regulations of the Commission issued thereunder with respect to any part of the Registration Statement, including this opinion as an Exhibit.

	Very truly yours,

	/s/ Bradley Arant Boult Cummings, LLP